Simultaneous polarization and Doppler observations of severe convective storms in central Alberta

XF~ IOSPHERIC RESEARCH Atmospheric Research 33 (1994) 37-56

ELSEVIER

Simultaneous polarization and Doppler observations of severe convective storms in central Alberta A.R. H o l t a, P.I. Joe b, R. M c G u i n n e s s a, E. Torlaschi a'*, T. Nichols c, F. Bergwall d, D.A. H o l l a n d a aDepartment of Mathematics, University of Essex, Colchester, C04 3SQ, UK bKing Weather Radar Stn., Atmospheric Environment Service, RR#1, Jane Str., King City, Ont., Canada LOG IKO CAtmospheric Environment Service, 4999-98 Avenue, Edmonton, Alta., Canada T6B 2X3 dEnvironmental Research and Engineering, Alberta Research Council Edmonton. Alta., Canada T6H 5X2 (Received December 4, 1992; revised and accepted June 14, 1993 )

Abstract

Simultaneous observations of a series of mesocyclonic storms in central Alberta by both C-band Doppler and S-band circular polarization-diversity radars are reported. Attention is focussed on one particular cell which was a right mover. Data show that the circular polarization parameter "degree of polarization" clearly differentiates the storm inflow regions. This suggests that the use of polarization-diversity may be a help in determining storm structure, and therefore an aid in severe storm forecasting.

I. Introduction The Alberta Research Council (ARC) has an S-band circular polarizationdiversity radar, located at Penhold, Alberta, Canada approximately 150 km south of the city of Edmonton. The region in which the radar is located is subject to severe convective storms in summer. These can cause large hail to fall with consequent damage. This radar was used during the period 1968 to 1985 in conjunction with the Alberta Hail Project, a weather modification experiment. In 1989, the radar was used in a pilot experiment designed to obtain data to test the proposition that differential propagation phase is a better estimator of heavy rain rate *On leave from Dept. de Physique, Universit6 du Quebec h Montreal, Montreal, Que., Canada H3C 3P8. 0169-8095/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-8095 ( 93 ) E0076-B

38

A.R. Holt et aL /Atmospheric Research 33 (1994) 37-56

than is reflectivity (English et al., 1991 ). Since the results were encouraging, and since the radar was still available, a second experiment was held for three weeks in 1991, from 17 July. It was conducted jointly by ARC and the University of Essex, UK. The object of the experiment was to collect further polarization data from convective storms, together with concurrent ground observations of rainfall, hail, etc. It was also hoped to obtain a real-time display of the polarization data. In addition to, and colocated with, the S-band radar, ARC have a C-band reflectivity radar. In the southern and eastern sectors of its scan, the C-band radar is partially blocked by the S-band antenna or buildings, but to the west and north it has clear vision. The major value of the C-band was for surveillance. The Atmospheric Environment Service (AES), of Environment Canada, has an operational C-band Doppler radar, situated at Carvel, some 45 km west of Edmonton. From the ARC radar, Carvel is at a range of approximately 155 km at azimuth 352 ° (Fig. 1 ). The range of the Carvel radar in Doppler mode extends to 110 km. There is therefore a considerable region in which the coverage areas of the two radars overlap. It was, however, intended that the range of the field experiment be restricted to approximately 70 km from Penhold. In this paper we describe observations on July 29, 1991. All timings are given in local time, M D T ( - 6 h UTC). At 06:00 MDT, a short wave trough oriented north-south over central British Columbia was moving eastward at 13 m/s. Winds from the surface to 2000 m above sea level (ASL) were light and variable while winds above were west-southwesterly and increasing to more than 40 m / s at the tropopause. Although a surface front was not evident, a west to east baroclinic zone lay across central Alberta. ........... .160[km ......... .... ,', . ........... ~........... ":,,C-band ,."" .'.,.... ...--.-i ....... '":,': ""..,

Fig. 1. The relative positions of the Carvel and Penhold radars showing the approximate storm tracks of the three cells considered. Range rings are at 20 km intervals.

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56

39

The 06:00 temperature and dew point sounding.from Rocky Mountain House shows that the air in the morning was already quite unstable ( - 7 lift index). By early afternoon, the low pressure center associated with the short wave had moved eastward away from the mountains, allowing a northwesterly flow to become established in the low levels over western Alberta together with the onset of strong vertical shear (northwesterly winds 10 to 15 m / s in the low levels, southwesterly 15 m / s at 3000 m ASL and stronger southwesterly aloft). The increasing divergence aloft with the short wave and the intensification and the southeastward motion of the front, allowed strong convection to develop during the afternoon. Early in the afternoon, five large, intense and persistent isolated storms developed and, later on, produced very large hail (greater than 6 cm diameter), possibly one tornado, but no excessive rain was reported. They moved across the region between the two radars, their tracks being from the west in a direction just south of east. Radar observations show for all of these storms features that usually occur in supercell storms, as is evidenced by their long lasting high reflectivity cores, and in Section 5 we describe the three most intense storms (see Fig. I for their approximate tracks). The information from the two radars is seen to be complementary. As well as the radar observations, the field experiment had two chase vehicles equipped with both wedge and tipping bucket gauges. In addition, observations were made by a small network of volunteer observers, and rainfall measurements were made by two permanent tipping bucket gauges belonging to AES and Alberta Environment.

2. Radars

The AES radar at Carvel is a C-band radar manufactured by Enterprise Electronics Corporation. It was Dopplerized for the 1991 severe weather season. It is a coherent on receive magnetron system with a twelve foot dish and has a nominal 1 ° beamwidth. Reflectivity, mean radial velocity and mean Doppler spread are computed via the pulse pair technique using a RVP6 Doppler signal processor from Sigmet Inc. The dual P R F velocity unfolding technique is used to extend the unambiguous velocity interval to + 48 m/s. Radar image products are produced at the radar site by a microvax (Digital Equipment Corporation ) processing system developed by the King City radar group (Crozier et al., 1991 ). Scanning mode consists of a 24 elevation reflectivity-only volume scan with 2 km resolution to a range of 256 km taking 5 minutes to perform, and then a 3 elevation Doppler scan (0.5 °, 1.5 °, 3.5 ° ) with 1 km resolution to a maximum range of 110 km occupying the next 5 minutes. The ARC S-band polarization diversity radar was installed in 1967, and has digitally recorded four channel data since 1977. It can transmit any polarization, but has operationally always transmitted Left Hand Circular (LHC). It records data for every 1 ° in azimuth (approximately), and from a maximum of 147 range bins, each 1.05 km long. The first range bin commences 3.2 km from the radar. Its scan cycle consists a helical scan, rising 1 ° in elevation for every 360 ° in azi-

40

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

muth. In 199 l, the maximum elevation was 8 o, and the total scan cycle, consisting of 12 revolutions, lasted 88 s. For a fuller description of the radar see English etal. (1991). The principle of the ARC S-band polarization diversity radar is to transmit LHC polarized radiation, and make four measurements of the return signal (cf. McCormick and Hendry, 1975 ). These are the powers in the main and orthogonal channels, ff'2 and I ~ respectively, and the magnitude and phase of the complex correlation, if', of the main and orthogonal fields.

3. Theory Because precipitation particles are generally nonspherical, not only will the transmitted radiation be depolarized when reflected by precipitation, but also the polarization state of the transmitted wave will change as the radar beam penetrates the region of precipitation. The intrinsic scattering properties of the resolution volume are, therefore, coupled with the properties of the propagation medium, and both effects contribute to establish the return signal. According to Todaschi and Holt (1993) when S-band LHC polarization is transmitted, the intrinsic properties of the radar signal can be estimated by: ~ = arg{ I~2 2 l~l + j Im [ I~] } = arg I~3

(1)

I~21 ' = e a ( cOsh Aa I~2 +2 l~l + _ I I~3 I - s i n h Aa Re if')

(2)

2

cosh Aa Re

(3)

where • is the total differential phase, Aa is the two-way differential attenuation in Np units, and A is the two-way mean attenuation of the radar signal. McGuinness and Holt ( 1989 ) suggested that it is possible to estimate in rain the two-way attenuation A, and the two-way differential attenuation AA and consequently it is possible from the polarization data to obtain information on both forward and backward scattering from each resolution volume. Some of the radar observables which can be obtained from the radar echo are the following. (i) The equivalent reflectivity factor for circular polarization: 2 4 4rtr 2 Z c = r ? l K i 2 icl~W2

(4)

where IKI2=0.93, 2 is the wavelength, r the range and I CI 2 a radar related constant. (ii) The differential reflectivity:

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56

ff'z + #~ - 2 R e WI ZDR = 101ogt0 i~2 + i,~,l + 2R e +A,4

41

(5)

(iii) The correlation for circular polarization: I WI p = ( W 1 W2)1/2

(6)

(iv) The degree of polarization (cf. McGuinness et al., 1987 ): /,

4 ( ['V1 W2 - [ [~[ 2

(v) The gradient, KDp, of the two-way propagation differential phase, •DP, in gate n of length dr:. Ar

(8)

In rain regions, ~I)P is equal to q~. In regions containing hail they may differ due to differential backscatter phase. In this paper we have derived rainfall amounts from the differential propagation phase KDp using the relationship suggested by D.S. Zrnic (pers. commun., 1988): Rl.15 KDp = 3.1722 (cm) (9) The degree of polarization p is virtually unaffected by propagation effects (Bebbington, 1992) and it takes values between 0 and 1 (100%). If the scatterers are random, such as in clutter, then p will be low, since the return field will be unpolarized. But in the precipitation values are much greater, typically around 99% in rain, and 95% in hail (Bebbington et al., 1987). Differential reflectivity ZDR less than 1 dB as well as circular correlation p less than 50% in high reflectivity regions are indicative of hail (AI-Jumily et al., 1991 ).

4. Ground observations

The field experiment had two mobile chase crews, in radio contact with the control at the radar site. The crews were equipped with both wedge and tipping bucket rain gauges. One crew also had a video camera. Radar control was equipped with oscilloscopes giving PPI displays of reflectivity, albeit not range corrected, and Circular Depolarization Ratio (CDR= 10 log~oI~'2/I~, of. McCormick and Hendry, 1975 ). A small network of some 27 volunteer observers had been established for the duration of the experiment, each measuring rainfall twice a day. They were also asked to note any occurrence of hail. In practice, many of the

42

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

observers read their gauges additionally immediately precipitation ceased (during daylight hours). Readings were only useful when radar monitoring of the site occurred throughout the rainfall observation period. We subsequently were given access to the data recorded by the AES network of observers, and data from tipping bucket gauges in the project area (radius 70 km) belonging to AES and Alberta Environment. Red Deer College also gave us access to their meteorological data. The AES Weather Office at Edmonton provided forecast information, and regular contact was made between the Weather Office and ARC radar control on the day being discussed. Furthermore, we have subsequently been given data on hail reports made to the Alberta Hail and Crop Insurance Corporation.

5. Radar observations The first sign of activity from Penhold was around 13:45 h *, when a small area of weak reflectivity appeared just west of the radar. This was investigated by a chase crew who experienced just a few drops of rain. This cell was moving in a direction slightly north of due east, and subsequently intensified. However, it was never an intense cell whilst within the confines of the project area, and appeared to diminish after 17:00 h. It will not be considered in this paper (except that part of its track appears, together with some hail reports in Figs. 8 and 9 nearly due east of the radar at a distance of more than 100 km). Subsequently, four further isolated cells developed in the region between the two radars (cf. Figs. 2 and 3 ). The first three of these resulted in heavy hail falling to the ground, and were accompanied by high winds. Whereas all three had similar echo characteristics, only the first two were within the Doppler range of Carvel. Thus although all three appeared to be mesocyclonic, only two can be clearly identified as such. (i) Storm 1: Due to essential maintenance, the S-band was not operational until 15:49 hrs, at which time the first convective cell was approximately 45 km from the radar along azimuth 350 ° . At this stage, heavy hail had already been experienced by one chase crew, and the second chase crew was already experiencing hail. The ARC radar data shows that there were reflectivities in excess of 55 dBZ at 1 ° elevation, whilst the differential reflectivities were less than 1 dB throughout the high reflectivity region. This is indicative of hail (A1-Jumily et al., 1991 ). It is also noteworthy that the circular correlation was less than 30% in the same region. Viewed from Carvel the storm was at range 100 km and azimuth 175 ° at 15:50. The largest low-level reflectivities were located in the southwest side of the storm. The highest reflectivity gradients, and a reflectivity notch, were located in the right front quadrant, indicative of the storm inflow region. The images of the storm seen from Penhold, show that at this stage the highest reflectivity region was close to the surface. Over the next fifteen minutes this decreased in intensity, and a marked overhang was seen towards the southeast in vertical cross-sections taken along a N W - S E line. The "overhang" is formed by *All times are given in M D T ( - 6 h U T C ) .

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

43

high reflectivity values aloft, with little reflectivity vertically below in the lower elevations. From 16:10 h the storm intensity grows again. By 16:21 hrs reflectivity of 65 dBZ is present in the 1 ° elevation PPI, and a clear i n f o w notch is visible. At 16:30 hrs reflectivities in excess of 65 dBZ are evident up to 3 ° elevation PPI, and the inflow notch is also visible at that elevation. During the period 16:3016:40 hrs the storm is characterized, as seen from Penhold, by a core region in excess of 65 dBZ which is in the lower elevations, and a distinct forward notch in the lower level PPI's. A vertical cross-section through the central core and the forward notch shows that typically the "overhang" is of 8 km in length and has reflectivities in excess of 50 dBZ, at around 4 km or more in height. In the region around the notch, the return echo, though weak ( ~ 20 dBZ), is almost as great in the "orthogonal" channel as in the "main" channel (CDR ~ - 4 dB). Furthermore, the values of circular correlation are very low (approx. 20% ). Together this suggests that the scatterers were nearly randomly oriented and depolarising. Throughout the period the anvil from the storm streamed towards the northeast, with a low reflectivity region extending in two directions (as in Fig. 2 ) forming the V-shaped echo pattern. Two line echoes are seen in the S-band reflectivity PPIs during this period. At 15:50, a very weak echo is seen at around 1.3 ° elevation in the southeast quadrant at a distance of around 25 km. It is very short-lived. It is parallel to the second echo which can be seen in Fig. 2 in the northwest quadrant at around 50 km distance. This echo subsequently moved towards the southeast, passing through the radar site. It was not characterized by a noticeable change in wind. Initially it was only visible in the reflectivity data, but as it moved it developed a strong differential reflectivity characteristic (clearly apparent by 16:30 hrs). The echo can be seen up to 2 km in height, and is longlasting - - remnants of it can still be seen 40 km southeast of the radar at 18:40 hrs. In Fig. 4 we show the 1 ° elevation PPI of degree of polarization through the storm at 16:30 hrs. Notice that the ground clutter and the line echo exhibit low values of p, but notice also that on the southeastern side of the storm, p is very low. The origin of the line echo is not clear. Many have been seen previously in polarisation data from the S-band (e.g. Holt et al., 1988 ), and several have since been seen in data from Carvel. Whereas it is not impossible that they are generated by gust fronts from storms, on balance we are inclined to believe that the line echos reveal a disturbance along which the storms developed. The Doppler data from Carvel shows a weak rotational structure (radial velocity difference of about 12 m / s over a distance of 3 km) within the weak echo region, indicative o f a mesocyclone. The storm was at the maximum range of the Doppler radar. It was tracking in the direction shown in Fig. 1 at a speed of 70 km/h. (ii) Storm 2: This had a similar reflectivity structure to the first storm. It can be seen in its early stages in Fig. 2 in the northwestern corner. At 16:48 hrs it was centered on a position at azimuth 340 ° and range 72 km with respect to Penhold. One of the chase crews observing this storm from a distance of approximately 16 km observed a possible touchdown. A "funnel" was observed to drop below tree-

55-65

....... ~. i .'~fi f- "~:

' ~

/

}

/

/

/

,/

f,-+

i

2

l

""

""/q

"

/ .' /",:: 9"';~,'I" LS"~

/ '":./".. "

.... -/L.

65-7O

45-55

n

35-45

15-25

25-35

mmmmm

Reflectivity (dBZ)

4

70-90

90-94

".

+-:~ +"'. ,,\

-"

",~

......~'~ ~ llll".,

e '''~

~'~"

, >.J,IiIIar.+.-,~ ', \

,,,¢" . , " 6

m 94-100

• ~ '\.f ,,~Sw-~ +'-.Q , !~j...~I~,

0-70

m m

Degree of Polarization (%)

•

'

.w Radial V e l o c i t y [m/s] 1 7 3 0 M D T

IlOKM

Reflectivity [dBZ]1730MI)T .....

....

3¢

,)

~T76 IT2,~

tli

I

M

P

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56

45

level for 30-60 s. At this stage the high reflectivity region was composed of two parts separated by a few kilometers and oriented WSW-ENE. The one to the west reaches a height of nearly 10 km, its western edge is near vertical, but it is only about 2 km from west to east. The one to the east does not reach the same height (up to 4 km) but is much broader from east to west. The time development of the storm shows the more westerly section becoming the dominant center at low level. An "overhang" develops on the eastern edge, as in the first storm, and by 17:10 the anvil from Storm 3 has already joined Storm 2 at a height of 5 km. The 1 ° elevation PPI shows that Storm 2 has developed evidence of two low reflectivity areas, one to the north and one to the northwest. This is partially seen in Fig. 3. During 17:30-17:45 hrs, this storm passed over a chase crew positioned at range 57 km and azimuth 13 °. As it approached, at 17:23 hrs, the crew reported strong rotation visible. In the early stages of the sampling there was little rain but sparse, soft, hail, baseball sized, was experienced, as the video record shows. It is possible that the data from the radar bin vertically above the position of the chase crew is not the appropriate data to compare with the ground observations due to the effect of wind. By 17:34 the hail had reduced in size to grape size, and to pea size by 17:41 hrs. The ARC radar data show that a line echo first became evident in the S-band data around 17:20 hrs, but it had appeared earlier, by 17:15 hrs in the (ARC) Cband. It seemed to start from Storm 2, link to Storm 3, and stream out to the southwest again (cf. Fig. 3 ). At 17:23 hrs the S-band reflectivities were generally in the range 10-20 dBZ at an elevation of I km; ARC C-band reflectivities were not easily comparable since there was no scan at this height, and the beam width is wider. It is also reasonable to believe that the echo did not fill either resolution volume. Such line echoes are typical of regions of strong convergence (Wilson and Schreiber, 1986; Wilson et al., 1992). Whereas the earlier line echo was hardly noticeable as it passed over the radar site, this line echo caused an abrupt change in wind speed, and considerable turbulence. It passed through the radar site at 18:05 hrs. At 18:00 hrs the log at the airport control tower, close to the radar, records the windspeed as 8 knots ( 15 k m / h ) , whilst at 19.00 hrs the mean speed was 25 knots (46 k m / h ) with gusts of 43 knots (80 k m / h ) . The record at Red Fig. 2.1 ° elevation PPI of circular reflectivity at 15:51 hrs. Range rings are at 10 km intervals. Storms 1 and 2 are visible (Storm 2 on the right). Fig. 3. As Fig. 2, but Storms 2, 3 and 4 are seen at 17:26 hrs (right to left respectively). Fig. 4. 1 ° elevation PPI of degree of polarization, p at 16:30 MDT. Fig. 6. As Fig. 4, but at 17:24 MDT. Fig. 7. (a) Reflectivity (dBZ), and (b) radial velocity PPI of Storm 2 at 17:30 MDT, as seen from the AES radar at Carvel. Range rings at 20 km out to 100 km, and at 110 km. A indicates velocities "away from", and T indicates "towards", the radar. The turquoise and yellow storm regions in (b) indicate the rotational couplet, and the dark blue the linear convergence feature.

46

A.R. Holt et aL /Atmospheric Research 33 (1994) 37-56

Deer College, situated 8 km nort-northeast of the radar, is almost exactly the same. There was a sharp drop in dew-point temperature from 13.4°C to 6.9°C, and in dry bulb temperature from 21.9 °C to 12.6 °C between 18:00 and 19:00 hrs at the radar site. As the line echoes passed through the region in which the radar detects them, they initially had little polarization return, but finally had strong differential reflectivity. As they passed through the radar site, some long pieces of wild grass were deposited, and the radar picture is consistent with the possibility that grasses sheared o f f b y the turbulent winds were the scattering mechanism on this occasion. Certainly there was no evidence on the ground of insects. The visual impression of the line echoes approaching the radar site was of a long low roll cloud isolated from, and some distance below, the main cloud (Fig. 5 ). In Fig. 6 we show the 1 ° elevation PPI o f p at 17:24 hrs, when the storm appeared to be at maximum low level S-band reflectivity. Again, p is low to the southeast of the storm. The observations of cloud base rotation and strong convergence in the storm inflow were confirmed by the Doppler radar observations (Fig. 7). The radial velocity image shows velocity couplets as well as a linear convergence feature at the southeastern edge of the storm. This latter feature is at the same location as the low p feature from the polarization data. The velocity couplets were azimuthally offset which is also indicative of strong convergence. It should be also noted that the convergence feature is consistent in space with the extension of the line echo seen from Penhold. (iii) Storm 3: This cell was unfortunately beyond the Doppler range of the

ii~i~!~!~i~i¸¸ili~iiii~!!ii%~...... Fig. 5 Low roll cloud, associated with a line echo, seen from the ARC radar site.

'~

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

47

Carvel radar. The storm structure was similar to cells 1 and 2. As seen in Fig. 3 the line echo linked to this cell at a region of high reflectivity at a height of 1 km. At this time, there was a "hook" structure to the reflectivity structure at that height, and the link point is at the southern end of the "hook". It should be noted that this is also a region of very high reflectivity gradient. The vertical structure of the cell is of an "overhang" region of high reflectivity lying to the south of the cell. In the lower elevations the cell wall is vertical. As the storm developed, its low elevation image became corrupted by the ground clutter of the radar. A chase crew observed mesocyclonic activity at 17:46 hrs, and experienced hail up to golf ball in size. Hailshafts in this storm were visible from the radar site.

6. Analysis To date, polarization has been shown to be useful for two purposes. These are firstly the identification of regions containing hail, and secondly the measurement of rainfall during periods of high rain rate. There is evidence from this data that a third purpose for polarization may be the identification of aspects of storm structure. The cells on this day turned out to produce many reports of hail, but the accompanying rainfall was relatively light. To demonstrate the value of the polarization returns we therefore consider these purposes in turn.

6.1. Identification of hail and measurement of rainfall To illustrate the value of polarization information in identifying the presence of hail we consider firstly the time series of data at a single location together with the observations of a chase crew situated on the ground vertically below the radar bin, and secondly the plot of all radar locations in which the polarization parameters suggest hail together with the set of locations in which we have information that hail was recorded on the ground. The time series data also give information on the local rainfall. In Table l we give the values of Z, ZDR, P and Kop (increase in ~DP per km ), and rain rate derived from gDp for the period 17:30-17:45 where the chase crew was monitoring the second cell. We also give the notes of the chase crew about the precipitation being experienced. In the early part of the storm there was little rain, but then there was a time period in which hail was mixed with rain, when it is difficult to judge rain intensity by eye. We also give the total rainfall measured over the period using a wedge gauge. From the data it can be clearly seen that in the early part of the storm when there was sparse but very large hail, the reflectivity was less than 50 dBZ, but ZDR and p were very low. Reflectivity alone could not have identified the region as containing hail, but the polarization parameters are strongly indicating hail (in rain p should be greater than 75%, and ZDR should certainly be greater than 1.0 for these values of Z). As the record progresses we see that first Z increases with ZDR and p remaining small, but then Z reduces

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56

48

Table 1 Observations at 17:30-17:45 hrs; July 29, 1991. Position from Penhold: Az. 12 °, range 57 km Time (MDT)

Z

ZDR

P

KDv

Rainrate

(daZ)

(dB)

(%)

(deg/ km)

(mm/ h)

17:30:27 17:31:47 17:33:15 17:34:49 17:36:17 17:37:37 17:39:05 17:40:32 17:42:07 17:43:27

48.2 49.7 59.8 61.1 57.1 53.9 53.9 49.3 41.7 28.5

1.03 0.74 0.33 0.28 0.48 0.75 1.52 2.10 2.14 1.36

15.6 13.3 16.4 18.7 28.9 44.8 72.3 71.1 87.5 50.3

1.3 0.1 1.7 1.9 3.7 4.3 0.1 -

26 3 34 37 66 75 2 -

Ground time

Ground information

17:31 17:32

Baseball hail (sparse) Golf ball hail

17:35

Grape-size hail

17:37

Grape/pea hail and light rain

17:42 17:45

Pea hail and light rain rain stopped

KDp estimate of rainfall 6 mm. Wedge gauge rainfall 8 ram.

IUNT 1- ¢ 5-8 >8

Fig. 8. Cumulative plot of locations where S-band reflectivity > 50 dBZ during the period 15:5018:50 hrs density gives the number of times the criterion is exceeded at a given location. Data taken from l ° elevation PPI.

again to 53 dBZ and ZORand p both increase, indicative of rain. The total rainfall predicted from the radar is in quite good agreement with that collected in the wedge gauge. In Figs. 8 and 9 we plot regions in which the radar data predicts that hail may well have fallen, together with the locations of ground reports of hail. It should be stressed that only a limited number of ground reports are available, and that it is not therefore a sufficiently large enough dataset on which to draw definite conclusions. Those within 70 km have come from the network of observers, and their locations are precise. Those at a greater distance (to the east in Fig. 9 ) have

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56

49

Fig. 9. As Fig. 8, but locations where Z > 50 dBZ, ZDR~<1.2 dB, p~< 30% simultaneously. Also shown are locations at which hail was reported on the ground. See text for symbols.

come from insurance claims, and only a limited land location is available; location is only known to a given 9.6 by 9.6 km square. These reports should not therefore be regarded as precise. In Fig. 8 we plot regions in which the S-band reflectivity values exceed 50 dBZ. In Fig. 9 we plot the regions in which the following conditions are all fulfilled: - Z>_-50 dBZ, ZDR ~<0.8 dB, p < 30%. For each of Figs. 8 and 9 we have run through the whole S-band radar database at 1 ° elevation and noted the number of times that the criteria are fulfilled in each location; the density of the plot gives this number. In Fig. 9 we also plot the locations at which hail was reported, giving the maximum size at each location ( × =shot, or pea, ~ =grape, or walnut, + =golfball, or larger). Almost all the hail reports occur at locations at which the criteria for Fig. 9 are satisfied. It will, however, be noted that in these storms the regions of maximum reflectivity are not significantly different from those in which polarization parameters are also invoked (cf. Figs. 8 and 9 ). The greatest differences are to be seen in the swath corresponding to Storm 3. This is consistent with the fact that the volunteers' raingauges did not collect much rain during the passage of the storms - - the maximum was 6 mm from Storm 3. There is virtually no trace of Storm 4 in Fig. 9. This may be partly due to the fact that at that distance from the radar the beam was sampling higher in the storm, but it should be noted that when this storm passed over a chase crew, the precipitation was almost entirely rain. Also indicated in Table 2 is the identification of a hail spike signature in Storm 2 (Fig. 10). Several other storms on this day showed hail spikes. As suggested by Wilson and Reum ( 1988 ) this could be an artifact of three-body scattering where the part of the energy scattered from the hail storm strikes the underlying wet ground, reflects back up to strike the hail core, and back to the radar to produce a spike-like signature on the far side of the storm. However, the hail spike shown in Fig. 10 has an extension of more than 30 km in range which cannot be easily

1.53 1.43 1.33 1.24 1.14 1.09 1.14 1.13 1.14 1.18 1.24 1.33 1.43

0.5 ° scan

Height (km)

5 5 4 4 5

10 15 12 10 5

24 30 30 24

36 48 40 40 40

D (km)

28

AV (m/s)

3.6 3.2 3.3 4.0 8.0

4.8 7.5 7.5 4.8

5.6

90 180 120 100 50

30 30 30 30

35

Shear p (m/s/km) (m/skm)

3.36 3.17 2.99 2.81 2.63 2.52 2.63 2.59 2.63 2.70 2.81 2.99 3.17

Height (kin)

D (km)

30 30 28 28 16 16 30 30 30 48 40 36 22

5 5 4 4 5 10 10 6 8 10 10 10 12

1.5 ° scan

AV (m/s)

6.0 6.0 7.0 7.0 3.2 1.6 3.0 5.0 3.8 4.8 4.0 3.6 1.8

38 38 28 28 20 40 75 45 60 120 100 90 66

Shear p (m/s/km) (m/skm)

hs hs hs hs hs+ hs+ hs+ hs+ hs+ hshshs+ hsi

l

t t t RM t t

Commentsa

aComments: hs: hail spike signature ( < 10 km long); h s - : weak signature (maybe); hs+: well developed signature ( > 10 km long); hsi: hail spike intermittent long signature; t also observed on echo top map; RM: right mover.

12 12 10 10 10 12 14 14 12 12 14 14 14

226 224 220 216 212 208 200 192 186 182 178 172 166

15:30 15:40 15:50 16:00 16:10 16:20 16:30 16:40 16:50 17:00 17:10 17:20 17:30

105 100 95 90 85 82 85 84 85 87 90 95 100

Azim. TOPS (deg) (km)

Time R (MDT) (kin)

Table 2 Mesocyclone analysis of Storm 2 Carvel Doppler Radar - - 15:30 MDT to 17:30 MDT

o~

e~

e~

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56

....... ,:.iii~s~. ~i: '

i iii 3

iiiiil

51

iii

iilili

I

64 :32

£1 " 25

MAX R mm/hr 1640MDT Fig. 10. Data from Carvel showingmaximum rainrate in mm/h, derived from reflectivity. Storm 1 is to be seen in the center, with Storm 2 towards the left. Storm 3 is on the left-hand edge. Range rings at 40 km. justified by the three-body scattering hypothesis alone. Comments in Table 2 indicate that the hail spike took on several forms in the radar imagery. The significance to this paper is that from 15:30 to 17:30 MDT, a spike signature could be observed with Storm 2, qualitatively indicating the hail bearing nature of this particular storm, which supports the polarization classification results. 6.2. S t o r m structure

We shall consider particularly Storm 2, since its development was monitored for some long period by both radars, and especially since it was within the Doppler range of Carvel whilst at its peak. Fig. 11 shows the track of Storm 2 moving to the right at the time of maximum development, and at the time of the possible touchdown of the funnel cloud. The storm was clearly mesocyclonic. Fig. 12 and Table 2 show the time evolution of the mesocyclone from the Doppler data. The intensity of the mesocyclone is described by two parameters, shear and momentum (Zrnic et al., 1985). Shear is defined as the maximum velocity difference across the velocity couplet divided by the distance between them, and momentum is the product of these two quantities. The time history appears to show two regimes separated at about 16:20 M D T when the storm became a right mover. Initially, the storm exhibited low momentum and high shear, and then at 16:20

52

A.R. Holt et al. /Atmospheric Research 33 (1994) 37-56 Storm 2 Track -60

~ -70

1

5

3

0

~

~

A -80

630MDT

-9(3

I

-100 -100

I

-75

I

I

-50 -25 WEST-> EASTIkm]

0

25

Fig. 11. Detailed track of Storm 2 derived from Carvel data.

Storm 2 150

i m ~10(

i

0 15.5

I

I

I

16,0

16.5 TIME [MDT]

17.0

17.5

12

15.5

' 16.0

I

16.5 TIME [MDT]

I

17.0

17.5

Mesocyclone characteristics at 3 kln level.

Fig. 12. Time plot of momentum and shear for Storm 2. Time scale in hrs MDT.

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

53

MDT the momentum began to increase and the shear to decrease. This temporal evolution seems to contradict the pattern found elsewhere (Joe and Crozier, 1988; Burgess et al., 1988) for tornado producing storms. The difference appears to originate from the re-organization of the storm. As the storm became a right mover, enhanced convergence could be interpreted from the degree of polarization and radial velocity data. Coincident with this enhanced convergence, the mesocyclone appeared to be better organized, to intensify, and to enlarge in size (Table 2 ). This is reflected in larger momentum and the increase in velocity difference is insufficient to offset the increase in size, and therefore the shear diminishes. The low degree of polarization areas within the storms appears to be the storm inflow areas, where, air converges to form the main updraft of the storm. It is possible that this air contains randomly oriented grasses, such as were deposited on the ground at the radar site after the passage of the line echo through it. It is important to note that not all low reflectivity areas have low p. However, low p ( < 70%) is generally found only in areas with Z < 45 dBZ. Thus, the low p information is providing us with different information about storm structure and hence, perhaps, storm development, which perhaps may be used for severe storm foreI

I

JUL 29 1991 T]ME 16:35:Q AZ 30 DEG RANGE 55 Krn AZ 46 DE¢ RANGE55 Km CORRECTED

ZC (dBZ) :::~:: < 35 i!!i!~!! 35-55

II ss-6s

•

>65

DEG OF POL ....::: 0-70% i lii:i

iLiiiiii!li

iliiii 70-90%

II~ 9o-%% •

94-100~

Fig. 13. Vertical cross-section through Storm 2 at 16:35 M D T , showing both S-band reflectivityand degree of polarization.

54

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

casting. This is illustrated in Fig. 13, which is a vertical cross-section through Storm 1 at 16:35 MDT, showing the reflectivity and degree of polarization structure. The low p ( < 70%) area can be seen within the weak echo region (WER) of the storm underneath an overhang defined by the 55 dBZ echo. Several minutes later at 16:42 MDT (Fig. 14) the overhang is still quite prominent but the low p area is diminished. Other observations (not shown) show that while the echo overhang and the low degree of polarization region are related, they evolve differently - - the low p region collapses before the collapse of the weak echo vault. The low p parameter is providing information about the structure of the inflow region, as opposed to the area of strong reflectivity, which is a new way of looking at storm structure. This interpretation is consistent with the Doppler information from Carvel (Fig. 7). The radial velocity imagery shows a linear area of toward (Carvel) radial velocities in the inflow region of the storm, which is indicative of a convergence region, at the same location as low p from the Penhold radar. The single Doppler observation of this convergence region is dependent on the viewing angle, and can have an ambiguous interpretation since it is only the radial component of the true velocity. On the other hand, the low p region appears to have a clear interpretation. Observations of a time sequence ofp images of Storm 2 indicate an association JUL 29 1991 T]ME 16:4-2:1t AZ 51 DEGRANGE60 kr~ AZ 48 ~O RANGE 60 Km CORRECTED

zc (d~Z) if::

< 55

iiii!i!iiii 55-55 55-65

• ]

>65 DEGOF POL 0-70%

iii;ili 70-90%

90-94%

•

Fig. 14. As Fig. 13, but at 16:42 MDT.

9#-100%

A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

55

with the development of the reflectivity pattern. Storm 2 was an isolated single cell storm. A classical V pattern in reflectivity developed at about 17:00 MDT, indicative of obstacle flow around the prime storm center. After about 17:40 MDT, the V-shaped pattern began to collapse. Therefore the storm reached its peak around 17:20 MDT. Early in the evolution of the storm, at low elevation angles, the low p area began to form at the southernmost tip of the echo pattern ( 16:32 MDT). At this time, the values ofp were generally > 94% in most of the storm and > 70% at the southern tip. As time developed, the low p area became entrenched by high p as the hook pattern developed. At about 17:21 MDT, values ofp dropped below 70%, which approximately corresponds to the time of peak development, and were present until about 17:37 MDT.

7. Conclusions In this paper we have reported on joint S-band polarization and C-band Doppler observations of a series of mesocyclonic storms in central Alberta. Comparison of the two data sets has shown that the polarization data, through the parameter Degree of polarization, has revealed aspects of storm structure in differentiating the inflow region. Since the polarization scanning is routine, and does not require slow scanning, this suggests that it may have some advantages in providing a more frequent time series of some aspects of storm development than is available through Doppler monitoring. It should be noted that the Penhold radar is capable of scanning up to 8 ° elevation in a scan cycle of 90 s. We would suggest that this work shows that an operational test of degree of polarization is warranted. One question of further interest is whether the degree of polarization information can be related to threshold wind speeds in the inflow region. If so, this could have a major impact on forecasting because of the promising potential use of helicity for severe storm classification.

Acknowledgements This research was crucially dependent on the support of C. Richmond of the Environmental Research and Engineering group at the Alberta Research Council. We thank him, and many other members of the group for their support, and also Dr. R. Charlton of the University of Alberta. Dr. R.G. Humphries gave us some of his own time to check the calibration of the radar. One of us (A.R. Holt) received support from the Royal Society of London and NSERC, Canada. We are also grateful for assistance from the Atmospheric Environment Service, Edmonton; Red Deer College; Alberta Environment; and Alberta Hail and Crop Insurance.

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A.R. Holt et al. / Atmospheric Research 33 (1994) 37-56

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