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<br />SEPTEMBER 1988 <br /> <br />MARK F. HEGGLI AND ROBERT M. RAUBER <br /> <br />999 <br /> <br />storms in this synoptic category, only three had com- <br />plete radiometric datasets. Supercooled water was <br />present in distinct regions of these three storms. The <br />primary region was within the orographic cloud system, <br />which remained after the onset of upper tropospheric <br />subsidence associated with the passage of the upper- <br />level jet. This event occurred 0 to 4 hours before the <br />surface frontal passage. In all three storms, increases <br />in supercooled water similar to the 26-29 March case <br />were observed, implying decreased precipitation effi- <br />ciency, which may be attributed to unfavorable ice nu- <br />cleation conditions. The duration of supercooled water <br />events varied from 24 to 45 hours. Sustained super- <br />cooled water (0.30-0.60 mm with peaks to 1.10 mm) <br />was typically present for 7-10 hours. In all cases, the <br />radiometer trace was suggestive of embedded convec- <br />tion. The second region where supercooled water was <br />observed was during the passage ofthe warm front. In <br />general, supercooled water measurements varied be- <br />tween 0.05 and 0.15 mm during warm frontal passage. <br />The supercooled water measurements were generally <br />steady, suggesting that the water was produced by more <br />uniform lifting rather than by convection. More sub- <br />stantial supercooled water measurements (0.40-0.60 <br />mm) were recorded for a period of6 h in one case (7- <br />10 February 1985). A third region where supercooled <br />water was sometimes observed was near the end of the <br />storm as isolated convective cells moved over the ra- <br />diometer site. These cells frequently produced isolated <br />peaks to 0.50 mm. <br /> <br />b. Moderate amplitude short wave associated with an <br />occluded storm: 23-25 February 1984 <br /> <br />The 23-25 February 1984 storm system was an oc- <br />cluded storm, its center located off the coast of central <br />Washington. Figure 7 shows the 1200 UTC 24 Feb- <br />ruary 500 mb chart and a satellite photograph taken <br />at 1215. Time sections of rawinsonde data taken at <br />Sheridan during the storm period are shown in Fig. 8. <br />Figure 9 shows the radiometric measurements and <br />precipitation measurements at Kingvale during the <br />storm system passage. Precipitation occurred in asso- <br />ciation with a cold frontal system, which extended <br />southward through the project area. <br />A closed circulation center at 500 mb was present <br />over the East Pacific near Washington at 1200 UTC <br />24 February. A deep 984 mb surface low was centered <br />off the coast. The cloud field associated with this system <br />wrapped around the center of circulation, a pattern <br />typical of many deep occlusions. <br />The thermal structure and moisture fields associated <br />with the cold frontal system on 24 February were very <br />similar to the split-front model discussed by Browning <br />and Monk (see Reynolds 1986). The storm system was <br />preceded by a wide cirrus shield. The cirrus clouds were <br />located on the anticyclonic side of the jet, and the back <br /> <br />edge of the cirrus shield was aligned very closely with <br />the jet core in the upper troposphere. An upper-level <br />baroclinic zone was present near the jet core. This zone <br />marked the boundary of subsiding air on the cyclonic <br />side of the jet. Low-level moisture was first observed <br />to move into the lower Sierra foothills around 1200, <br />approximately 8 h ahead of the surface front. The top <br />of this moist layer was confined by the subsidence in- <br />version. Based on satellite photographs and rawinsonde <br />data, the passage of the jet core and onset of low-level <br />moisture occurred at approximately 1500 at Kingvale, <br />in association with the formulation of an orographic <br />cloud. Weak embedded convection occurred within the <br />system associated with weak convective instability near <br />cloud top (Huggins et al. 1986). Supercooled water <br />measurements had a steady background value of 0.05 <br />mm with peaks from 0.20 to 0.50 mm. Snowfall was <br />steady at 2 mm h -I for approximately 5 h. The cloud <br />system began to dissipate over Kingvale at 2030. The <br />surface front passed Sheridan at 2000 and Kingvale at <br />2300. At 2230, a weak cold frontal rainband associated <br />with the surface front moved through Kingvale. Pre- <br />cipitation rates increased to 4 mm h-I. The cloud sys- <br />tem became much more convective, and convective <br />cells continued to develop for several hours, producing <br />rain and snow showers through the Sierra Nevada. <br />Supercooled water increased substantially after the <br />passage of the cold-frontal rainband. The radiometric <br />measurements contained many peaks typical of con- <br />vective clouds. By 0330 UTC 25 February, only a shal- <br />low nonprecipitating orographic cloud remained. This <br />cloud system remained over the Sierra Nevada for <br />nearly 12 h before dissipating. Supercooled water was <br />present continuously in this cloud in quantities from <br />0.20 to 0.50 mm. Satellite photographs and radiometric <br />data suggested that convection weakened in intensity <br />with time and that an orographic cloud remained over <br />the Sil:rra Nevada crest. <br />Discussion. The 23-25 February storm was one of <br />12 storm systems in which the center of the storm was <br />occluded, near the coast, and north of 420 latitude. At <br />the latitude of the project area, the storms occurred in <br />association with the passage of a cold front. The du- <br />ration of precipitation at higher altitudes was generally <br />10-15 h, although cloud cover was often present for <br />24 to 48 hours. In general, the cold front was preceded <br />by a wide cirrus shield. The back edge of the cirrus <br />shield was often, but not always, aligned with the upper <br />troposphere jet axis. Depending on the evolution of <br />the storm, this axis and the cirrus boundary were either <br />aligned with or east of the surface cold front. The latter <br />case was more common. The cloud systems associated <br />with the fronts followed a typical evolution: Cirrus first <br />moved over the region. Low-level clouds developed <br />rapidly and precipitation began to fall as the front ap- <br />proached. A very distinct cold frontal rainband moved <br />through the Sierra Nevada within a few hours after the <br />onset of precipitation. After the rainband passed, pre- <br />