Precipitation characteristics within several Canadian east coast winter storms

Atmospheric Research, 25 (1990)293--316

293

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Precipitation Characteristics within Several C a n a d i a n East Coast Winter S t o r m s R.E. STEWART, R.W. CRAWFORD and N.R. DONALDSON

Atmospheric Environment Service, 4905 Du[ferin Street, Downsview, Ont. M3H 57'4 (Canada) {Received July 5, 1988; accepted after revision June 30, 1989 )

ABSTRACT Stewart, R.E., Crawford, R.W. and Donaldson, N.R., 1990. Precipitation characteristics within several Canadian East Coast winter storms. Atmos. Res., 25: 293-316. Precipitation frequently varies between snow, freezing rain, ice pellets and rain in Canadian East Coast winter storms. Snow is commonly in aggregate form, often rimed, and the largest of these reach up to 5 cm in diameter. Deep 0 °C layers in the atmosphere, in which aggregation readily occurs, are associated with these latter cases. Ice pellets are commonly frozen water drops; some of the frozen drops have spicules. Precipitation-type evolution during the transition from snow to rain is similar to predictions. Mesoscale circulations and thermodynamic features within such precipitation situations have been substantially affected by precipitation phase changes. RESUME Dans les perturbations hivernales de la c6te canadienne Atlantique, ilest habituel que les prdcipitations passent de la neige aux gouttes geldes, au gr~sil ou ~ la pluie. La neige se prdsente couramment sous forme d'agr~gats, souvent givr~s, les plus gros atteignant jusqu'h 5 c m de diambtre. A cette situation sont assocides des couches ~paisses de l'atmosph~re h 0 °C dans lesquelles le processus d'agrdgation peut facilement se d~velopper. Le gr~sil est commundment constitud de gouttes d'eau gelSes, dont quelques-unes pr~sentent des spicules. L'~volution du type de la precipitation lors de la transition de la neige h la pluie est conforme ~ la prdvision. Les circulations moyenne ~chelle ainsi que les processus thermodynamiques dans de telles situations sont fortement influences par les changements de phase des prdcipitations.

INTRODUCTION

Canadian winter storms typically produce snow, freezing precipitation and rain. The effect of such storms is largely dependent upon the type of precipitation experienced at the ground. To eventually improve the prediction of precipitation and its consequences, the nature of the precipitation in all its forms must be understood. During the winter of 1986 along the East Coast of Canada, the Canadian 0169-8095/90/$03.50

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Atlantic Storms Program (CASP) field project was undertaken {Stewart et al., 1987). Sixteen storms passed through the observation area and many of these produced substantial amounts of precipitation. Detailed precipitation data were collected during 6 of these storms at Canadian Forces Base (CFB) Shearwater, Nova Scotia. All of these storms produced rain, snow, and freezing precipitation. The purpose of this article is to use the CASP information to document and explain characteristics of the precipitation within these six storms in which the surface temperature passed through 0 ° C. Explanations will be based on microphysical processes and coupled mesoscale and thermodynamic features. EXPERIMENTAL

APPROACH

As part of the project, a Particle Measuring Systems {PMS) 2D-G Ground Probe was deployed at CFB Shearwater. This probe recorded images (in a 3level grey scale) of the precipitation particles and hence their sizes could be determined. The probe has a resolution of 0.15 mm and a maximum sampling width of 9.6 mm. Visual observations of the particles were also carried out using three approaches. Regular observations from the adjacent weather office were the prime source of precipitation-type information. This was supplemented with visual precipitation observations by the operator of the ground probe and in a few cases by photographs of individual particles. OVERVIEW OF PRECIPITATION

PARTICLES

The precipitation experienced at CFB Shearwater proceeded from snow to rain or drizzle in five of the storms. In one of these (February 22), a double transition was experienced from snow to rain and back to snow. TABLE I O c c u r r e n c e o f p r e c i p i t a t i o n types1 Precipitation type

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Fig. 2. Surface and 50 kPa analyses at 1200 UTC on February 22, 1986. The surface pressure (kPa) is shown by the solid line and the 50 kPa height (din) is shown by the dotted line. The dot to the northeast of the low centre indicates the location of CFB Shearwater.

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34 30 2G 22 18 14 10 Fig. 4. 1.5 km CAPPI plot from the Halifax radar at 1010 U T C on February 22, 1986. Reflectivities are in units of dB (Ze). The dot indicates the location of Shearwater.

Table I indicates that freezing rain and ice pellets commonly occurred. Ice pellets were noted in all storms, freezing rain occurred in five, whereas snow pellets were only observed in two. Precipitation evolving between snow and rain is therefore also associated with these other forms of precipitation. The occurrence of the various precipitation types and combinations observed during the six storms is shown in Fig. 1. Snow with ice pellets was the most common combination. Snow and freezing rain, ice pellets and freezing rain, and ice pellets and rain were somewhat less common. Snow was the type most likely to occur in combination with other precipitation types. This common occurrence of combinations of precipitation types has been noted previously by Hogan (1985). To illustrate common features of the precipitation within the storms, observations from two storms are discussed at length in the following section. These cases are February 22 and March 11. The observations from both these storms as well as from the others are synthesized in the discussion section.

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CASE S T U D I E S

February 22, 1986 A surface synoptic analysis for February 22 is shown in Fig. 2. The analysis shows that a system and its associated warm front tracked south of Nova Scotia. Surface temperatures were within 1 ° C of 0 ° C during the entire period between the occurrences of snow alone (Fig. 3). During this period, near-saturated conditions prevailed, wind speed slowly increased, and the pressure decreased slowly before rising after 1800 UTC. Significant wind directional changes at 0800 and 1900 UTC were associated with surface temperatures passing through 0 ° C. Radar information revealed that precipitation bands were present during this case (Fig. 4 ). The band producing the precipitation at Shearwater strad-

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Fig. 6. Visual particle observations on February 22, 1986. Crystal types were classified according technique.

to the Magono and Lee (1966)

dled the coastline and was 25-50 km across. The end of precipitation was associated with the m o v e m e n t of this feature from the region. A vertical profile of radar reflectivity as a function of time was also constructed over Shearwater. Radar echo tops were rather low with m a x i m u m heights at 4.0 km; the highest echo tops were associated with snow. Fig. 5 shows rawinsonde data associated with the precipitation. The lower atmosphere was characterized by saturated conditions and by deep layers close to 0 ° C. The deepest near 0 ° C layers were over 2 km thick. Mid-level inversions with m a x i m u m temperatures up to 2 ° C were centred near 0900 and 1800 U T C and were linked at the surface to the occurrence of freezing rain and ice pellets. Fig. 6 illustrates detailed precipitation information, as noted by visual observations. Ice pellets corresponded to transparent disks, and frozen single or coalesced raindrops. Some of the frozen raindrops were clear, whereas others had opaque cores. Large snowflakes, up to 5 cm diameter, occurred for 2 min starting shortly after the cessation of ice pellets. (Although not reported in the figure, large snowflakes were also reported at 1200 U T C for a short time some 15 km inland from Shearwater. ) The trailing snow was comprised of a number of crystal habits and a few of these were rimed. Illustrative images from the 2D-G grey probe which was operating after 1132

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UTC are shown in Fig. 7. At 1702 UTC, frozen and liquid raindrops up to 3 mm diameter were present and were reported as rain and ice pellets. No spicules were evident in the 2D-G images, although they were observed visually from 1713 to 1717 UTC. At 1746 UTC, the co-existence of frozen raindrops with non-spherical particles is evident, although all of the precipitation was reported as ice pellets. W h e n the observer noted the occurrence of 5 cm diameter snowflakes at 1922 UTC, the probe could only indicate that particles >> 9.6 m m were present. No constituent crystal habits could be discerned but there were "holes" through m a n y of the aggregates. Small particles were coexisting with these giant snowflakes. Illustrative size spectra are shown in Fig. 8. There was often an excess of large particles in comparison to the concentration predicted by, for example, Marshall and Palmer (1948). Such spectra occurred in rain, freezing rain, and ice pellets. There were also a number of cases in which the concentration of small particles ( < 1 ram) exceeded the Marshall-Palmer predictions. March 11, 1986 The synoptic analysis illustrated t h a t a warm front was associated with a

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transition from snow to drizzle at Shearwater. Throughout the precipitation event, the temperature rose, saturated conditions persisted, the surface pressure dropped, and wind speeds gradually decreased (Fig. 9). The wind direction was almost constant until the warm frontal passage near 1200 UTC. The warm front was associated with temperatures near 0 ° C. The precipitation was again associated with a well-defined precipitation band (Fig. 10). In this case, the band was about 75 km across. A vertical profile of radar reflectivity over Shearwater indicated that the highest echo tops were at 3.5 km and were associated with the occurrence of snow at the surface. Echoes associated with other precipitation types only reached 2.0 kin. Atmospheric features measured by rawinsonde are shown in Fig. 11. A substantial inversion was present during much of the precipitation period. Temperatures first exceeded 0°C at 0300 UTC at a height of 1.8 km. The onset of these temperatures was closely tied to the onset of ice pellets and freezing rain. The 0°C level lowered with time until it reached the surface at 1000 UTC. During much of the transition, near-saturated conditions prevailed to temperatures colder than - 12 ° C. The period of freezing drizzle or drizzle which began

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at 0600 UTC was associated with a major decrease in the depth of saturated conditions. The detailed precipitation habits are shown in Fig. 12. Rimed needles, plates and sectors predominated from about 2300 UTC to 0200 UTC. The sectors often had broken or missing branches. Heavily rimed frozen raindrops ( < 1 ram) were reported as ice pellets from 0308 UTC to 0315 UTC. Near 0330 UTC, the smaller ice pellets were opaque and the larger ones tended to be clear. Rimed plates and sectors were not observed after 0430 UTC so that the final period of snow was comprised of rimed needles and irregular rime particles. Photographs of crystals taken at two times during the transition are shown in Fig. 13. The crystals at 0122 UTC were rimed, although the stellar crystals exhibited the heaviest riming. The ice pellet at 0520 UTC was a frozen drop which had been fractured. Simultaneous 2D-G probe information indicated that the maximum ice pellet diameter was about 3 mm throughout the 03450520 UTC duration of this precipitation type.

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D I S C U S S I O N AND S Y N T H E S I S OF O B S E R V A T I O N S

Mesoscale and thermodynamic characteristics Precipitation bands were associated with all the precipitation events. The presence of these bands is consistent with a mesoscale circulation driven by the gradient of melting across the precipitation area which in all the storms in this study included a transition between snow and rain (Lin and Stewart, 1986). Such a circulation causes updrafts to preferentially occur over the snow region. As evidence of this, the deepest echoes were over the snow in the two cases discussed above as well as in all the other four cases. Such circulations also explain surface wind changes in association with temperature changes through 0 °C that were typically also analyzed as the warm front. The particles altered tropospheric temperatures through their changes of phase. Melting aloft cools the air towards 0°C; freezing to form ice pellets or to refreeze partially melted snow warms the air towards 0 ° C. The magnitude of these processes can be estimated. Stewart and King (1987a) showed that the depth of water D (in mm) required to eliminate an inversion is given by:

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the maximum temperature in the inversion, zt is the upper height with a 0 °C temperature, Zbis the lower height with a 0 ° C temperature, Lf is the latent heat of fusion, and Pw is the density of water. Eq. 1 was applied to observed temperature fields to determine the significance of the phase changes of precipitation on the temperatures aloft. Results showed that cooling due to melting would have significantly influenced the decay of inversions. For example, it was found that 2 mm of precipitation would be needed to eliminate the inversion occurring near 1000 UTC on February 22. At the observed average precipitation rate of 5 mm h-1, this would occur over 0.4 h. Refreezing to form ice pellets or to refreeze partially melted snowflakes would also cause significant effects. For example, near 0600 UTC on March 11, eq. 1 predicts that temperatures in the low-level subfreezing region would have risen 0.5°C h -1. This represents approximately 25% of the observed warming. At least partially because of these latent heat exchanges associated with phase changes, the lower atmosphere in many of the storms remained near 0 ° C during much of the precipitation. Deep layers near 0 ° C were first noted by Stewart {1984); their formation is also influenced by horizontal warm and cold air advection (Szeto et al., 1988a,b). The evolution of the lower tropospheric temperature in turn, contributes substantially to the general characteristics of the precipitation at the surface such as its type (Stewart, 1985). Understanding processes contributing to precipitation characteristics at the surface is therefore intertwined with understanding conditions aloft during precipitating periods of the storms. Particle nature and evolution

The precipitation-type transitions somewhat followed the systematic progression predicted in an evolving inversion (Stewart and King, 1987b). In addition, snow and ice pellets or snow pellets sometimes alternated before the final transition to rain (Fig. 14). No structure related to this additional variability could be discerned from the radar information which in each case just showed a single band in the precipitation region passing over the observation site. The reason for this additional variability must be small-scale fluctuations in the low-level winds and temperatures that were not sampled adequately with the sequential rawinsonde releases. A special note needs to be made of the occurrence of freezing rain and snow. This combination is only predicted by the theory of Stewart and King {1987b) if complete refreezing of semi-melted particles does not occur. When this combination is occurring, there should be particles which are a combination of liquid and solid. These particles would have a temperature of 0 °C and would freeze more rapidly on structures than pure freezing rain since less latent heat would need to be liberated for the same amount of resultant ice mass.

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Fig. 14. The precipitation-type paths followed between only snow and only rain in the Shearwater cases. The second transition on February 22 is reversed. Drizzle is combined with rain, and freezing drizzle is combined with freezing rain. The predicted evolution of Stewart and King ( 1987b )

is shownby the solid,horizontalline. Precipitationtype symbolsare definedin Table I. Fig. 15 illustrates the transitions in precipitation types observed at Shearwater. The transitions were normalized in order to discern whether a consistent pattern in the duration of particular precipitation types was present. No such pattern was evident although this may be at least partially a result of variations in the observing technique itself. Snow in combination with other precipitation types ranged from 25% to 80% of the transitions. Freezing rain alone varied from being present for over 60% of the period in the March 11 case to being totally absent on February 2 and February 5. During the occurrence of snow and freezing rain on February 22 and March 7, the base of the above freezing portion of the inversion was less than 500 m above the surface. This is consistent with insufficient time being available for ice pellet formation. At Shearwater, 60% of the visually-observed crystals were rimed. This is quite consistent with the presence of low-level updrafts in the snow region. Such updrafts in the saturated environment lead to the formation of droplets by condensation; ice crystals falling through this updraft would collect these droplets as rime. Such evidence for updrafts in the snow region also supports the assertion that melting-induced circulations were operating. A variety of habits occurred in the regions with snow or snow mixed with other precipitation types. However, needles were most common and they occurred in 50% of the visual precipitation observations. In contrast only 20% of the visual observations showed sectors and plates and 15% contained dendrites. The predominance of needles over colder-temperature crystals such as

CANADIAN EAST COAST WINTER STORMS

DATE

309 FRACTION OF DURATION

DURATION (mln)

0.0 I

~-

0.2

0.4

0.6

0.8

t

I

i

I

1.0

IP P

FEB. 2

52

m

FEB. 22A

133

ip

FEB. 5

167

IP

Ip p

MAR. 7

232 sp

sp z

Sp S

$P $

u) MAR. 14

iT,:

R

FEB. 22B

,os ~; I

MAR. 11

IP P

I 0.0

I

I

I

I

I

0.2

0.4

0.6

0.8

1.0

Fig. 15. Fraction of time spent within different precipitation types during all the transitions at Shearwater. The transitions are ordered in terms of increasing duration. The beginning time is the end of snow alone; the ending time is the beginning of rain alone. The second transition on February 22 is reversed. Drizzle is combined with rain, and freezing drizzle is combined with freezing rain. Precipitation type symbols are defined in Table I.

plates and dendrites cannot be attributed to greater nucleation. The presence of the needles may be a reflection of ice multiplication processes occurring favourably around - 5 ° C. The process described initially by Hallet and Mossop (1974) is one possible explanation. This process requires collisions between crystals and supercooled droplets which occurred in these situations as indicated by the extent of riming. In addition, Stewart et al. (1984) used aircraft information to show that ice multiplication was probably occurring close to an elevated melting layer in stratiform clouds. A similar situation may be occurring when the melting layer is associated with the precipitation-type transition region. As evidenced by the spectra that were observed during the present cases, there was typically an excess of large particles during freezing rain and ice pellets. Similar observations have been reported by Ohtake (1970), Iwai (1970) and Kimura and Kajikawa (1984). Such an observation has also been reported in rain by, for example, Stewart et al. (1984), List et al. (1987) and Steiner and V~aldvogel (1987). Time is required for the equilibrium spectra of Mar-

310

R.E. STEWART ET AL.

shall-Palmer to evolve. Until this happens, large snowflakes would have melted to form raindrops that were larger than predicted by the formula. The proximity of the melting level to the surface in freezing rain and ice pellet situations only accentuates this factor. The proximity of melting to the surface suggests an explanation for the excess of small drops sometimes observed. Until they have completely melted, large particles will be a combination of liquid and ice; the ice would hold the particles together by reducing internal circulation and by resisting deformation by the passing airflow. The melting of these particles could lead to a rapid drop evolution as surface tension alone could not sustain them; they would spontaneously breakup into many small drops.

Ice pellet considerations Reports of ice pellets were typically associated with"the occurrence of frozen drops. Some of these frozen drops had spicules or were shattered. Spicules on frozen drops have been noted and explained previously by Bally (1935), Dorsey (1948) and Blanchard (1951). They found that spicules can develop if a drop freezes after formation of an ice shell. Such a shell would be expected to occur if contact nuclei had activated the freezing process or if freezing had been initiated through a collision with another ice particle. The latter possibility further implies that particle interactions during the refreezing process will affect the precipitation types observed at the surface and in particular these interactions will tend to lessen the amount of freezing rain as suggested by Hogan ( 1985 ) and Stewart and King (1987b). The probability that a significant number of collisions would occur in which freezing raindrops were involved was examined in more detail. If one considers geometric sweepout alone, the average distance H required for a drop of diameter D1 to collide with an ice pellet or snowflake of diameter D2 is given by: H = 4 v~ [n AvTt(D1+D2) 2]

(2)

Here, v is the drop terminal velocity, n is the concentration of ice pellets or snowflakes, and Av is the difference in terminal velocity between the drop and the other particle. If embedded within a field of snowflakes having a diameter of i cm, falling at 1 m s - ' and occurring in a concentration of 10 m -3, a drop falling at 5 m s - 1 would expect to collide with a snowflake every 1.6 km. If the snowflake diameter was 2 cm, this distance would be only 400 m. For a drop embedded in a field of ice pellets having a diameter of 2 mm, falling at 4 m s- ' and occurring in a concentration of 100 m -3, the distance between collisions would be 500 m. Such distances for collisions are quite comparable to the observed depths of the subfreezing regions. As suggested by Hogan (1985) and Stewart and King (1987b), collisions would therefore be expected to signifi-

CANADIAN EAST COAST W I N T E R S T O R M S

311

cantly diminish the concentration of freezing rain drops and enhance the concentration of ice pellets. To test whether the development of ice pelletsfrom partialmelting aloftand refreezing near the surface could be simulated numerically, a model based on the basic governing equations from Pruppacher and Klett (1978) was developed. The rate at which freezing occurs is governed by several factors. These include the ventilation rate at which heat is exchanged with the environment. This is given by: d h / d t = F h K ( Tp -

Ta)D/2

(3)

Here, h corresponds to the heat content, Fh is the heat ventilation coefficient, K is the thermal diffusivity, Tp is the particle temperature, and T~ is the air temperature. As the particle falls, the vapour density at its surface will differ from the vapour density in the surrounding atmosphere. The change in heat due to mass exchange is determined by the following equation:

dh/dt= LcFvDv(ps - # )D/2

(4)

Here, Lc is the latent heat of condensation, Fv is the vapour ventilation coefficient, Dv is the diffusivity of water vapour in air, p is the vapour density, and ps is the saturation vapour density at the particle surface. To account for the changing terminal velocity of a particle as it froze, its diameter and density were continuously determined. The particle's terminal velocity was assumed to be that of a sphere of the diameter corresponding to its proportion of liquid and solid. The model was applied to the observation of ice pellets on March 11 near 0515 UTC. The model predicted that particles with a melted diameter of 2 mm only finish melting as they reach the bottom of the deep inversion present at this time. The deep subfreezing region below this inversion is sufficient to allow complete freezing to form an ice pellet. Particles with an effective diameter less than 2 m m melt completely within the upper inversion; they would fall to the surface as supercooled drops. Particles somewhat larger than 2 mm, including the 3 m m observed particles, would be able to refreeze in the lower subfreezing region and be observed at the surface as ice pellets. The model therefore is quite capable of simulating the ice pellet observations in at least this instance. Ice pellets may form through another process as well. In particular, Kajikawa et al. (1988) discussed the production of rain and ice pellets in conditions in which temperatures were everywhere less than 0 ° C. Warm rain processes were responsible for the growth of drops some of which later froze at colder temperatures near the surface to produce ice pellets. The largest particle diameters in their case were 0.4 ram; diameters to 3 m m were common in the

312

R.E. STEWART ET AL.

East Coast storms. Furthermore, their precipitation rates were < 0.1 mm h - 1; rates in excess of 1 mm h-1 occurred in the East Coast storms. These observations suggest that large ice pellets associated with higher precipitation rates will be formed through a melting-refreezing process. Relatively little water is available at subfreezing conditions to form large drops; the warm-rain mode of producing supercooled drops and their later freezing to form ice pellets in certain conditions could be an important process however in some cases with light precipitation. The freezing of drops produced through warm-rain processes would not need to be through nucleation at cold temperatures as in the study of Kajiwawa et al. (1988); collisions of these drops with ice particles could also induce the freezing process.

Large snowflakes Large snowflakes were observed in several of the storms (Table II). When noted, these snowflakes consisted of aggregates of needles or dendrites and lasted for only a few minutes. The largest snowflake, 5 cm in diameter, is however small in comparison with greater than 10 cm diameter snowflakes reported in the literature (Lowe, 1887; Plunket, 1891; Denning, 1912; Abbe, 1915; Hawke, 1951; Auer, 1971; Corliss, 1984; Pike, 1988). All of the large snowflake observations, including the present 5 cm one, occurred at surface temperatures near 0 ° C. Rawinsonde observations near the time of the 5-cm particles furthermore indicated that in the lower 2 km the atmosphere was within 2 degrees of 0 ° C and that the winds were continually < 10 m s - 1. Such temperatures are ideal for aggregation and the light winds would not lead to the breakup of the particles nor would it carry them away from the near-0 °C environment and the observing site. The growth of large snowflakes was examined through the use of a simple mathematical analysis. It was assumed that all particles were spherical, colT A B L E II Occurrence of large snowflakes at t h e surface Date

Time (UTC)

Temp. (°C)

Humidity (%)

Size (cm)

Habits

02/02 02 / 15 02/22 03/02 03/14

1935-1940 2018 - 2022 1922-1924 1553 - 1557 0358-0402 0528-0532 0828-0832 0848-0852 1048-1052

- 0.9 - 0.5 0.0 - 2.1 - 3.0 - 3.0 - 2.4 -0.5 - 2.3

99.0 96.9 96.0 92.6 98.5 99.2 99.8 99.0 99.8

0.8 1.0 5.0 1.0 1.0 1.0 1.0 1.0 2.0

needles, dendrites dendrites dendrites needles needles

needles

CANADIAN EAST COAST WINTER STORMS

313

lected crystals were all the same size, and aggregation was through geometric sweepout. The basic growth equation is given by: dM dt - C E r c ( D + Dc)2(Va -vc)ncrnc/4

(5)

Here, M refers to the mass of the aggregate, CE is the collection efficiency, D is the aggregate diameter having an initial diameter Do, Dc is the crystal diameter, va is the aggregate terminal velocity, vc is the crystal terminal velocity, nc is the crystal concentration, and rn~ is the crystal mass. The mass of the aggregate and each crystal is determined by assuming that they have densities ofpa and #~, respectively. If two simplifying assumptions are made, the equation can be rearranged to determine the distance H' required for a given diameter to be reached. By assuming that the aggregate is much larger than the crystal and that the aggregate terminal velocity does not change as it falls, the equation can be written as"

H' - 2 p a ( D - D o ) (va) CEn~m¢ (Va -- Vc )

(6)

Predictions of eq. 6 are consistent with the observed heights and snowflake characteristics. The following reasonable values were used in the equation: collection efficiency of 1.0, crystal terminal velocity of 0.5 m s - 1, crystal concentration of 20 l- 1, crystal mass of 0.01/~g, and initial diameter of I cm. Using these values, the equation predicts that 4 km is required to produce a 5-cm diameter snowflake if its density if 10 kg m - 3 and 2 km is required if its density is 5 kg m -3. A 2-cm diameter snowflake is produced in I km if its density is 10 kg m -3. As shown by the observations, conditions near 0°C extended up to 2 km. Furthermore, the snowflakes were composed of dendrites and needles; these habits yield low-density aggregates which most readily increase their size. These calculations and the observations collectively indicate that ideal conditions for the growth of large snowflakes, such as temperatures near 0°C, cannot just occur near the surface; they must extent through a substantial depth of the atmosphere. The common observation that large snowflakes occur for a short period is at least partially a consequence of the particles themselves. The collection of crystals by each snowflake leaves fewer crystals for subsequent ones and these cannot grow as large. This concept was examined using the equation. Snowflake concentration was assumed to be 1 m -3 as calculated by Auer (1971). Over a 3 km fall, snowflakes falling 10 rain after the first ones had diameters a factor of 2 smaller. Shorter periods were produced if, for example, the crystal concentration was increased. The time scales to significantly decrease sizes are then a few minutes, as observed in this study and as noted in the earlier studies as well.

314

R,E. STEWART ET AL.

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HORIZONTAL DISTANCE (km) Fig. 16. Conceptual model of the precipitation cases discussed in the article. Solid lines show temperature in ~C. The shaded area refers to the region with enhanced upwards motion. T h e horizontal and vertical scales are only approximate.

Synthesis Many of the precipitation characteristics which have been discussed within this Section have been incorporated into Fig. 16. Cold crystals are produced within the deepest clouds which preferentially occur over the surface snow region. This region is also characterized by updrafts in which riming occurs on the crystals falling from aloft. Because of the occurrence of riming near - 5 ° C, it is probable that some ice multiplication process such as described by Hallett and Mossop (1974) is occurring so that needles are the most common crystal at the surface. W h e n horizontal winds are light, deep near-0 ° C layers serve as an ideal environment for the production of large snowflakes at surface temperatures also near 0 oC. If an inversion with sufficiently cold air beneath is present, ice pellets are formed. Even in the rain region, the melting layer is only a short distance above the surface. Consequently, there is typically insufficient time for the liquid precipitation to evolve into a steady state size distribution in this region. CONCLUDING REMARKS

A study has been conducted of the precipitation characteristics within six Canadian East Coast winter storms in which surface temperatures passed through 0 ° C. This has led to a number of observations and conclusions.

CANADIANEASTCOASTWINTERSTORMS

315

The precipitation was banded with the highest echo tops associated with snow. Such observations are quite consistent with circulations associated with the atmospheric response to melting snow. The tropospheric temperature profiles varied systematically in association with the precipitation. As indicated by the common occurrence of temperatures aloft near 0 ° C, the troposphere had furthermore been significantly influenced by the phase changes of the precipitation itself. A number of features associated with the precipitation particles were documented. This includes the common occurrence of needles, the degree of aggregation, the presence of riming, the nature and number of ice pellets, the occurrence of large snowflakes, and the deviation of raindrop spectra from steady state. It is evident that an understanding of precipitation characteristics must consider the complex interactions occurring within the storms. For example, the depth of clouds producing crystals is influenced by the precipitation itself, precipitation-type evolution is governed by temperatures affected b'y the phase changes of the precipitation, and large snowflakes sometimes occur within deep 0 ° C layers developed through the melting or freezing of other particles. In summary, this investigation has illustrated that consistent characteristics occur in association with the precipitation within Canadian East Coast winter storms in which surface temperatures pass through 0 ° C. Such characteristics are nevertheless explainable in terms of the underlying microphysical, thermodynamic, and dynamic processes. ACKNOWLEDGEMENTS

This research was supported by the Federal Panel on Energy Research and Development (PERD). The authors would like to thank Ken Macdonald for providing the synoptic analyses used in this study. The authors would also like to thank Andy Madej, Walter Strapp, Mohammed Wasey, Tom Low, and Arthur Di Leo for assisting in the preparation of the manuscript.

REFERENCES Abbe, C.A. Jr., 1915. Gigantic snowflakes. Mon. Weather, Rev., 43: 73. Auer, A.H., 1971. Some large snowflakes. Weather, 26: 121-122. Bally, O., 1935. [)ber eine eigenartige Eiskrystallbildung. Helv. Chim. Acta, 18: 475-476. Blanchard, D.C., 1951. A verification of the Bally-Dorsey theory of spicule formation on sleet particles. J. Meteorol., 8: 268-269. Corliss, W.R., 1984. Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena. The Sourcebook Project, Glen Arm, MD, 196 pp. Denning, W.F., 1912. Large snowflakes. Symon's Meteorol. Mag., 47: 22. Dorsey, N.E., 1948. The freezing of supercooled water. Trans. Am. Philos. Soc., 38: 247-328.

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Hallett, J. and Mossop, S.C., 1974. Production of secondary ice particles during the riming process. Nature, 249: 26-28. Hawke, E.L., 1951. Outsize snowflakes. Weather, 6: 254. Hogan, A.W., 1985. Is sleet a contact nucleation phenomenon? Proc. Eastern Snow Conf., Montreal, Que., pp. 290-294. Iwai, K., 1970. Size distribution of raindrops at surface temperature below 0 ° C. Bull. Inst. Natl. Educ., Shiga Heights (Sinshu Univ. ), 9: 93-99. Kajikawa, M., Sakurai, K. and Kikuchi, K., 1988. Characteristic features of supercooled raindrops in the midwinter season of Arctic Canada. J. Meteorol. Soc. Jpn., 66: 393-398. Kimura, T. and Kajikawa, M., 1984. An observation of ice pellets. J. Meteorol. Soc. Jpn., 62: 802808. Lin, C.A. and Stewart, R.E., 1986. Mesoscate circulations initiated by melting snow. J. Geophys. Res., 91: 13299-13302. List, R., Donaldson, N.R. and Stewart, R.E., 1987. Temporal evolution of drop spectra to collisionat equilibrium in steady and pulsating rain. J. Atmos. Sci., 44: 361-372. Lowe, E.J., 1887. Snowstorm of January 7, 1887. Nature (London), 35: 271. Magono, C. and Lee, C.W., 1966. Meteorological classification of natural snow crystals. J. Fac. Sci., Hokkaido Univ., Ser. 7, 2: 321-335. Marshall, J.S. and Palmer, N., 1948. The distribution of raindrops with size. J. Meteorol., 5: 165166. Ohtake, T., 1970. Factors affecting the size distribution of raindrops and snowflakes. J. Atmos. Sci., 27: 804-813. Pike, W.S., 1988. Unusually-largesnowflakes. J. Meteorol., 13, 3-16. Plunket, J.D., 1891. Snowfalls. Mon. Weather, Rev., 19:11. Pruppacher, H.R. and Keltt, J.D., 1978. Microphysics of Clouds and Precipitation. R. Reidel, Dordrecht, 714 pp. Steiner. M. and Waldvogel, A., 1987. Peaks in raindrop size distributions. J. Atmos. Sci., 44:31273133. Stewart, R.E., 1984. Deep 0°C isothermal layers within precipitation bands over southern Ontario. J. Geophys. Res., 89: 2567-2572. Stewart, R.E., 1985. Precipitation types in winter storms. PAGEOPH, 123:597 609. Stewart, R.E. and King, P., 1987a. Rain/snow boundaries over southern Ontari(~. Mon. Weather Rev., 115: 1894-1907. Stewart, R.E. and King, P., 1987b. Freezing precipitation in winter storms. Mon. Weather, Rev., 115: 1270-1279. Stewart, R.E., Marwitz, J.D., Pace, J.C. and Carbone, R.E., 1984. Characteristics through the melting layer of stratiform clouds. J. Atmos. Sci., 41: 3227-3237. Stewart, R.E., Shaw, R.W. and Isaac, G.A., 1987. Canadian Atlantic Storms Pr~gram: the meteorological field project. Bull. Am. Meteorol. Soc., 68: 338-345. Szeto, K.K., Lin, C.A. and Stewart, R.E., 1988a. Mesoscale circulations forced by melting snow, Part I. Basic simulations and dynamics. J. Atmos. Sci., 45: 1629-1641. Szeto, K.K., Stewart, R.E. and Lin, C.A., 1988b. Mesoscale circulations forced by melting snow. Part I I. Application to meteorological features. J. Atmos. Sci., 45: 1642-1650.