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1292 POST-FRONTAL LIQUID WATER PRE-FRONTAL LIQUID WATER (WEST TO SOUTHWESTERLY FLOW ALOFT) (NORTHWEST TO NORTHERLY FLOW ALOFT) t&:& &&& t&&& &&& ~ &&& I ........ &&& t&&& &&& & & & & & ~~::& & & & & & ~........: ~&& & & & ~&&& & & FIG. 2. Schematic depiction of low-level windflow with respect to the cold front for (a) storms traveling in a southwesterly flow aloft with liquid water occurring post-frontally and (b) storms traveling in a northwest to northerly flow aloft with liquid water occurring pre- frontally. SPLIT-FRONT MODEL Km. 6 3 o FIG. 3. Conceptual model of a split front and indicated regions of seeding opportunity. (After Browning and Monk 1982). in SLW (determined from radiometric measurements) associ- ated with a corresponding drop in cloud top, a decrease in radar reflectivity, and a decrease in precipitation rate at this same location. Competing processes can occur within shallow orographic cloud systems. Release of latent instability by forced ascent can develop pockets of natural ice. Strong evaporative cooling near cloud top induced by mixing can lead to natural areas of high concentrations of ice crystals. In addition, regions of co- alescence-grown drops can lead to rapid glaciation by the Hal- let-Mossop (1974) process. These processes together lead to the high variability of SLW and ice observed in these cloud types. For storms moving in from a more northerly direction (me- ridional flow), SLW generally occurs prefrontally. Here the winds are more perpendicular to the barrier, causing an in- creased low-level condensate supply and increased SLW. Post- frontally, the winds veer to a northerly direction or "parallel" to the mountain (see figure 2b). When SLW exists it is necessary to know its amounts, du- ration, and its vertical and horizontal extent. Aircraft obser- , """"~- Vol. 69, No.1 1. November 1988 26- 27 MAR 1985 330 II~____~---- ------ .---------.--TiJ~ 10 ---------~ /----~- ---,--------~---- 320 (A) .. ~ ~d ;:::E 5 f~ "~ .~ .~ ~'" "," ~--~_ . ._/290 01 I --'-~I I lill!:" i ;1,1 ,." ", .n >0' '" r I"' 0.0 -,-ii, ~,lJW~L,<\_'__,-uJ~~ E 6.0.. ~'~ i ~ ~ 4 0 _ 0 ~~.~9 _n_ "- ~~ ~ . _ :~~:~: _ j)~_ - _~ ~ -. ~ ~_o_Q ~ra~ ~~020-,-, , ,"3~2~, , , HI : ~lrlIGI1'-~:~~I~)llllllllill_ljlfJ"'m'T~ I~_'_I_ (D) 15 12 09 06 03 00 21 18 15 12 09 T I ME (UTe) (e) FIG. 4. Composite temporal analysis for the 26-27 March storm crossing the central Sierra Nevada (time increases to the left), from top to bottom; (A) 8, time-height from the Sheridan rawinsonde. Fea- tures noted, WF (warm front), CF (cold front), J (jet maxima), and scalloped line indicates cloud boundaries as estimated using relative humidity, satellite cloud top temperatures, and the "Sheridan-radar" cloud tops. Computed 2500 condensate supply rate integrated from surface to 5 km over Sheridan is shown at the time of the soundings. (B) Vertically integrated liquid water from the dual-channel microwave radiometer at Kingvale. (C) Time-height radar-reflectivity profile over Kingvale as determined by the Sheridan radar. Minimum detectable signal over Kingvale is 8 dBZ. (D) Precipitation rate (mm . h-I) measured by a weighing-type precipitation gauge at Kingvale (Reynolds and Kuciauskas ] 988). 'lations have been used extensively to determine the spatial variability of SLW, but only to within 500 to 1000 m of the underlying terrain. The application of the radiometer within various mountain locations allows determination of the tem- poral variability of SLW and the direct comparison among different geographical locations of integrated SLW observed from cloud base through cloud top. Figure 5 shows the cu- mulative distribution of SLW observed at three separate geo- graphic locations.' Note that approximately 85 percent of the time SLW is observed, it has a value below 0.2 mm. Using a 2 For this graph, data availability varied depending on the project. For SCPP, 775 hours of SLW observations were available from four seasons, ]983-84 through ]986-87. for CRADP, 958 hours of ob- servations were available for the period November through December ]983 and January through March ]985. For Utah, 7]9 hours of ob- servations were used for the period February through March] 985. This distribution is similar to one produced using ] 79 hours of radiometer data from the ]983 Utah field project.