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FEBRUARY 1988 DA VID W. REYNOLDS AND ARUNAS P. KUCIAUSKAS 143 TABLE 1. Key instrumentation utilized. Measurement Location Temporal frequency Integrated water vapor (em) KGV 24 h day-I Integrated liquid water (mm) 2 min avg Reflectivity to 125 km range SHR 24 h day-I 15 or 5 min volume scans Reflectivity (dBZ) Cool, CA During storms Radial velocity (m S-I) Temperature, moisture SHR 24 h day-SHR Wind 1000-100 mb BLU 12 h day-BLU, KGV KGV 3 h intervals during storms Daylight visible (hardcopy) AUB , 24 h day-I Day-night infrared (hardcopy) Forecast Office 30 min interval Precipitation rate (mm h-I) ARB 24 h day-I 5 min interval Temperature, humidity, windspeed, CIS (2020 m) 24 h day-I direction SQP (2675 m) 5 min interval Instrument Dual-channel microwave radiometer 5 (im radar NCAR CP-4, 5-cm Doppler radar Rawinsonde Satellite imagery Weighing type Precipitation gauges (30) Remote mountaintop weather stations the constructed radar time-height profile over Kingvale (Fig. 2c) shows only weak e9ho and Kingvale precipi- tation rates were under 2 mm h -I. Kingvale precipi- tation rates will be used in this paper as they more closely represent cloud precipitation processes that would affect radiometer observed SL W. The second stage of the precipitation event occurred between 1800 on the seventh and 0000 on the eighth. It began with the passage of a well-defined upper jet streak accompanying an upper-level cold surge/hu- rnidity front similar to what was described by Hobbs (1978) for precipitation systems over the Cascades. The passage of this feature had a distinct impact on the precipitation process. It is hypothesized that the cir- culation associated with the upper jet streak produced the subsidence field on the cyclonic shear side of the jet. The low-level jet of 25m S-I observed over Sheridan at 1800 was associated with the indirect circulation of , the upper jet (Uccellini and Johnson, 1979). There is a rather distinct warming of cloud tops (as seen in the 2130 satellite image in Fig. 3) in the vicinity of the barrier. This corresponds to the drying aloft be- hind theupperjet. The association between the upper jet streak and discontinuity in CTT has been observed and documented by Oliver et al. (1964) and Weldon (1979). The low-level jet associated with a zone oflow- level convergence provides the low-level forcing pro- ducing the rainband that was observed to pass the Au- burn precipitation gauge site at 1600 (Fig. 4). This band of heavier precipitation could not be tracked up the barrier either in the precipitation data or radar data because the band merged with the enhanced orographic cloud and began to dissipate with the loss of low-level convergence over the barrier. Several other significant features were noted to occur during the passage of the upper jet. First, drying aloft temporarily removed the upper cloud deck which had been acting as a natural seeder cloud. Second, the sur- face to 5 krn CSR increased as low-level winds turned perpendicular to the barrier behind the upper front. Third, aOe/az (hereby referred to as potential insta- bility) showed decreased stability in the region between 3 and 6 km induced by the drying aloft. This desta- bilization aided in producing embedded convection and increasing the local condensate supply rate. These three factors lead to a recovery of the SL W within the cloud even though precipitation rates were also in- creasing. This increase in SLW was directly observed by examination of the snow crystals falling at Kingvale and by the project aircraft which were conducting a randornized "no-seed" experiment at this time. Crystals at Kingvale showed a distinct increase in riming along with the onset of large frozen water drops up to 500 JLm. Project aircraft reported severe degradation of air- craft performance due to these large drops accreting to the underside of the wings (Sand et a1., 1984). The third stage of this storm occurred several hours after passage of the upper jet streak and upper-level cold surge/hurnidity front. At this time a deep, stable, warm overrunning precipitation period began. This third stage produced the bulk of the precipitation from this event. Key points to note here are the high CSR (almost double those from 0900-1800 on the seventh), decreased SLW, deep and extensive echo over King- vale, and moderate precipitation rates of 5-8 mrn h-I with indications of embedded band features yielding the highest precipitation rates such as at 0600 and 1500. Another important feature noted was the presence of