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The characteristics of precipitation observed over Cypress Mountain during the SNOW-V10 campaign
Accepted Manuscript The characteristics of precipitation observed over Cypress Mountain during the SNOW-V10 campaign
H.W. Stephen Berg, Ronald E. Stewart, Paul I. Joe PII: DOI: Reference:
S0169-8095(16)30647-0 doi: 10.1016/j.atmosres.2017.06.009 ATMOS 3973
To appear in:
Atmospheric Research
Received date: Revised date: Accepted date:
24 November 2016 15 May 2017 9 June 2017
Please cite this article as: H.W. Stephen Berg, Ronald E. Stewart, Paul I. Joe , The characteristics of precipitation observed over Cypress Mountain during the SNOW-V10 campaign, Atmospheric Research (2017), doi: 10.1016/j.atmosres.2017.06.009
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The Characteristics of Precipitation Observed over Cypress
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Mountain during the SNOW-V10 Campaign
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*H.W. Stephen Berga ([email protected])
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Ronald E. Stewarta ([email protected])
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Paul I. Joeb ([email protected])
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a – Department of Environment and Geography, 470 Wallace Building, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2
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b – Cloud Physics and Severe Weather Research Section, Environment and Climate Change Canada, 4905 Dufferin St., Toronto, ON, Canada, M3H 5T4 Current affiliation: World Meteorological Organization Current email address: [email protected]
* Corresponding author Corresponding author no longer works at the University of Manitoba. Secondary email address is [email protected]. Postal address is: 162 Bartlet Ave Winnipeg, Manitoba, Canada R3L 0Z4 1
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ABSTRACT
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Cypress Mountain, north of Vancouver, is a 1440-m high coastal barrier for moisture-laden onshore airflow and is consequently subject to substantial precipitation. Unprecedented data from various precipitation events were obtained between January and April 2010 during the SNOW-V10 (Science of Nowcasting Olympic Weather for Vancouver 2010) field campaign occurring in conjunction with the 2010 Winter Olympics. Information was collected from specialized radar, enhanced surface weather stations, and operational observing systems. During this period, overall precipitation amounts were similar to long-term averages. Some precipitation events lasted ≥ 24 h, although periods with heavier precipitation rates lasted ≤ 6 h. Temperatures were generally above 0°C at an observation site near the base of the mountain so snow was almost entirely absent, which was sometimes the case at other sites higher and nearer its peak. Precipitation amounts from some events were similar at the base and near the summit but other events showed much more precipitation at higher elevations. Most of these latter cases were linked with strong, sustained upward particle velocities on the upwind side of the mountain and this flow was also suggested to be a contributing factor for freezing rain occurring near the peak on occasion.
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Key words: mountain precipitation; rain; snow; freezing rain
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ACCEPTED MANUSCRIPT 1. Introduction
Mountainous areas are often subjected to substantial precipitation due to the upward forcing of air which promotes increased condensation. This enhanced precipitation is especially likely if the mountains are adjacent to a large body of water allowing significant onshore moisture advection (Houze, 2012). The precipitation falling on the mountains can be liquid or
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solid and, although seemingly straightforward, many factors affect what type of precipitation
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actually occurs (Stoelinga et al., 2013).
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One mountainous region of importance in Canada is the Lower Mainland of British Columbia (BC). This region was of particular significance during the 2010 Winter Olympics
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when freestyle skiing and snowboarding events at Cypress Mountain (north of Vancouver) were delayed or postponed numerous times, due to a shift towards rainfall instead of snowfall
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(Guttsman, 2010). Simultaneously, an intensive field campaign, the Science of Nowcasting Olympic Weather for Vancouver 2010 (SNOW-V10), was conducted to improve understanding
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of precipitation processes in this region and mountain meteorology in general (Isaac et al., 2014). Numerous SNOW-V10 studies have been undertaken to contribute to knowledge of
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precipitation in this region with most of the attention paid to the meteorology over the Whistler area. Little attention has been paid to Cypress Mountain, and none of the articles within the
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special SNOW-V10 issue of Pure and Applied Geophysics published in January 2014 (see for example Isaac et al. and Joe et al.) specifically focused on this mountain. This article begins to
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address this gap. In particular, its objective is to improve understanding of the distribution and production of precipitation over Cypress Mountain during the SNOW-V10 period, given the
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recent advances in knowledge from other field campaigns like the Mesoscale Alpine Programme (MAP; Bougeault et al., 2001) and Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE; Stoelinga et al., 2003). This objective will be achieved by first characterizing the precipitation and then examining its associated atmospheric conditions. Cypress Mountain, located approximately 20 km northwest of downtown Vancouver (Fig. 1), has a pyramidal shape with a summit elevation of 1440 m above sea level (ASL). Substantial precipitation occurs on the mountain, with average annual amounts ≥ 2500 mm near its peak, although this is highly elevation dependent. Oke and Hay (1998) developed a 3
ACCEPTED MANUSCRIPT climatology for the Vancouver region which showed an approximate 100 mm increase in annual
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precipitation per 100 m increase in elevation over Cypress Mountain.
Fig. 1. Location of Cypress Mountain (yellow pin) with respect to the BC Lower Mainland
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including the City of Vancouver. The locations of the Aldergrove and Mount Sicker radars, as well as the Quillayute, WA, rawinsonde site (all referred to within the article) are indicated with
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red pins. (Adapted from Google Earth, 2015, with data from SIO, NOAA, U.S. Navy, NGA, GEBCO, LDEO Columbia, NSF, NOAA, Landsat / Copernicus).
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This article is structured as follows. Section 2 describes the instruments on Cypress Mountain as well as other datasets used in this study. The meteorological conditions, specifically temperature and precipitation, are described in Section 3. This is followed by analyses of days with significant precipitation (≥ 10 mm on Cypress Mountain) and two illustrative case studies in Section 4. Interpretations and implications are discussed in Section 5 and concluding remarks follow in Section 6.
2. Data and Methodology 4
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Several datasets were utilized in this study, which is based on measurements made from January to April 2010. To characterize large-scale circulation and synoptic patterns, satellite imagery from GOES-11, surface analyses from Environment Canada, and data from re-analyses performed by the Earth System Research Laboratory (2015, using the methodologies of Kalnay et al., 1996, and Kistler et al., 2001) were examined. Data from the Environment Canada
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Doppler radar situated at Aldergrove and Mount Sicker, BC, from January 13 to April 30 were
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analyzed to describe precipitation patterns over the Cypress Mountain area. Rawinsonde data
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from Quillayute, Washington (WA; University of Wyoming, 2015) were utilized to determine the presence of blocking via the inverse Froude number technique employed by Colle et al.
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(2013) and Fargey et al. (2014). Variables from the lowest 100 m ASL and the nearest value to 1440 m ASL were utilized in these calculations.
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As indicated in Fig. 2, meteorological observation station data from three stations were examined to quantify surface temperature, humidity, wind, and precipitation (including rates,
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types, and amounts): West Vancouver (WWA, located at the base of the mountain at 168 m ASL), Cypress Bowl South (VOG, located on the mountain at 886 m ASL), and Cypress Bowl
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North (VOE, located on the mountain at 953 m ASL). Multi-year precipitation records from Environment Canada (2012) exist at both West Vancouver (1992-2013) and Cypress Bowl North
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(1955-1995 nearby at Hollyburn Ridge) and these are used for calculating long-term precipitation amounts even though the time periods are quite different. Data from a vertically-
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pointing Micro Rain Radar (MRR) at West Vancouver were obtained between January 22 and April 29 to examine the vertical structure of precipitating systems (Joe et al., 2014).
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Different methods of inferring precipitation were utilized. The Pluvio precipitation gauge was used at West Vancouver and Cypress Bowl North (Joe et al., 2014). Precipitation Occurrence Sensor System (POSS) instruments were utilized at West Vancouver and Cypress Bowl South to infer precipitation types (Joe et al., 2014). (Precipitation types at Cypress Bowl South were therefore utilized for Cypress Bowl North.) The same types of instruments were employed at Whistler during SNOW-V10, and Boudala et al. (2014) and Gultepe et al. (2014) noted that corrections for under-catch were needed with respect to wind speed. The correction provided by Goodison et al. (1998) was subsequently applied in this study. Precipitation
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approximately 1.5 m/s.
Fig. 2. Topographical view of Cypress Mountain. The three sites indicated in red were the
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primary meteorological stations at Cypress Mountain for SNOW-V10. The site indicated in brown is Hollyburn Ridge (at Hollyburn Mt., 930 m ASL). Sites in yellow indicate important
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operational or tourist sites. Mt. Strachan is the main summit at Cypress Mountain, whereas Hollyburn Mt. and Black Mt. are secondary summits.
3. Meteorological Conditions 3.1. Temperatures, Synoptic Conditions, and El Niño – Southern Oscillation
Mean temperatures from January to April 2010 were above the 1992-2013 average for West Vancouver and above the 1955-1995 average for Cypress Bowl North. Mean temperatures 6
ACCEPTED MANUSCRIPT at West Vancouver and Cypress Bowl North were 2.3°C and 1.3°C above average, respectively. The ranges of monthly temperature anomalies at West Vancouver and Cypress Bowl North were +1.0°C to +3.9°C and -0.6°C to +3.8°C, respectively. At both stations, the lowest anomalies occurred in April and the highest anomalies occurred in January. Numerous low pressure systems developed over the northeast Pacific Ocean during the period. An El Nino event influenced the development of these large scale circulation systems
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Mountain (Seager et al., 2010; Mo et al., 2014; Doyle, 2014).
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and generated substantial moisture advection and anomalously warm temperatures over Cypress
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3.2. Precipitation
From January to April 2010, West Vancouver experienced near-average precipitation
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amounts, whereas Cypress Bowl North experienced substantially higher than average amounts. Precipitation amounts measured at West Vancouver and Cypress Bowl North were 797 mm and
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1616 mm, respectively, a difference of 819 mm. This increase of just over 100 mm per 100 m in elevation slightly exceeded the annual climatological average found by Oke and Hay (1998).
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Monthly precipitation amounts at West Vancouver (Fig. 3a) and Cypress Bowl North (Fig. 3b) were not extreme. West Vancouver recorded near-average monthly precipitation (+20
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mm on average with a monthly range of -49 mm to +31 mm), whereas Cypress Bowl North experienced greater than average precipitation (+565 mm on average with a monthly range of
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+76 mm to +236 mm). At West Vancouver, precipitation amounts were ≥ 100 mm less than long-term maximum values for every month of examination, whereas at Cypress Bowl North,
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positive precipitation anomalies of 236 mm and 154 mm were recorded for January and April, respectively, although these precipitation amounts were 76 mm and 57 mm less than long-term maximum values.
A substantial portion of the precipitation at West Vancouver occurred in January, in particular over a 5-day period (January 11-15) during which 174 mm was measured (Fig. 4a). Even so, monthly precipitation amounts for January were barely above the long-term average. Precipitation amounts were substantially greater at Cypress Bowl North for the same 5-day period: 318 mm (Fig. 4b). In terms of daily precipitation records, both West Vancouver and Cypress Bowl North experienced 7 days of precipitation which equalled or exceeded the 7
ACCEPTED MANUSCRIPT maximum recorded values, although the period of record at West Vancouver (21 years) was half that of Hollyburn Ridge (41 years). Many precipitation events occurred, with 78 and 84 days having ≥ 0.2 mm of precipitation at West Vancouver and Cypress Bowl North, respectively. The former value was close to the long-term average for West Vancouver of 72 days, whereas the latter value exceeded the long-term average of 67 days. West Vancouver and/or Cypress Bowl North recorded ≥ 10
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mm of precipitation on 44 days: 29 days at West Vancouver and 42 days at Cypress Bowl North.
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On 2 days, ≥ 10 mm of precipitation was recorded at West Vancouver but not at Cypress Bowl
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North.
The evolution of the cumulative precipitation at West Vancouver and Cypress Bowl
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North from January to April 2010 and their difference are shown in Figs. 5 and 6. The step-wise progression of differences illustrates substantial variation. On some days, there were small
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differences in precipitation amounts, whereas other days had large differences with some exceeding 50 mm.
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The amount of rain and snow also varied substantially between West Vancouver and Cypress Bowl North. Of the 797 mm total precipitation at West Vancouver, 489 mm fell as rain,
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whereas of the 1616 mm at Cypress Bowl North, 256 mm occurred as snow and 810 mm fell as rain.
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The duration of each precipitation type at West Vancouver and Cypress Bowl South was also substantially different. At West Vancouver, snow was never inferred as the dominant
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precipitation type. A total of 425 h of rain was inferred, with a maximum duration of 9 h. At Cypress Bowl South, 107 h of snow and 428 h of rain were inferred. The maximum duration of
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snow was 8 h, whereas the longest duration of rain was 13 h. Some individual precipitation events lasted ≥ 24 h (with some brief pauses of < 1 h), but periods with heavier precipitation rates (≥ 5 mm/h) lasted ≤ 6 h. (No comparisons can be made between West Vancouver and Cypress Bowl South precipitation types and long-term averages, since similar precipitation type data were not collected beyond the SNOW-V10 field campaign.)
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Fig. 3. Monthly precipitation amounts at (a) West Vancouver and (b) Cypress Bowl North for January to April 2010 compared to long-term maximum, average, and minimum values. West Vancouver values were compared to the 1992-2013 record, whereas Cypress Bowl North values were compared to the 1955-1995 record at Hollyburn Ridge (Environment Canada, 2012).
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Fig. 4. Daily precipitation amounts at (a) West Vancouver and (b) Cypress Bowl North from January to April 2010 compared to highest recorded and average values. West Vancouver values were compared to the 1992-2013 record, whereas Cypress Bowl North values were compared to the 1955-1995 record at Hollyburn Ridge (Environment Canada, 2012). The yellow line indicates the beginning of MRR operations.
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Fig. 5. Cumulative precipitation (in mm) at West Vancouver and Cypress Bowl North and their
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difference (Cypress Bowl North – West Vancouver) from January to April 2010. The yellow
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line indicates the beginning of MRR operations.
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Fig. 6. Difference in daily precipitation amounts (Cypress Bowl North – West Vancouver) on
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days with daily precipitation amounts of ≥ 10 mm at either site from January to April 2010. The
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yellow line indicates the beginning of MRR operations.
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4. Precipitation Events
4.1. Specific Events
As described above, the daily amount of precipitation and the relative amount between sites varied greatly. The 44 days with ≥ 10 mm of precipitation at West Vancouver and/or Cypress Bowl North were examined to determine key factors leading to the variable precipitation. Two days were chosen to illustrate the distinct patterns regarding forcing factors present on days with small or large differences between these sites. February 2 had a small difference in 12
ACCEPTED MANUSCRIPT precipitation amounts (2 mm), whereas April 3 had a large difference (52 mm). Substantial differences in precipitation amounts were also present between the two days at each site; 16 mm and 3 mm were recorded at West Vancouver on February 2 and April 3, respectively, whereas 18 mm and 55 mm were measured at Cypress Bowl North on February 2 and April 3, respectively. Precipitation amounts for February 2 and April 3 were also separated by precipitation type inferred by the POSS. On February 2, 13 mm (out of 16 mm) fell as rain at West
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Vancouver, whereas 4 mm (out of 18 mm) fell as rain and 11 mm fell as snow at Cypress Bowl
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North. On April 3, 3 mm (out of 3 mm) fell as rain at West Vancouver, whereas 37 mm (out of
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55 mm) fell as rain, 1 mm fell as snow, and the other 17 mm occurred when the POSS at Cypress Bowl South were identified as “indeterminate” which indicates that the precipitation may have
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been mixed, drizzle, or snow (Sheppard and Joe, 2000). There is also evidence for the presence of freezing rain on April 3. Freezing rain was inferred for 2 h at Cypress Bowl South with
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temperatures of approximately -0.4°C to -0.8°C at Cypress Bowl North over the 04-06 UTC period. Precipitation amounts of 20 mm (included in the 37 mm rainfall total) were measured
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during the 2 h of freezing rain.
Infrared satellite imagery and surface analyses (Figs. 7 and 8) depict different circulation
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patterns for February 2 and April 3. A weak 998 mb low pressure system was present on February 2 southwest of the Cypress Mountain area with cloud top temperatures of
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approximately -40°C over the region. Conversely, a deep 979 mb cyclonic system was situated over the region at the beginning of April 3, with cloud top temperatures ≤ -50°C. Moisture
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advection was much stronger on April 3 than on February 2, due to the much stronger circulation. Inverse Froude numbers calculated from Quillayute, WA, rawinsonde data for the
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two days were ≤ 0.2, indicating the presence of unblocked flows in both cases. (See glossary near the end of the article for a brief description of the inverse Froude number.)
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Fig. 7. Enhanced GOES-11 infrared satellite imagery for (a) 1300 UTC February 2 and (b) 0300
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UTC April 3, 2010.
Radar reflectivity imagery (Fig. 9) infers less precipitation on February 2 compared to
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April 3 over Cypress Mountain. Reflectivity values rarely exceeded 25 dBZ over Cypress Mountain on February 2, whereas values of ≥ 35 dBZ were frequently detected on April 3. The
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spatial extent of the precipitation was also much smaller on February 2 than on April 3. Radial velocity imagery from Aldergrove Doppler radar (Fig. 10) shows a substantial
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difference in the strength of the low level airflow. Southerly to west-southwesterly flows of ≤ 5 m/s were common over Cypress Mountain on February 2 (shown in dark red in Fig. 10a),
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whereas on April 3, strong southwesterly airflows, sometimes ≥ 20 m/s, were detected over the mountain (shown in light orange and yellow in Fig. 10b). Mean daily 850 mb wind speeds over Cypress Mountain from re-analyses were 5 m/s and 10 m/s on February 2 and April 3, respectively.
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Fig. 8. Surface analyses for (a) 1200 UTC February 2 and (b) 0000 UTC April 3 provided by Environment Canada.
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m/s (indicative of rain) below 1000 m AGL (1168 m ASL).
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(indicative of snow) within the layer 1000-3000 m AGL (1168-2168 m ASL) to values of 4-9
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The MRR detected widely varying values of upward particle Doppler velocities of precipitation. The Nyquist velocity limits of the MRR are 0 m/s and 12 m/s. Positive Doppler
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velocity values are interpreted as downward particle fall speeds in the absence of strong vertical air motions. When there is substantial upward air motion of ≥ 1 m/s, the particle Doppler
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velocities will decrease and can be negative and are aliased or folded, as shown in imagery such as Fig. 11b (see direction of arrows within image). On February 2, the MRR detected minimal
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upward velocities (shown in Fig. 11a with yellow colours folding over to 11.5-12 m/s from 1600-3200 m AGL (1768-3368 m ASL)) and the melting layer was from 400-800 m AGL (568-
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968 m ASL). Conversely, on April 3, the MRR detected 7.5 h of upward velocities, with maximum values ≥ 5 m/s (shown in Fig. 11b with red and orange colours folding over to ≤ 7 m/s
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(368-768 m ASL).
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from 600-1400 m AGL (768-1568 m ASL)), and the melting layer was from 200-600 m AGL
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Fig. 9. Illustrative Vancouver composite 2-km CAPPI reflectivity radar imagery for (a) 1310 UTC February 2 and (b) 0310 UTC April 3, 2010. Range rings are 40 km. Cypress Mountain is located within the black circle (diameter approximately 40 km).
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Fig. 10. Doppler 0.5° PPI radial velocity imagery from Aldergrove for (a) 1310 UTC February 2 and (b) 0310 UTC April 3, 2010. Range rings are 20 km. Cypress Mountain is located within the white circle (diameter approximately 40 km). Over Cypress Mountain, the elevation of the radar beam is approximately 1000 m ASL. Red colours are away from the radar and blue colours are towards the radar. White arrows indicate the direction of airflow.
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ACCEPTED MANUSCRIPT Strong updrafts generated from strong onshore flows over Cypress Mountain sometimes produced large upward particle Doppler velocities (≥ 2 m/s). These updrafts were generally continuous rather than sporadic. This implies a steady, upward flow as opposed to convective, highly variable flow. One example of this is the steady particle motion patterns detected by MRR between 00 UTC and 05 UTC on April 3 in Fig. 12a (for which Nyquist velocity folding is shown, with particle Doppler velocities of 7-11 m/s detected). Strong, low-level, onshore flows
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which frequently generated rapid upward particle velocities (≥ 2 m/s).
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(sometimes ≥ 20 m/s) towards Cypress Mountain were present on April 3 (shown in Fig. 10b),
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Substantial changes in precipitation patterns occurred on the mountain as a consequence of these differences in updrafts and upward particle Doppler velocities. As mentioned earlier,
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large precipitation amount differences were measured between the two days and between the two sites. On February 2, a day with minimal to non-existent upward particle velocities, precipitation
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amounts at West Vancouver and Cypress Bowl North were similar. Maximum precipitation rates of 2 mm/h and 3 mm/h were measured at West Vancouver and Cypress Bowl North,
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respectively. Conversely, on April 3, a day with a long duration of rapid upward particle velocities, much less precipitation fell near the base (at West Vancouver) compared to at higher
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elevations (at Cypress Bowl North). Maximum precipitation rates of 2 mm/h and 23 mm/h were measured at West Vancouver and Cypress Bowl North, respectively, occurring at the same time
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maximum upward particle Doppler velocities were ≥ 5 m/s.
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Fig. 11. Doppler spectrum data from MRR at West Vancouver for (a) 1310 UTC February 2, 2010 and for (b) 0310 UTC April 3, 2010. Heights are indicated as above ground level (AGL), which is 168 m lower than the above sea level (ASL) heights referenced in the text. Violet arrows in (b) indicate direction of motion of precipitation particles. 20
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Fig. 12. Time-height plots of (a) particle Doppler velocity and (b) reflectivity from MRR at West Vancouver on April 3, 2010.
4.2. Comparison with other Events during SNOW-V10
The features present on February 2 and April 3 occurred on other days as well. Substantial precipitation amounts (≥ 50 mm) fell on 7 days at Cypress Bowl North when the 21
ACCEPTED MANUSCRIPT mean daily 850 mb winds were ≥ 10 m/s as inferred from re-analysis information over Cypress Mountain (Table 1; Earth System Research Laboratory, 2015). The largest precipitation amounts, between 70 mm and 125 mm, occurred with the strongest mean daily 850 mb winds (≥ 14 m/s). Mean daily 850 mb winds were between westerly and southeasterly (i.e. upslope on Cypress Mountain) for all of the days included in Table 1. Low-level directional wind shear was present on each of these days, with wind directions at West Vancouver and Cypress Bowl North
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typically differing by 60-90°. Speed wind shear of ≥ 2 m/s between the surface and 850 mb over
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the Cypress Mountain region was also typically inferred from re-analysis information on these
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days.
Some days with strong onshore flows had low precipitation amounts on Cypress
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Mountain. Onshore flows of ≥ 10 m/s were present on five days with ≤ 20 mm of precipitation at both West Vancouver and Cypress Bowl North. Surface relative humidity values were near-
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saturation (≥ 90%) for ≥ 12 h on each of these days.
Other contributing factors were present on some days when precipitation amounts were ≥ 10 mm lower at West Vancouver than at Cypress Bowl North. On eight days, long durations of
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upward particle Doppler velocities (≥ 2 h) were detected by MRR. Mean daily 850 mb wind
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speeds were ≥ 10 m/s on each of these days except for on April 8. On 15 days, maximum upward velocities ≥ 2 m/s were also detected by MRR.
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Precipitation amounts at West Vancouver were also ≥ 10 mm lower than at Cypress Bowl North when orographic enhancement of precipitation was detected by Aldergrove radar.
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Orographic enhancement occurred on 25 days according to our definition, which is that radar reflectivity values had to increase by ≥ 10 dBZ from 5-10 km upstream of Cypress Mountain to
Table 1.
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above and downstream of the mountain as evident on 1.5 km CAPPI imagery.
Days with precipitation amounts ≥ 10 mm on Cypress Mountain for January to April 2010. “NA” in upward particle Doppler velocity categories denotes when the MRR was not operational. Wind speeds at 850 mb are mean daily values. The two case studies (February 2 and April 3) are italicized and in bold. In the flag column, “O” denotes days with strong onshore flows (mean daily 850 mb winds ≥ 10 m/s) with ≤ 20 mm of precipitation on Cypress Mountain, “L” denotes days with long durations of upward particle Doppler velocities (≥ 2 h), “M” denotes days with maximum upward particle Doppler velocities ≥ 2 m/s, and “E” denotes days with orographic enhancement of reflectivity on radar (increases of ≥ 10 dBZ over the mountain 22
ACCEPTED MANUSCRIPT compared to 5-10 km upstream at approximately 2 km ASL). Days which did not meet these criteria and experienced < 1 h of upward particle Doppler velocities and < 10 m/s mean daily 850 mb wind speeds were excluded.
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850 mb Wind Speed (m/s) 12 10 10 14 16 12 8 11 16 13 13 15 5 12 9 13 11 10 8 11 6 4 14 7 13 15 8 13 10 10 11 8 9 10
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Maximum Upward Particle Velocity (m/s) NA NA NA NA NA NA NA NA NA NA NA 2.0 0.0 0.5 1.0 2.0 4.0 2.0 2.0 0.5 1.0 0.5 2.0 3.0 4.5 5.0 2.0 5.0 5.0 1.0 2.0 3.0 1.5 4.0
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Upward Particle Velocity Duration (h) NA NA NA NA NA NA NA NA NA NA NA 0.8 0.0 0.2 0.2 2.3 2.0 1.2 1.5 0.7 0.8 0.2 1.0 1.5 2.0 4.0 1.0 4.0 7.5 0.3 2.8 3.5 0.7 1.7
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01-Jan 02-Jan 08-Jan 09-Jan 11-Jan 12-Jan 13-Jan 14-Jan 15-Jan 17-Jan 18-Jan 25-Jan 02-Feb 05-Feb 11-Feb 12-Feb 13-Feb 14-Feb 16-Feb 26-Feb 27-Feb 28-Feb 11-Mar 13-Mar 28-Mar 29-Mar 30-Mar 02-Apr 03-Apr 05-Apr 07-Apr 08-Apr 24-Apr 27-Apr
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Date
West Cypress Vancouver Bowl North Precipitation Precipitation Amount Amount (mm) (mm) 22 50 17 16 9 13 31 39 58 103 15 27 10 24 20 38 71 125 12 32 10 22 7 33 16 18 8 14 10 36 6 27 11 35 18 36 21 46 10 25 28 66 7 23 40 77 9 36 4 16 27 78 5 24 33 69 3 55 14 10 9 21 27 43 9 19 30 47
Flag
O O
E E E E E ME O E LME LME ME ME E E E ME ME OLME LME ME LME LME O LME LME E ME
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These features were not all necessary for substantial and large differences in precipitation amounts on Cypress Mountain. On February 27, substantial precipitation amounts at Cypress Bowl North (66 mm) and a large difference in precipitation amounts between West Vancouver and Cypress Bowl North (38 mm) were recorded. Orographic enhancement of precipitation was
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detected on radar, but the mean 850 mb wind speed was 6 m/s, the duration of upward particle
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Doppler velocities was < 1 h, and maximum upward particle velocities were 1 m/s. A large
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cyclone was present near the region, which advected the substantial moisture necessary for precipitation towards Cypress Mountain. A frontal system would have provided general lifting, condensation, and precipitation formation aloft and it also would have generated the stability and
5. Discussion
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5.1. Summary of Precipitation Conditions
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wind fields that were critical for varying precipitation differences between the two stations.
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Warmer than average temperatures were present over the region, attributed to the El Niño event (Seager et al., 2010; Mo et al., 2014; Doyle, 2014). These warmer conditions resulted in a
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shift in precipitation from snow to rain. Mean temperatures were 2.3°C above average at West Vancouver and 1.3°C above average at Cypress Bowl North. Snow was never inferred as the
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dominant precipitation type at West Vancouver, whereas snow made up < 20% of cumulative precipitation inferred at Cypress Bowl North (compared to the long-term average of 45%).
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Collectively, precipitation amounts during the January-April 2010 period were somewhat typical but also illustrated some departures from the long-term. Cumulative precipitation at West Vancouver and Cypress Bowl North was 797 mm and 1616 mm, respectively. This represents a difference of just over 100 mm per 100 m of elevation, similar to the average annual precipitation climatology by Oke and Hay (1998). Precipitation amounts at West Vancouver were just 20 mm above average, whereas they were > 500 mm above average at Cypress Bowl North.
5.2. Flow Regime 24
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Unblocked flows were present for most of January to April 2010. Inverse Froude numbers were < 0.1 for 138 and < 0.3 for 219 out of the 240 total launches from Quillayute, WA, indicating the presence of unblocked flows was typical (Fig. 13). Both short and long duration upward particle Doppler velocity events and stronger upward particle Doppler velocities occurred when the inverse Froude number was < 0.2. These conditions were typically present
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during periods with higher precipitation amounts on Cypress Mountain. Upward particle
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Doppler velocities were not detected when the inverse Froude number was ≥ 0.2.
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Just 11 launches showed inverse Froude numbers > 0.5, indicating more blocked rather than unblocked flows. Little precipitation occurred in these conditions; the maximum 12-h
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precipitation amount at Cypress Bowl North was 5 mm.
Fig. 13. Number of rawinsonde launches arranged according to inverse Froude numbers at Quillayute, WA for January-April 2010. 25
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A relationship was described by Jiang (2003), which estimates the precipitation rate from unblocked flows over mountains in non-sheared environments, depending on the speed of onshore flow and the moisture content and stability of the air. This relationship predicts an increasing precipitation rate with height on a mountain given stronger onshore airflows and greater moisture content and instability, but the rate of increase in the precipitation rate decreases
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above a specific height depending on the same factors. However, this relationship cannot be
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utilized here, given the presence of low-level speed and directional wind shear between surface
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measurement stations on days with precipitation amounts of ≥ 10 mm.
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5.3. Upward Particle Doppler Velocities
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The presence of upward particle Doppler velocities was an important aspect on days with ≥ 10 mm precipitation on Cypress Mountain. For the January 22 to April 29 operational period
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for the MRR, 15 days with ≥ 1 h of upward particle Doppler velocities were detected for a large proportion of the precipitation. There was also a positive correlation between highest upward
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particle velocities and largest differences in precipitation. On each of these days, differences in precipitation amounts of ≥ 10 mm between West Vancouver and Cypress Bowl North were
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recorded (Figs. 14 and 15).
Strong updrafts are required to achieve upward particle Doppler velocities, particularly in
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the case of rain. For raindrops 1 mm and 3 mm in diameter to reach upward velocities of 4 m/s, updraft speeds of approximately 8 m/s and 12 m/s, respectively, are required. The MRR inferred
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raindrops exhibiting upward velocities at Cypress Bowl South for 27 h. For snow to reach upward particle velocities of 4 m/s, however, updraft speeds of approximately 5-6 m/s are necessary. The MRR inferred snow exhibiting upward velocities at Cypress Bowl South for 20 h.
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Fig. 14. Daily precipitation differences (Cypress Bowl North – West Vancouver) on days with ≥
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10 mm of precipitation as functions of duration of upward particle Doppler velocities detected by
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MRR at West Vancouver. The two days discussed in Section 4 are shown in red.
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Fig. 15. Daily precipitation differences (Cypress Bowl North – West Vancouver) on days with ≥
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10 mm of precipitation as functions of maximum upward particle Doppler velocity detected by
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MRR at West Vancouver. The two days discussed in Section 4 are shown in red.
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5.4. Fall Trajectory for Freezing Rain Occurrence
Freezing rain was inferred on Cypress Mountain on 6 days between January and March, typically for short periods: March 11, March 13, March 14, March 29, March 30, and March 31. Of the 5 h of freezing rain with precipitation rates ≥ 1 mm/h, 4.5 h occurred with upward particle Doppler velocities. The average precipitation rate at Cypress Bowl North under these conditions was 3 mm/h, but for 2 h on April 3 (see Section 4), freezing rain was inferred with an average precipitation rate of 10 mm/h in association with maximum upward particle velocities of 2-3 m/s. The longest duration of freezing rain with precipitation rates ≥ 1 mm/h was 2.25 h on March 30. 28
ACCEPTED MANUSCRIPT Based on these observations, a conceptual model of the trajectory and evolution of a precipitation particle was developed (Fig. 16). The particle, initially a snowflake, begins to fall upstream of the mountain within weak updrafts but strong horizontal winds. The particle falls below the melting level, melts, and becomes a raindrop. As the raindrop approaches the mountain, given sufficient upslope flows, its downward velocity decreases and eventually reverses to move upwards. The raindrop is then carried up the mountain until it either reaches
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the mountainside (in the scenario shown) or is carried downstream of the mountain. The drop is
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embedded in air that is saturated and is therefore cooling at approximately 6°C/km as it ascends.
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For a raindrop reaching the mountainside, if the air temperature is < 0°C, it falls as freezing rain.
Fig. 16. Conceptual model of the trajectory of a falling precipitation particle which reaches the
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surface on Cypress Mountain as freezing rain. The upslope flow component is shown in beige and the top of the 0°C isotherm is shown in red.
6. Concluding Remarks
Precipitation occurring over Cypress Mountain from January to April 2010 has been examined. By mainly using two observation sites (West Vancouver near the base and Cypress Bowl North nearer the summit) with an elevation difference of 785 m, several observations and conclusions can be made. 29
ACCEPTED MANUSCRIPT Substantial precipitation occurred on Cypress Mountain. West Vancouver recorded approximately half of the precipitation of that of Cypress Bowl North (797 mm and 1616 mm, respectively), which roughly followed the annual “rule of thumb” of Oke and Hay (1998), but over a 4-month period. West Vancouver experienced near-average precipitation, whereas aboveaverage precipitation was measured at Cypress Bowl North. This precipitation occurred over numerous, although a near-average number of, days. Daily records were matched or set on 7
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days at both sites. Precipitation fell nearly exclusively as rain near the base, but some snow
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occurred near the peak.
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There was a large variation in daily precipitation amount differences (up to 58 mm) between West Vancouver and Cypress Bowl North and inverse Froude number analysis showed
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that unblocked flows were predominant. West Vancouver and Cypress Bowl North precipitation amounts were typically within 5 mm on days with weak or non-existent upslope flows and
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minimal or no upward particle Doppler velocities, including when blocked flows were present. Precipitation differences of ≥ 50 mm only occurred on days with strong moisture advection as
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well as unblocked and strong upslope flows which produced upward particle Doppler velocities that sometimes exceeded 5 m/s. These upslope flows and upward particle Doppler velocities
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may limit precipitation reaching the ground. On five days with strong and nearly saturated onshore flows, precipitation amounts were ≤ 20 mm at both West Vancouver and Cypress Bowl
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North, suggesting that the ensuing strong vertical air motions acted to limit precipitation from reaching the surface anywhere on the mountain.
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Freezing rain was inferred on Cypress Mountain. Of the 5 h of freezing rain at Cypress Bowl South with precipitation rates ≥ 1 mm/h, 4.5 h occurred with upward particle Doppler
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velocities. It is suggested that falling snow melted upwind of the mountain and the ensuing raindrops were lifted upwards by strong upslope flows before reaching the mountainside at subfreezing temperatures.
Current operational forecast models with approximately 10 km resolution do not resolve the critical spatial scales at Cypress Mountain where stations are 6 km apart. Additionally, the associated large horizontal and vertical air motions within the events studied here imply that improvements in current microphysical parameterizations are needed to account for the rapid conversion of water vapour to rain and snow as well as the trajectories of these precipitation
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ACCEPTED MANUSCRIPT particles as they fall to the surface (Morrison and Milbrandt, 2015; Morrison et al., 2015; Milbrandt and Morrison, 2016). Collectively, precipitation at Cypress Mountain from January to April 2010 exhibited large temporal and elevation variations that were linked with variable upslope flows which also contributed to the production of freezing rain.
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Glossary
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Blocked flow: a weak or stable airflow which is unable to be advected over a topographic boundary.
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Inverse Froude number: an index which determines the presence of blocked or unblocked flows, including moist static stability, the terrain height, and the upstream flow velocity.
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An inverse Froude number < 0.5 indicates an unblocked flow, whereas an inverse Froude number > 0.5 indicates a blocked flow.
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Nyquist velocity: the range of velocity that can be uniquely detected by radar. Folding or aliasing occurs when particles exhibit a velocity beyond this range. For example, if the
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Nyquist velocity of a vertically-pointing radar is downwards at 0-12 m/s and the radar detects a particle which is rising at 2 m/s, folding would occur and the radar would
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interpret the velocity of the particle as downwards at 10 m/s. Particle Doppler velocity: the vertical velocity of precipitation particles relative to the ground
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including the vertical movement of the surrounding air. A positive velocity indicates towards the radar (downwards) and a negative velocity indicates away from the radar
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(upwards).
Unblocked flow: a strong or more unstable airflow which may be brought over a topographic boundary.
Acknowledgements
The authors would like to thank John Hanesiak, Ruping Mo, Sudesh Boodoo, David Hudak, Ivan Heckman, and George Isaac for their assistance. The authors would also like to
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ACCEPTED MANUSCRIPT acknowledge the Natural Sciences and Engineering Research Council of Canada for financial support.
References
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snowfall measurements in complex terrain made during the 2010 Winter Olympics in
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Bougeault, P. & Coauthors, 2001. The MAP Special Observing Period. Bull. Am. Meteorol. Soc. 82, 433-462.
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Colle, B.A., Smith, R.B., & Wesley, D.A., 2013. Theory, observations, and predictions of orographic precipitation. Mountain Weather Research and Forecasting, F. Chow et al.,
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Eds., Springer Atmospheric Sciences, 291-344, doi:10.1007/978-94-007-4098-3_6. Doyle, C., 2014. The impact of weather forecasts of various lead times on snowmaking
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decisions made for the 2010 Vancouver Olympic Winter Games. Pure Appl. Geophys., 171, 87-94, doi:10.1007/s00024-012-0609-y.
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Earth System Research Laboratory, 2015. Daily Climate Composites. Physical Sciences Division. NOAA. http://www.esrl.noaa.gov/psd/data/composites/day/. (Accessed:
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http://www.climate.weatheroffice.gc.ca/climate_normals/index_e.html. (Accessed: June
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Fargey, S., Hanesiak, J., Stewart, R., & Wolde, M., 2014. Aircraft observations of orographic cloud and precipitation features over southern Baffin Island, Nunavut, Canada. Atmos.Ocean, 52 (1), 54-76, doi: 10.1080/07055900.2013.855624. Goodison, B.E., Louie, P.Y.T., & Yang, D., 1998. WMO Solid Precipitation Measurement Intercomparison: Final Report. Instruments and Observing Methods Report No. 67. World Meteorological Organization. WMO/TD No. 872. Gultepe, I., Isaac, G.A., Joe, P., Kucera, P.A., Thériault, J.M., & Fisico, T., 2014. Roundhouse (RND) mountain top research site: measurements and uncertainties for winter alpine weather conditions. Pure Appl. Geophys., 171, 59-85, doi:10.1007/s00024-012-0582-5. 32
ACCEPTED MANUSCRIPT Guttsman, J., 2010. Olympics - Worsening weather threatens more postponements. Reuters, February 23, 2010. http://www.reuters.com/article/2010/02/23/olympics-weatheridUSN2312070820100223. (Accessed: June 26, 2012.) Houze Jr., R.A., 2012. Orographic effects on precipitating clouds. Rev. Geophys., 50, RG1001, doi:10.1029/2011RG000365. Isaac, G.A. & Coauthors, 2014. Science of Nowcasting Olympic Weather for Vancouver 2010
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(SNOW-V10): a World Weather Research Programme project. Pure Appl. Geophys., 171,
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Jiang, Q., 2003. Moist dynamics and orographic precipitation. Tellus, 55A, 301-316. Joe, P. & Coauthors, 2014. The monitoring network of the Vancouver 2010 Olympics. Pure
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Kalnay, E. & Coauthors, 1996. The NCEP/NCAR reanalysis 40-year Project. Bull. Am.
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and documentation. Bull. Am. Meteorol. Soc., 82, 247-267. Milbrandt, J.A. & Morrison, H., 2016. Parameterization of cloud microphysics based on the
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prediction of bulk ice particle properties. Part III: introduction of multiple free categories. J. Atmos. Sci., 73, 975-995.
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Mo, R., Joe, P.I., Doyle, C., & Whitfield, P.H., 2014. Verification of an ENSO-based long-range prediction of anomalous weather conditions during the Vancouver 2010 Olympics and
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Paralympics. Pure Appl. Geophys., 171, 323-336, doi:10.1007/s00024-012-0523-3. Morrison, H. & Milbrandt, J.A., 2015. Parameterization of ice microphysics based on the
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prediction of bulk particle properties. Part I: scheme description and idealized tests. J. Atmos. Sci., 72, 287-311. Morrison, H., Milbrandt, J.A., Bryan, G.H., Ikeda, K., Tessendorf, S.A., & Thompson, G., 2015. Parameterization of cloud microphysics based on the prediction of bulk ice particle properties. Part II: case study comparisons with observations and other schemes. J. Atmos. Sci., 72, 312-339. Oke, T. & Hay, J., 1998. The Climate of Vancouver. BC Geographical Series, No. 50. University of British Columbia, 84 pp.
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ACCEPTED MANUSCRIPT Seager, R., Kushnir, Y., Nakamura, J., Ting, M., & Naik, N., 2010. Northern Hemisphere winter snow anomalies: ENSO, NAO, and the winter of 2009/10. Geophys. Res. Lett., 37, L14703, doi:10.1029/2010GL043830. Sheppard, B.E. & Joe, P.I., 2000. Automated precipitation detection and typing in winter: a twoyear study. J. of Atmos. Ocean. Tech., 17, 1493-1507. Stoelinga, M.T. & Coauthors, 2003. Improvement on Microphysical Parameterization through
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Observational Verification Experiment. Bull. Am. Meteorol. Soc., 1807-1826,
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doi:0.1175/BAMS-84-12-1807.
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Stoelinga, M.T., Stewart, R.E., Thompson, G., & Thériault, J.M., 2013. Microphysical processes within winter orographic cloud and precipitation systems. Mountain Weather Research
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and Forecasting. F. Chow et al., Eds., Springer Atmospheric Sciences, 345-408, doi:10.1007/978-94-007-4098-3_7.
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University of Wyoming, 2015. Atmospheric Soundings. Department of Atmospheric Science.
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http://weather.uwyo.edu/upperair/sounding.html. (Accessed: October 4, 2015.)
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ACCEPTED MANUSCRIPT Highlights
Near- to above-average precipitation occurred at Cypress Mountain for January-April 2010.
Above-average temperatures from El Niño led to more rain and less snow than average.
Much greater daily precipitation amounts occurred at higher elevation (by up to 54 mm)
Strong upslope and unblocked flows produced upwards precipitation velocities,
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during stronger upslope flows. sometimes ≥ 5 m/s, which on occasion produced little precipitation.
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Freezing rain sometimes occurred, probably due to highly variable particle trajectories
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including upward motions.
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H.W. Stephen Berg, Ronald E. Stewart, Paul I. Joe PII: DOI: Reference:
S0169-8095(16)30647-0 doi: 10.1016/j.atmosres.2017.06.009 ATMOS 3973
To appear in:
Atmospheric Research
Received date: Revised date: Accepted date:
24 November 2016 15 May 2017 9 June 2017
Please cite this article as: H.W. Stephen Berg, Ronald E. Stewart, Paul I. Joe , The characteristics of precipitation observed over Cypress Mountain during the SNOW-V10 campaign, Atmospheric Research (2017), doi: 10.1016/j.atmosres.2017.06.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The Characteristics of Precipitation Observed over Cypress
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Mountain during the SNOW-V10 Campaign
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*H.W. Stephen Berga ([email protected])
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Ronald E. Stewarta ([email protected])
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Paul I. Joeb ([email protected])
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a – Department of Environment and Geography, 470 Wallace Building, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2
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b – Cloud Physics and Severe Weather Research Section, Environment and Climate Change Canada, 4905 Dufferin St., Toronto, ON, Canada, M3H 5T4 Current affiliation: World Meteorological Organization Current email address: [email protected]
* Corresponding author Corresponding author no longer works at the University of Manitoba. Secondary email address is [email protected]. Postal address is: 162 Bartlet Ave Winnipeg, Manitoba, Canada R3L 0Z4 1
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ABSTRACT
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Cypress Mountain, north of Vancouver, is a 1440-m high coastal barrier for moisture-laden onshore airflow and is consequently subject to substantial precipitation. Unprecedented data from various precipitation events were obtained between January and April 2010 during the SNOW-V10 (Science of Nowcasting Olympic Weather for Vancouver 2010) field campaign occurring in conjunction with the 2010 Winter Olympics. Information was collected from specialized radar, enhanced surface weather stations, and operational observing systems. During this period, overall precipitation amounts were similar to long-term averages. Some precipitation events lasted ≥ 24 h, although periods with heavier precipitation rates lasted ≤ 6 h. Temperatures were generally above 0°C at an observation site near the base of the mountain so snow was almost entirely absent, which was sometimes the case at other sites higher and nearer its peak. Precipitation amounts from some events were similar at the base and near the summit but other events showed much more precipitation at higher elevations. Most of these latter cases were linked with strong, sustained upward particle velocities on the upwind side of the mountain and this flow was also suggested to be a contributing factor for freezing rain occurring near the peak on occasion.
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Key words: mountain precipitation; rain; snow; freezing rain
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ACCEPTED MANUSCRIPT 1. Introduction
Mountainous areas are often subjected to substantial precipitation due to the upward forcing of air which promotes increased condensation. This enhanced precipitation is especially likely if the mountains are adjacent to a large body of water allowing significant onshore moisture advection (Houze, 2012). The precipitation falling on the mountains can be liquid or
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solid and, although seemingly straightforward, many factors affect what type of precipitation
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actually occurs (Stoelinga et al., 2013).
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One mountainous region of importance in Canada is the Lower Mainland of British Columbia (BC). This region was of particular significance during the 2010 Winter Olympics
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when freestyle skiing and snowboarding events at Cypress Mountain (north of Vancouver) were delayed or postponed numerous times, due to a shift towards rainfall instead of snowfall
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(Guttsman, 2010). Simultaneously, an intensive field campaign, the Science of Nowcasting Olympic Weather for Vancouver 2010 (SNOW-V10), was conducted to improve understanding
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of precipitation processes in this region and mountain meteorology in general (Isaac et al., 2014). Numerous SNOW-V10 studies have been undertaken to contribute to knowledge of
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precipitation in this region with most of the attention paid to the meteorology over the Whistler area. Little attention has been paid to Cypress Mountain, and none of the articles within the
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special SNOW-V10 issue of Pure and Applied Geophysics published in January 2014 (see for example Isaac et al. and Joe et al.) specifically focused on this mountain. This article begins to
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address this gap. In particular, its objective is to improve understanding of the distribution and production of precipitation over Cypress Mountain during the SNOW-V10 period, given the
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recent advances in knowledge from other field campaigns like the Mesoscale Alpine Programme (MAP; Bougeault et al., 2001) and Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE; Stoelinga et al., 2003). This objective will be achieved by first characterizing the precipitation and then examining its associated atmospheric conditions. Cypress Mountain, located approximately 20 km northwest of downtown Vancouver (Fig. 1), has a pyramidal shape with a summit elevation of 1440 m above sea level (ASL). Substantial precipitation occurs on the mountain, with average annual amounts ≥ 2500 mm near its peak, although this is highly elevation dependent. Oke and Hay (1998) developed a 3
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precipitation per 100 m increase in elevation over Cypress Mountain.
Fig. 1. Location of Cypress Mountain (yellow pin) with respect to the BC Lower Mainland
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including the City of Vancouver. The locations of the Aldergrove and Mount Sicker radars, as well as the Quillayute, WA, rawinsonde site (all referred to within the article) are indicated with
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red pins. (Adapted from Google Earth, 2015, with data from SIO, NOAA, U.S. Navy, NGA, GEBCO, LDEO Columbia, NSF, NOAA, Landsat / Copernicus).
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This article is structured as follows. Section 2 describes the instruments on Cypress Mountain as well as other datasets used in this study. The meteorological conditions, specifically temperature and precipitation, are described in Section 3. This is followed by analyses of days with significant precipitation (≥ 10 mm on Cypress Mountain) and two illustrative case studies in Section 4. Interpretations and implications are discussed in Section 5 and concluding remarks follow in Section 6.
2. Data and Methodology 4
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Several datasets were utilized in this study, which is based on measurements made from January to April 2010. To characterize large-scale circulation and synoptic patterns, satellite imagery from GOES-11, surface analyses from Environment Canada, and data from re-analyses performed by the Earth System Research Laboratory (2015, using the methodologies of Kalnay et al., 1996, and Kistler et al., 2001) were examined. Data from the Environment Canada
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Doppler radar situated at Aldergrove and Mount Sicker, BC, from January 13 to April 30 were
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analyzed to describe precipitation patterns over the Cypress Mountain area. Rawinsonde data
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from Quillayute, Washington (WA; University of Wyoming, 2015) were utilized to determine the presence of blocking via the inverse Froude number technique employed by Colle et al.
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(2013) and Fargey et al. (2014). Variables from the lowest 100 m ASL and the nearest value to 1440 m ASL were utilized in these calculations.
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As indicated in Fig. 2, meteorological observation station data from three stations were examined to quantify surface temperature, humidity, wind, and precipitation (including rates,
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types, and amounts): West Vancouver (WWA, located at the base of the mountain at 168 m ASL), Cypress Bowl South (VOG, located on the mountain at 886 m ASL), and Cypress Bowl
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North (VOE, located on the mountain at 953 m ASL). Multi-year precipitation records from Environment Canada (2012) exist at both West Vancouver (1992-2013) and Cypress Bowl North
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(1955-1995 nearby at Hollyburn Ridge) and these are used for calculating long-term precipitation amounts even though the time periods are quite different. Data from a vertically-
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pointing Micro Rain Radar (MRR) at West Vancouver were obtained between January 22 and April 29 to examine the vertical structure of precipitating systems (Joe et al., 2014).
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Different methods of inferring precipitation were utilized. The Pluvio precipitation gauge was used at West Vancouver and Cypress Bowl North (Joe et al., 2014). Precipitation Occurrence Sensor System (POSS) instruments were utilized at West Vancouver and Cypress Bowl South to infer precipitation types (Joe et al., 2014). (Precipitation types at Cypress Bowl South were therefore utilized for Cypress Bowl North.) The same types of instruments were employed at Whistler during SNOW-V10, and Boudala et al. (2014) and Gultepe et al. (2014) noted that corrections for under-catch were needed with respect to wind speed. The correction provided by Goodison et al. (1998) was subsequently applied in this study. Precipitation
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approximately 1.5 m/s.
Fig. 2. Topographical view of Cypress Mountain. The three sites indicated in red were the
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primary meteorological stations at Cypress Mountain for SNOW-V10. The site indicated in brown is Hollyburn Ridge (at Hollyburn Mt., 930 m ASL). Sites in yellow indicate important
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operational or tourist sites. Mt. Strachan is the main summit at Cypress Mountain, whereas Hollyburn Mt. and Black Mt. are secondary summits.
3. Meteorological Conditions 3.1. Temperatures, Synoptic Conditions, and El Niño – Southern Oscillation
Mean temperatures from January to April 2010 were above the 1992-2013 average for West Vancouver and above the 1955-1995 average for Cypress Bowl North. Mean temperatures 6
ACCEPTED MANUSCRIPT at West Vancouver and Cypress Bowl North were 2.3°C and 1.3°C above average, respectively. The ranges of monthly temperature anomalies at West Vancouver and Cypress Bowl North were +1.0°C to +3.9°C and -0.6°C to +3.8°C, respectively. At both stations, the lowest anomalies occurred in April and the highest anomalies occurred in January. Numerous low pressure systems developed over the northeast Pacific Ocean during the period. An El Nino event influenced the development of these large scale circulation systems
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Mountain (Seager et al., 2010; Mo et al., 2014; Doyle, 2014).
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and generated substantial moisture advection and anomalously warm temperatures over Cypress
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3.2. Precipitation
From January to April 2010, West Vancouver experienced near-average precipitation
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amounts, whereas Cypress Bowl North experienced substantially higher than average amounts. Precipitation amounts measured at West Vancouver and Cypress Bowl North were 797 mm and
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1616 mm, respectively, a difference of 819 mm. This increase of just over 100 mm per 100 m in elevation slightly exceeded the annual climatological average found by Oke and Hay (1998).
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Monthly precipitation amounts at West Vancouver (Fig. 3a) and Cypress Bowl North (Fig. 3b) were not extreme. West Vancouver recorded near-average monthly precipitation (+20
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mm on average with a monthly range of -49 mm to +31 mm), whereas Cypress Bowl North experienced greater than average precipitation (+565 mm on average with a monthly range of
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+76 mm to +236 mm). At West Vancouver, precipitation amounts were ≥ 100 mm less than long-term maximum values for every month of examination, whereas at Cypress Bowl North,
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positive precipitation anomalies of 236 mm and 154 mm were recorded for January and April, respectively, although these precipitation amounts were 76 mm and 57 mm less than long-term maximum values.
A substantial portion of the precipitation at West Vancouver occurred in January, in particular over a 5-day period (January 11-15) during which 174 mm was measured (Fig. 4a). Even so, monthly precipitation amounts for January were barely above the long-term average. Precipitation amounts were substantially greater at Cypress Bowl North for the same 5-day period: 318 mm (Fig. 4b). In terms of daily precipitation records, both West Vancouver and Cypress Bowl North experienced 7 days of precipitation which equalled or exceeded the 7
ACCEPTED MANUSCRIPT maximum recorded values, although the period of record at West Vancouver (21 years) was half that of Hollyburn Ridge (41 years). Many precipitation events occurred, with 78 and 84 days having ≥ 0.2 mm of precipitation at West Vancouver and Cypress Bowl North, respectively. The former value was close to the long-term average for West Vancouver of 72 days, whereas the latter value exceeded the long-term average of 67 days. West Vancouver and/or Cypress Bowl North recorded ≥ 10
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mm of precipitation on 44 days: 29 days at West Vancouver and 42 days at Cypress Bowl North.
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On 2 days, ≥ 10 mm of precipitation was recorded at West Vancouver but not at Cypress Bowl
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North.
The evolution of the cumulative precipitation at West Vancouver and Cypress Bowl
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North from January to April 2010 and their difference are shown in Figs. 5 and 6. The step-wise progression of differences illustrates substantial variation. On some days, there were small
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differences in precipitation amounts, whereas other days had large differences with some exceeding 50 mm.
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The amount of rain and snow also varied substantially between West Vancouver and Cypress Bowl North. Of the 797 mm total precipitation at West Vancouver, 489 mm fell as rain,
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whereas of the 1616 mm at Cypress Bowl North, 256 mm occurred as snow and 810 mm fell as rain.
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The duration of each precipitation type at West Vancouver and Cypress Bowl South was also substantially different. At West Vancouver, snow was never inferred as the dominant
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precipitation type. A total of 425 h of rain was inferred, with a maximum duration of 9 h. At Cypress Bowl South, 107 h of snow and 428 h of rain were inferred. The maximum duration of
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snow was 8 h, whereas the longest duration of rain was 13 h. Some individual precipitation events lasted ≥ 24 h (with some brief pauses of < 1 h), but periods with heavier precipitation rates (≥ 5 mm/h) lasted ≤ 6 h. (No comparisons can be made between West Vancouver and Cypress Bowl South precipitation types and long-term averages, since similar precipitation type data were not collected beyond the SNOW-V10 field campaign.)
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Fig. 3. Monthly precipitation amounts at (a) West Vancouver and (b) Cypress Bowl North for January to April 2010 compared to long-term maximum, average, and minimum values. West Vancouver values were compared to the 1992-2013 record, whereas Cypress Bowl North values were compared to the 1955-1995 record at Hollyburn Ridge (Environment Canada, 2012).
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Fig. 4. Daily precipitation amounts at (a) West Vancouver and (b) Cypress Bowl North from January to April 2010 compared to highest recorded and average values. West Vancouver values were compared to the 1992-2013 record, whereas Cypress Bowl North values were compared to the 1955-1995 record at Hollyburn Ridge (Environment Canada, 2012). The yellow line indicates the beginning of MRR operations.
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Fig. 5. Cumulative precipitation (in mm) at West Vancouver and Cypress Bowl North and their
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difference (Cypress Bowl North – West Vancouver) from January to April 2010. The yellow
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line indicates the beginning of MRR operations.
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Fig. 6. Difference in daily precipitation amounts (Cypress Bowl North – West Vancouver) on
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4. Precipitation Events
4.1. Specific Events
As described above, the daily amount of precipitation and the relative amount between sites varied greatly. The 44 days with ≥ 10 mm of precipitation at West Vancouver and/or Cypress Bowl North were examined to determine key factors leading to the variable precipitation. Two days were chosen to illustrate the distinct patterns regarding forcing factors present on days with small or large differences between these sites. February 2 had a small difference in 12
ACCEPTED MANUSCRIPT precipitation amounts (2 mm), whereas April 3 had a large difference (52 mm). Substantial differences in precipitation amounts were also present between the two days at each site; 16 mm and 3 mm were recorded at West Vancouver on February 2 and April 3, respectively, whereas 18 mm and 55 mm were measured at Cypress Bowl North on February 2 and April 3, respectively. Precipitation amounts for February 2 and April 3 were also separated by precipitation type inferred by the POSS. On February 2, 13 mm (out of 16 mm) fell as rain at West
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Vancouver, whereas 4 mm (out of 18 mm) fell as rain and 11 mm fell as snow at Cypress Bowl
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North. On April 3, 3 mm (out of 3 mm) fell as rain at West Vancouver, whereas 37 mm (out of
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55 mm) fell as rain, 1 mm fell as snow, and the other 17 mm occurred when the POSS at Cypress Bowl South were identified as “indeterminate” which indicates that the precipitation may have
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been mixed, drizzle, or snow (Sheppard and Joe, 2000). There is also evidence for the presence of freezing rain on April 3. Freezing rain was inferred for 2 h at Cypress Bowl South with
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temperatures of approximately -0.4°C to -0.8°C at Cypress Bowl North over the 04-06 UTC period. Precipitation amounts of 20 mm (included in the 37 mm rainfall total) were measured
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during the 2 h of freezing rain.
Infrared satellite imagery and surface analyses (Figs. 7 and 8) depict different circulation
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patterns for February 2 and April 3. A weak 998 mb low pressure system was present on February 2 southwest of the Cypress Mountain area with cloud top temperatures of
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approximately -40°C over the region. Conversely, a deep 979 mb cyclonic system was situated over the region at the beginning of April 3, with cloud top temperatures ≤ -50°C. Moisture
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advection was much stronger on April 3 than on February 2, due to the much stronger circulation. Inverse Froude numbers calculated from Quillayute, WA, rawinsonde data for the
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two days were ≤ 0.2, indicating the presence of unblocked flows in both cases. (See glossary near the end of the article for a brief description of the inverse Froude number.)
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Fig. 7. Enhanced GOES-11 infrared satellite imagery for (a) 1300 UTC February 2 and (b) 0300
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UTC April 3, 2010.
Radar reflectivity imagery (Fig. 9) infers less precipitation on February 2 compared to
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April 3 over Cypress Mountain. Reflectivity values rarely exceeded 25 dBZ over Cypress Mountain on February 2, whereas values of ≥ 35 dBZ were frequently detected on April 3. The
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spatial extent of the precipitation was also much smaller on February 2 than on April 3. Radial velocity imagery from Aldergrove Doppler radar (Fig. 10) shows a substantial
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difference in the strength of the low level airflow. Southerly to west-southwesterly flows of ≤ 5 m/s were common over Cypress Mountain on February 2 (shown in dark red in Fig. 10a),
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whereas on April 3, strong southwesterly airflows, sometimes ≥ 20 m/s, were detected over the mountain (shown in light orange and yellow in Fig. 10b). Mean daily 850 mb wind speeds over Cypress Mountain from re-analyses were 5 m/s and 10 m/s on February 2 and April 3, respectively.
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Fig. 8. Surface analyses for (a) 1200 UTC February 2 and (b) 0000 UTC April 3 provided by Environment Canada.
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m/s (indicative of rain) below 1000 m AGL (1168 m ASL).
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(indicative of snow) within the layer 1000-3000 m AGL (1168-2168 m ASL) to values of 4-9
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The MRR detected widely varying values of upward particle Doppler velocities of precipitation. The Nyquist velocity limits of the MRR are 0 m/s and 12 m/s. Positive Doppler
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velocity values are interpreted as downward particle fall speeds in the absence of strong vertical air motions. When there is substantial upward air motion of ≥ 1 m/s, the particle Doppler
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velocities will decrease and can be negative and are aliased or folded, as shown in imagery such as Fig. 11b (see direction of arrows within image). On February 2, the MRR detected minimal
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upward velocities (shown in Fig. 11a with yellow colours folding over to 11.5-12 m/s from 1600-3200 m AGL (1768-3368 m ASL)) and the melting layer was from 400-800 m AGL (568-
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968 m ASL). Conversely, on April 3, the MRR detected 7.5 h of upward velocities, with maximum values ≥ 5 m/s (shown in Fig. 11b with red and orange colours folding over to ≤ 7 m/s
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(368-768 m ASL).
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from 600-1400 m AGL (768-1568 m ASL)), and the melting layer was from 200-600 m AGL
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Fig. 9. Illustrative Vancouver composite 2-km CAPPI reflectivity radar imagery for (a) 1310 UTC February 2 and (b) 0310 UTC April 3, 2010. Range rings are 40 km. Cypress Mountain is located within the black circle (diameter approximately 40 km).
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Fig. 10. Doppler 0.5° PPI radial velocity imagery from Aldergrove for (a) 1310 UTC February 2 and (b) 0310 UTC April 3, 2010. Range rings are 20 km. Cypress Mountain is located within the white circle (diameter approximately 40 km). Over Cypress Mountain, the elevation of the radar beam is approximately 1000 m ASL. Red colours are away from the radar and blue colours are towards the radar. White arrows indicate the direction of airflow.
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which frequently generated rapid upward particle velocities (≥ 2 m/s).
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(sometimes ≥ 20 m/s) towards Cypress Mountain were present on April 3 (shown in Fig. 10b),
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Substantial changes in precipitation patterns occurred on the mountain as a consequence of these differences in updrafts and upward particle Doppler velocities. As mentioned earlier,
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large precipitation amount differences were measured between the two days and between the two sites. On February 2, a day with minimal to non-existent upward particle velocities, precipitation
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amounts at West Vancouver and Cypress Bowl North were similar. Maximum precipitation rates of 2 mm/h and 3 mm/h were measured at West Vancouver and Cypress Bowl North,
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respectively. Conversely, on April 3, a day with a long duration of rapid upward particle velocities, much less precipitation fell near the base (at West Vancouver) compared to at higher
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elevations (at Cypress Bowl North). Maximum precipitation rates of 2 mm/h and 23 mm/h were measured at West Vancouver and Cypress Bowl North, respectively, occurring at the same time
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maximum upward particle Doppler velocities were ≥ 5 m/s.
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Fig. 11. Doppler spectrum data from MRR at West Vancouver for (a) 1310 UTC February 2, 2010 and for (b) 0310 UTC April 3, 2010. Heights are indicated as above ground level (AGL), which is 168 m lower than the above sea level (ASL) heights referenced in the text. Violet arrows in (b) indicate direction of motion of precipitation particles. 20
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Fig. 12. Time-height plots of (a) particle Doppler velocity and (b) reflectivity from MRR at West Vancouver on April 3, 2010.
4.2. Comparison with other Events during SNOW-V10
The features present on February 2 and April 3 occurred on other days as well. Substantial precipitation amounts (≥ 50 mm) fell on 7 days at Cypress Bowl North when the 21
ACCEPTED MANUSCRIPT mean daily 850 mb winds were ≥ 10 m/s as inferred from re-analysis information over Cypress Mountain (Table 1; Earth System Research Laboratory, 2015). The largest precipitation amounts, between 70 mm and 125 mm, occurred with the strongest mean daily 850 mb winds (≥ 14 m/s). Mean daily 850 mb winds were between westerly and southeasterly (i.e. upslope on Cypress Mountain) for all of the days included in Table 1. Low-level directional wind shear was present on each of these days, with wind directions at West Vancouver and Cypress Bowl North
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typically differing by 60-90°. Speed wind shear of ≥ 2 m/s between the surface and 850 mb over
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the Cypress Mountain region was also typically inferred from re-analysis information on these
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days.
Some days with strong onshore flows had low precipitation amounts on Cypress
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Mountain. Onshore flows of ≥ 10 m/s were present on five days with ≤ 20 mm of precipitation at both West Vancouver and Cypress Bowl North. Surface relative humidity values were near-
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saturation (≥ 90%) for ≥ 12 h on each of these days.
Other contributing factors were present on some days when precipitation amounts were ≥ 10 mm lower at West Vancouver than at Cypress Bowl North. On eight days, long durations of
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upward particle Doppler velocities (≥ 2 h) were detected by MRR. Mean daily 850 mb wind
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speeds were ≥ 10 m/s on each of these days except for on April 8. On 15 days, maximum upward velocities ≥ 2 m/s were also detected by MRR.
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Precipitation amounts at West Vancouver were also ≥ 10 mm lower than at Cypress Bowl North when orographic enhancement of precipitation was detected by Aldergrove radar.
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Orographic enhancement occurred on 25 days according to our definition, which is that radar reflectivity values had to increase by ≥ 10 dBZ from 5-10 km upstream of Cypress Mountain to
Table 1.
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above and downstream of the mountain as evident on 1.5 km CAPPI imagery.
Days with precipitation amounts ≥ 10 mm on Cypress Mountain for January to April 2010. “NA” in upward particle Doppler velocity categories denotes when the MRR was not operational. Wind speeds at 850 mb are mean daily values. The two case studies (February 2 and April 3) are italicized and in bold. In the flag column, “O” denotes days with strong onshore flows (mean daily 850 mb winds ≥ 10 m/s) with ≤ 20 mm of precipitation on Cypress Mountain, “L” denotes days with long durations of upward particle Doppler velocities (≥ 2 h), “M” denotes days with maximum upward particle Doppler velocities ≥ 2 m/s, and “E” denotes days with orographic enhancement of reflectivity on radar (increases of ≥ 10 dBZ over the mountain 22
ACCEPTED MANUSCRIPT compared to 5-10 km upstream at approximately 2 km ASL). Days which did not meet these criteria and experienced < 1 h of upward particle Doppler velocities and < 10 m/s mean daily 850 mb wind speeds were excluded.
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850 mb Wind Speed (m/s) 12 10 10 14 16 12 8 11 16 13 13 15 5 12 9 13 11 10 8 11 6 4 14 7 13 15 8 13 10 10 11 8 9 10
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Maximum Upward Particle Velocity (m/s) NA NA NA NA NA NA NA NA NA NA NA 2.0 0.0 0.5 1.0 2.0 4.0 2.0 2.0 0.5 1.0 0.5 2.0 3.0 4.5 5.0 2.0 5.0 5.0 1.0 2.0 3.0 1.5 4.0
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Upward Particle Velocity Duration (h) NA NA NA NA NA NA NA NA NA NA NA 0.8 0.0 0.2 0.2 2.3 2.0 1.2 1.5 0.7 0.8 0.2 1.0 1.5 2.0 4.0 1.0 4.0 7.5 0.3 2.8 3.5 0.7 1.7
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01-Jan 02-Jan 08-Jan 09-Jan 11-Jan 12-Jan 13-Jan 14-Jan 15-Jan 17-Jan 18-Jan 25-Jan 02-Feb 05-Feb 11-Feb 12-Feb 13-Feb 14-Feb 16-Feb 26-Feb 27-Feb 28-Feb 11-Mar 13-Mar 28-Mar 29-Mar 30-Mar 02-Apr 03-Apr 05-Apr 07-Apr 08-Apr 24-Apr 27-Apr
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West Cypress Vancouver Bowl North Precipitation Precipitation Amount Amount (mm) (mm) 22 50 17 16 9 13 31 39 58 103 15 27 10 24 20 38 71 125 12 32 10 22 7 33 16 18 8 14 10 36 6 27 11 35 18 36 21 46 10 25 28 66 7 23 40 77 9 36 4 16 27 78 5 24 33 69 3 55 14 10 9 21 27 43 9 19 30 47
Flag
O O
E E E E E ME O E LME LME ME ME E E E ME ME OLME LME ME LME LME O LME LME E ME
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These features were not all necessary for substantial and large differences in precipitation amounts on Cypress Mountain. On February 27, substantial precipitation amounts at Cypress Bowl North (66 mm) and a large difference in precipitation amounts between West Vancouver and Cypress Bowl North (38 mm) were recorded. Orographic enhancement of precipitation was
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detected on radar, but the mean 850 mb wind speed was 6 m/s, the duration of upward particle
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Doppler velocities was < 1 h, and maximum upward particle velocities were 1 m/s. A large
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cyclone was present near the region, which advected the substantial moisture necessary for precipitation towards Cypress Mountain. A frontal system would have provided general lifting, condensation, and precipitation formation aloft and it also would have generated the stability and
5. Discussion
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5.1. Summary of Precipitation Conditions
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wind fields that were critical for varying precipitation differences between the two stations.
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Warmer than average temperatures were present over the region, attributed to the El Niño event (Seager et al., 2010; Mo et al., 2014; Doyle, 2014). These warmer conditions resulted in a
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shift in precipitation from snow to rain. Mean temperatures were 2.3°C above average at West Vancouver and 1.3°C above average at Cypress Bowl North. Snow was never inferred as the
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dominant precipitation type at West Vancouver, whereas snow made up < 20% of cumulative precipitation inferred at Cypress Bowl North (compared to the long-term average of 45%).
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Collectively, precipitation amounts during the January-April 2010 period were somewhat typical but also illustrated some departures from the long-term. Cumulative precipitation at West Vancouver and Cypress Bowl North was 797 mm and 1616 mm, respectively. This represents a difference of just over 100 mm per 100 m of elevation, similar to the average annual precipitation climatology by Oke and Hay (1998). Precipitation amounts at West Vancouver were just 20 mm above average, whereas they were > 500 mm above average at Cypress Bowl North.
5.2. Flow Regime 24
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Unblocked flows were present for most of January to April 2010. Inverse Froude numbers were < 0.1 for 138 and < 0.3 for 219 out of the 240 total launches from Quillayute, WA, indicating the presence of unblocked flows was typical (Fig. 13). Both short and long duration upward particle Doppler velocity events and stronger upward particle Doppler velocities occurred when the inverse Froude number was < 0.2. These conditions were typically present
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during periods with higher precipitation amounts on Cypress Mountain. Upward particle
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Just 11 launches showed inverse Froude numbers > 0.5, indicating more blocked rather than unblocked flows. Little precipitation occurred in these conditions; the maximum 12-h
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precipitation amount at Cypress Bowl North was 5 mm.
Fig. 13. Number of rawinsonde launches arranged according to inverse Froude numbers at Quillayute, WA for January-April 2010. 25
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A relationship was described by Jiang (2003), which estimates the precipitation rate from unblocked flows over mountains in non-sheared environments, depending on the speed of onshore flow and the moisture content and stability of the air. This relationship predicts an increasing precipitation rate with height on a mountain given stronger onshore airflows and greater moisture content and instability, but the rate of increase in the precipitation rate decreases
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above a specific height depending on the same factors. However, this relationship cannot be
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measurement stations on days with precipitation amounts of ≥ 10 mm.
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5.3. Upward Particle Doppler Velocities
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The presence of upward particle Doppler velocities was an important aspect on days with ≥ 10 mm precipitation on Cypress Mountain. For the January 22 to April 29 operational period
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for the MRR, 15 days with ≥ 1 h of upward particle Doppler velocities were detected for a large proportion of the precipitation. There was also a positive correlation between highest upward
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particle velocities and largest differences in precipitation. On each of these days, differences in precipitation amounts of ≥ 10 mm between West Vancouver and Cypress Bowl North were
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recorded (Figs. 14 and 15).
Strong updrafts are required to achieve upward particle Doppler velocities, particularly in
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the case of rain. For raindrops 1 mm and 3 mm in diameter to reach upward velocities of 4 m/s, updraft speeds of approximately 8 m/s and 12 m/s, respectively, are required. The MRR inferred
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raindrops exhibiting upward velocities at Cypress Bowl South for 27 h. For snow to reach upward particle velocities of 4 m/s, however, updraft speeds of approximately 5-6 m/s are necessary. The MRR inferred snow exhibiting upward velocities at Cypress Bowl South for 20 h.
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Fig. 14. Daily precipitation differences (Cypress Bowl North – West Vancouver) on days with ≥
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10 mm of precipitation as functions of duration of upward particle Doppler velocities detected by
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Fig. 15. Daily precipitation differences (Cypress Bowl North – West Vancouver) on days with ≥
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10 mm of precipitation as functions of maximum upward particle Doppler velocity detected by
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MRR at West Vancouver. The two days discussed in Section 4 are shown in red.
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5.4. Fall Trajectory for Freezing Rain Occurrence
Freezing rain was inferred on Cypress Mountain on 6 days between January and March, typically for short periods: March 11, March 13, March 14, March 29, March 30, and March 31. Of the 5 h of freezing rain with precipitation rates ≥ 1 mm/h, 4.5 h occurred with upward particle Doppler velocities. The average precipitation rate at Cypress Bowl North under these conditions was 3 mm/h, but for 2 h on April 3 (see Section 4), freezing rain was inferred with an average precipitation rate of 10 mm/h in association with maximum upward particle velocities of 2-3 m/s. The longest duration of freezing rain with precipitation rates ≥ 1 mm/h was 2.25 h on March 30. 28
ACCEPTED MANUSCRIPT Based on these observations, a conceptual model of the trajectory and evolution of a precipitation particle was developed (Fig. 16). The particle, initially a snowflake, begins to fall upstream of the mountain within weak updrafts but strong horizontal winds. The particle falls below the melting level, melts, and becomes a raindrop. As the raindrop approaches the mountain, given sufficient upslope flows, its downward velocity decreases and eventually reverses to move upwards. The raindrop is then carried up the mountain until it either reaches
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the mountainside (in the scenario shown) or is carried downstream of the mountain. The drop is
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embedded in air that is saturated and is therefore cooling at approximately 6°C/km as it ascends.
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For a raindrop reaching the mountainside, if the air temperature is < 0°C, it falls as freezing rain.
Fig. 16. Conceptual model of the trajectory of a falling precipitation particle which reaches the
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surface on Cypress Mountain as freezing rain. The upslope flow component is shown in beige and the top of the 0°C isotherm is shown in red.
6. Concluding Remarks
Precipitation occurring over Cypress Mountain from January to April 2010 has been examined. By mainly using two observation sites (West Vancouver near the base and Cypress Bowl North nearer the summit) with an elevation difference of 785 m, several observations and conclusions can be made. 29
ACCEPTED MANUSCRIPT Substantial precipitation occurred on Cypress Mountain. West Vancouver recorded approximately half of the precipitation of that of Cypress Bowl North (797 mm and 1616 mm, respectively), which roughly followed the annual “rule of thumb” of Oke and Hay (1998), but over a 4-month period. West Vancouver experienced near-average precipitation, whereas aboveaverage precipitation was measured at Cypress Bowl North. This precipitation occurred over numerous, although a near-average number of, days. Daily records were matched or set on 7
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days at both sites. Precipitation fell nearly exclusively as rain near the base, but some snow
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occurred near the peak.
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There was a large variation in daily precipitation amount differences (up to 58 mm) between West Vancouver and Cypress Bowl North and inverse Froude number analysis showed
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that unblocked flows were predominant. West Vancouver and Cypress Bowl North precipitation amounts were typically within 5 mm on days with weak or non-existent upslope flows and
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minimal or no upward particle Doppler velocities, including when blocked flows were present. Precipitation differences of ≥ 50 mm only occurred on days with strong moisture advection as
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well as unblocked and strong upslope flows which produced upward particle Doppler velocities that sometimes exceeded 5 m/s. These upslope flows and upward particle Doppler velocities
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may limit precipitation reaching the ground. On five days with strong and nearly saturated onshore flows, precipitation amounts were ≤ 20 mm at both West Vancouver and Cypress Bowl
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North, suggesting that the ensuing strong vertical air motions acted to limit precipitation from reaching the surface anywhere on the mountain.
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Freezing rain was inferred on Cypress Mountain. Of the 5 h of freezing rain at Cypress Bowl South with precipitation rates ≥ 1 mm/h, 4.5 h occurred with upward particle Doppler
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velocities. It is suggested that falling snow melted upwind of the mountain and the ensuing raindrops were lifted upwards by strong upslope flows before reaching the mountainside at subfreezing temperatures.
Current operational forecast models with approximately 10 km resolution do not resolve the critical spatial scales at Cypress Mountain where stations are 6 km apart. Additionally, the associated large horizontal and vertical air motions within the events studied here imply that improvements in current microphysical parameterizations are needed to account for the rapid conversion of water vapour to rain and snow as well as the trajectories of these precipitation
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ACCEPTED MANUSCRIPT particles as they fall to the surface (Morrison and Milbrandt, 2015; Morrison et al., 2015; Milbrandt and Morrison, 2016). Collectively, precipitation at Cypress Mountain from January to April 2010 exhibited large temporal and elevation variations that were linked with variable upslope flows which also contributed to the production of freezing rain.
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Glossary
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Blocked flow: a weak or stable airflow which is unable to be advected over a topographic boundary.
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Inverse Froude number: an index which determines the presence of blocked or unblocked flows, including moist static stability, the terrain height, and the upstream flow velocity.
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An inverse Froude number < 0.5 indicates an unblocked flow, whereas an inverse Froude number > 0.5 indicates a blocked flow.
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Nyquist velocity: the range of velocity that can be uniquely detected by radar. Folding or aliasing occurs when particles exhibit a velocity beyond this range. For example, if the
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Nyquist velocity of a vertically-pointing radar is downwards at 0-12 m/s and the radar detects a particle which is rising at 2 m/s, folding would occur and the radar would
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interpret the velocity of the particle as downwards at 10 m/s. Particle Doppler velocity: the vertical velocity of precipitation particles relative to the ground
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including the vertical movement of the surrounding air. A positive velocity indicates towards the radar (downwards) and a negative velocity indicates away from the radar
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(upwards).
Unblocked flow: a strong or more unstable airflow which may be brought over a topographic boundary.
Acknowledgements
The authors would like to thank John Hanesiak, Ruping Mo, Sudesh Boodoo, David Hudak, Ivan Heckman, and George Isaac for their assistance. The authors would also like to
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ACCEPTED MANUSCRIPT acknowledge the Natural Sciences and Engineering Research Council of Canada for financial support.
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Observational Verification Experiment. Bull. Am. Meteorol. Soc., 1807-1826,
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doi:0.1175/BAMS-84-12-1807.
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Stoelinga, M.T., Stewart, R.E., Thompson, G., & Thériault, J.M., 2013. Microphysical processes within winter orographic cloud and precipitation systems. Mountain Weather Research
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and Forecasting. F. Chow et al., Eds., Springer Atmospheric Sciences, 345-408, doi:10.1007/978-94-007-4098-3_7.
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University of Wyoming, 2015. Atmospheric Soundings. Department of Atmospheric Science.
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http://weather.uwyo.edu/upperair/sounding.html. (Accessed: October 4, 2015.)
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ACCEPTED MANUSCRIPT Highlights
Near- to above-average precipitation occurred at Cypress Mountain for January-April 2010.
Above-average temperatures from El Niño led to more rain and less snow than average.
Much greater daily precipitation amounts occurred at higher elevation (by up to 54 mm)
Strong upslope and unblocked flows produced upwards precipitation velocities,
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during stronger upslope flows. sometimes ≥ 5 m/s, which on occasion produced little precipitation.
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Freezing rain sometimes occurred, probably due to highly variable particle trajectories
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including upward motions.
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