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1002 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 II: 1.0 "'- t-e ~.! 0%0,5 ~ :it: 2~ ~I ~. ..J 0,0 -- Z 4 ~ t!~ ire 2 2- g: 0 12 09 06 03 (ff25) TIME (UTe) 18 15 21 12 09 (2/24) FIG. 9. As in Fig. 6, but for 0900 UTC 24 February 1984 to 1500 UTC 25 February 1984. rier-perpendicular orientation. However, in some cases, this effect was not as important because it was accom- panied by a reduction in wind speed. c. Split flow in the middle troposphere associating with a dissipating cyclonic storm: 22-23 December 1986 The 22-23 December storm was a weak system that produced only 16 mm of precipitation at Kingvale. The storm was associated with the passage of a weak cold front. Figure 10 shows an infrared satellite pho- tograph of the storm for 1300 UTC 22 December and the associated 500 mb chart at 1200. Time-height cross sections of the storm from Lincoln are shown in Fig. 11. Figure 12 shows a time section of rawinsonde data and the radiometric and precipitation data from King- vale during the storm, as well as icing rate data. The flow at 500 mb at 1200 was split, with the major belt of the westerlies moving northward over a ridge centered near the western United States. An occluded storm, with rapidly rising central low pressure, was lo- cated at 1350 W longitude off the coast of British Co- lumbia. A weak and shallow cold front extended southward through the latitude of the project area. A wide region of thin cirrus was present over California and Nevada ahead of the storm. No distinct upper- level subsidence region was evident during the storm passage. The jet stream remained north of the project area. Prior to the arrival of the cold front, a midlevel stra- tus deck, 1.5-2.0 km thick, moved over the Sierra. This deck appeared to be composed of supercooled water, based on the radiometric measurements and the ab- sence of precipitation. The surface front arrived at 1400 at Lincoln and at 1700 at Kingvale. It was marked by a single, narrow-frontal rainband. Low-level relative humidity increased substantially behind the front. After the band passed through the Sierra Nevada, an oro- graphic cloud remained. This cloud produced minor amounts of precipitation for several hours. Supercooled water was present in the storm throughout its existence: it was greatest (0.70-1.00 mm) in the leading midlevel stratus and least (0.15-0.50 mm) in the frontal band. Supercooled water was 0.20 to 0.50 mm continuously for 11 h in the postfrontal orographic cloud. Icing rate data from SQP and SIG confirmed that supercooled water was present near the crest from the onset of the cold frontal rainband until dissipation of the orographic cloud. Discussion. The 22 December 1986 storm was one of 12 storms that impacted the Sierra during the dis- sipation stages of weakening cyclones. These storms had many common characteristics. Clouds and pre- cipitation were associated with the passage of weak cold fronts. The storms all moved into a mean long-wave ridge located over the western United States. Split flow at middle and upper levels of the atmosphere was com- mon. The parent occluded-circulation centers asso- ciated with the cold fronts were typically located at 500 latitude or greater and were dissipating. The storms all occurred during extremely dry months, based on an 89-year climatology. No significant precipitation fell prior to the passage of the cold front in any storm except on 18 December 1986, when light (0.5-1.0 mm h-I) precipitation fell for several hours prior to cold frontal passage. Maxi- mum precipitation rates in all storms (2-6 mm h -I) were associated with the cold frontal passage. After cold frontal passage, orographic clouds typically formed. Embedded convection may have been present within these clouds, but it did not appear to be very important to the precipitation processes, as evidenced by steady precipitation rates (typically storm duration, based on measurable precipitation at Kingvale or the presence of supercooled water-bearing clouds, ranged from 12 to 24 hours. Complete radiometer datasets were available for five storms in this group. In all five storms, supercooled water was present in the clouds throughout the lifetime of the storms. The highest values were typically ob- served in the prefrontal midlevel stratus present in three storms. Values there ranged from 0.05 to 0.60 mm, depending on the storm. The duration of values of this magnitude ranged from 3 to 13 h. Supercooled water values generally ranged from 0.10 to 0.40 mm within the frontal bands and postfrontal orographic clouds. The postfrontal period lasted from 6 to 19 h depending on the storm. Small peaks were present in some cases, . .