Laserfiche WebLink
<br />l48 <br /> <br />JOURNAL OF APPLIED METEOROLOGY <br /> <br />VOLUME 27 <br /> <br /> <br />b. 24-25 March 1985 <br /> <br />The stI'Jlcture and organization of this precipitation <br />event was in rnost regards considerably different than <br />the 7-8 February storm. The predorninate forcing <br />mechanism for this case was a narrbw, slow moving <br />cold katafront and accompanying upper-level cold <br />surge interacting with a short-wave trough aloft pro- <br />viding several hours of fairly light precipitation. No- <br />ticeably lacking was any prefrontal warm overrunning <br />precipitation which provided the bulk of the precipi- <br />tation for 7-8 February. <br />The best description of this precipitation event is <br />through comparison of the SHR time-height profile <br />(Fig. 7a) with the satellite sequence shown in Fig. 8 <br />and the radar sequence shown in Fig. 9. The time- <br />height fie plot and the 1831 satellite image shows that <br />a very distinct cloud and moisture boundary marked <br />the leading edge of this event. Moisture moving in over <br />the barrier produced several hours of mid- and low- <br />level cloudiness containing SL W above background <br />levels before the main frontal precipitation began. The <br />frontal precipitation can be seen over the barrier from <br /> <br />24- 25 MAR 1985 <br /> <br />II <br /> <br />10 <br /> <br />325 <br />9 320 <br />315 <br />8 310 <br /> <br />both the radar time-,height produced over Kingvale <br />(Fig. 7c), Kingvale precipitation data (Fig. 7d) and the <br />radar PPI and RHI (Fig. 9). From these data it can be <br />observed that the ice crystals were originating from the <br />mid- and upper-levels of the' cloud, and then falling <br />into the low-level liquid water zone, suggesting a <br />seeder-feeder mechanism. This was verified by the <br />predominance oflightly rimed dendrites and dendritic <br />aggregates falling at Kingvale during this period. <br />This frontal precipitation stage ended with passage <br />of an upper-level cold surge associated with an upper- <br />level jetstreak, followed by the low-level cold katafront. <br />No accompanying low-level jet was noted for this case. <br />The 1730 PPI/RHI in Fig. 9 shows the upper cold surge <br />located approximately 15 to 20 km east of Sheridan <br />with the low~level cold katafront (a front could only <br />be defined above 1 krn MSL) just having passed Sher- <br />idan. These same two features passed Kingvale at 2000 <br />and 2100, respectively. The mountaintop weather sta- <br />tion at CIS (Fig. 1) indicated a wind shift at 2100. Pre- <br />cipitation rates were reduced to about 1-2 mm h-1 at <br />Kingvale after this time with only a slight recovery in <br />cloud SL W. Ice crystal habits at Kingvale showed a <br /> <br />1 <br /> <br />320 <br /> <br />c 7 315 <br />D <br />~ 6 <br />VJ <br />e- 310 <br />0-' <br />:~ 5 (A) <br />-g,e <br />'ii~ 295 <br />'" 4 <br />,- <br />"", <br />E .~ 290 <br />i=~ 3 <br />a>" <br /> <br />.05 .16 27 .46 .22 .57 1.46 3.83 4.12 1.86 .24 <br /> <br /> <br />;JJ~~J~~...~ <br /> <br />(8) <br /> <br />dBZ <br />010-19 <br />II 20.24 <br /> <br />~r!El:~D:Q_ _~a_~! _': :Q~ <br /> <br />(Cl <br /> <br />I . . I . . . . <br />; - - - - - : - - - - -: -. . - - ~ - - - - -: - - - - - : - - - ~ - : - - - - - : - - - - - <br />----- ~.....~-- -- -t----. ~ .---- ~-----~-- - -.:. ..-- <br />. . I . . . . <br />- - -....- - - ....... - - - t- - - - - r - - - - - r - - - - - ,- - -. -. - - - -- <br />., .... <br /> <br />(D) <br /> <br />15 12 09 06 03 00 21 18 15 12 09 <br />TIME (UTC) <br />FIG. 7. As in Fig. 2 but for 24-25 March 1985. <br />