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DA V1D W. REYNOLDS AND ARUNAS P. KUCIAUSKAS 145 :ISCPP 15MI7~TI.: .:..:. .:.. ~. .:.. ::~.:..:~. 24 2i 20 18 16 14 12 10 8 6 4 2 Om 2i m20 18 16 14 12 FEBRUARY 1988 .c.... .. . :. '. n: t...J ......-:..: (: "".. ~...:. I. . FIG. 3. One km resolution visible satellite image from the GOES- W satellite for 2131 UTC 7 February 1985. Enhanced infrared data replaces the visible data for CTT's colder than - 30oC. Gray-scale enhancement is shown at top of image. This second overrunning period is followed by pas- sage of a complex cold front consisting of an upper cold surgeJhurnidity front and a surface front having cold katafront characteristics in all but the lowest layers. Ragette (1984) described a storm over central Europe having very similar frontal characteristics and at- tempted, through use of serial soundings, to derive ver- tical motion fields associated with the frontal bound- aries. This technique showed strong subsidence behind the upper cold surge and above the surface cold front down to about 3 km MSL, similar to what would be inferred here from the rapid drying aloft shown in Fig. 2a. The radar PPI sequence shown in Fig. 5a illustrates the passage of the cold-frontal rainband with a well- defined back edge of echoing cloud approaching the radar at 1730 in close proximity to the upper-level cold 9: f ~ & t= ~ R' surge. By 1845 the back edge of this band was posi- tioned near Kingvale. The vertical cross section shown in Fig. 5b, corresponding to the short line segment shown in Fig. 5a, depicts the main band of precipitation as a stratiform echo with a well-defined back edge and tops generally 5 to 6 km.Behind the band, widely scat- tered echo tops were briefly seen to extend to only 3 to 4 km. Figure 2b shows that SL W increased with the band as strong vertical motions existed in the band's leading edge much like Hobbs (1978) describes. The SL W remained substantially above background levels in the shallow convectively enhanced orographic cloud remaining over the barrier until 2200, when rnixing of the drier air aloft eroded the cloud into individual con- vective elements. The satellite sequence in Fig. 6 shows the relationship between the CTT field and the position of the surface cold front. These can be compared to Fig. 2 and Fig. 5. The enhanced infrared image taken at 1500 shows the very cold CTT associated with the increased pre- cipitation rate between 1300 and 1700 at Kingvale. By 1700 the higher clouds passed Kingvale, with precip- itation decreasing slightly and SL W increasing slightly. The back edge of the upper cloud can best be associated with the upper-level jet which can, at times, be located as much as 60 to 120 n mi into the clear region or cyclonic shear side of the jet (Oliver et ai., 1964). Pre- cipitation rates increased again as the main rainband passed Kingvale between 1700 and 2000. The cold front appeared to lose speed from crossing the Sierra Nevada. This may, in part, be due to the barrier in- duced southerly jet and surface drag. This barrier effect may have contributed to splitting of the upper and lower cold front for this event. In summary, similarities have been found in the structure and organization of this precipitation event to Browning's (1985) split-front model and to rneso- scale features observed by Hobbs (1978) in the Pacific Northwest. Satellite and radar features are shown to be associated with upper-level wind maxima, their as- sociated vertical circulations, and frontal boundaries. These transitions have a significant impact on the cloud's precipitation process, and, in turn, lead to spe- cific increases or decreases in cloud SLW. Increases or higher values in SLW are associated with either gen- erally lower precipitation rates and decreasing cloud depth or an increasing CSR. rEB 7,1985 10 8 HOUR (UTe) FIG. 4. Precipitation rate as measured by the Auburn weighing gauge for 7-8 February 1985.