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'i ~~r ~ Reprinted from Preprint Vol ume: Conference on Si('rr,l Nevada Meteorology. June 19-21, 19 7R. S, Lake TAhoe. Calif. Published by American Meteorological Socit'ty, Boston, Mass. A MICROPHYSICS AND SYNOPTIC STUDY OF THE 14-15 DECEMBER 1977 SIERRA STORM James A. Moore U. S. Bureau of Reclamation Denver, CO 1. INTRODUCTION Radar. aircraft. and ground instrumentation were operated in the American River Basin of the North-Central Sierra Nevada during the winter of 1977-78 to investigate the micro- physics. dynamics, and transport and diffu~ioln characteristics of Sierra storms for the Slerra Cooperative Pilot Project (SCPP). The 14-15 December 1977 storm was selected for intensivle study because of its meteorological interest and because the instrumentation in this early peri od of the fi e 1 d effort operated well throlJgh most of the storm. Case studies of other storms from this field season with additional instrumentation will be selected. analyzed. and reported in the future. The synoptic situation on 14-15 December, 197:7 was characterized by a storm system which movl~d inland from the Pacific Northwest Coast into Southern Cal ifornia. Precipitation was wide- spread; rain fell throughout the valley and foothill areas with snow above 1830 m (6000 ft.) within the SCPP project area. The synopti~ patterns, cross-sections of important atmosphl!rlC variables and precipitation characteristics associated with this storm are described and the storm's potential for seeding are discussl!d. 2. SYNOPTIC DISCUSSION The storm of 14-15 December was initiated by Cl large trough at 50 kPa (500 mbar) which moved southward from the Gulf of Alaska (600N, 150011) and closed off near 500N, 1500W by 1200 GMT, (0400 pst) on Wednesday, 14 December. Figure 1 shows the 50 kPa heights and temperature regime for 0000 GMT, 14 December. There were no closed isobars and the central trough height was about 4960 m. However, by 1200 GMT (see Figure 2) a closed low center with heights of 4890 m was evident centered around 540N, 151011. By 0000 GMT, 15 December, Fi gure 3, the centrell trough height is about 4860 m. The core of the trough has shifted slightly northward and eastward, The short-wave trough which brought weather to California just appears as a minor wave centered near 400N, 1400W. It was SUppoy'ted by cold air advection at those heights indicating it would strengthen with time. Figure 4 shows the 50 kPa heights and temperatures at 1200 GMT, 15 December. The short-wave trough has reached landfall centered near 380N, 1220W. The major closed low remained centered near 550N, 1450W at 50 kPa. However, its central height rose from 4860 m at 0000 GMT to 4990 m at 1200 GMT, 15 December. By 0000 GMT, 16 December the short-wave trough passed east of California and the main low pressure area was centered at 550N, l420W with no closed height lines at 50 kPa. Figures 5 and 6 show the 70 kPa for 1200 GMT Wednesday 14 December and 1200 GMT Thursday, 15 December. Comparing Figure 5 with Figure 1, we can detect some northwestward tilt in the low pressure area centered near 500N 1500W at 50 kPa. The spawning of the short-wave trough affecting the project area occurred between 1200 GMT, 14 December and 0000 GMT, 15 December. The short-wave trough is an obvious feature in Figure 6 at the 70 kPa level at 1200 GMT, 15 December. Important features appear in the 30 kPa for 0000 GMT 14 December and 0000 GMT, 15 December (see Fig~res 7 and 8). We note in Figure 7 that the trough at 30 kPa supporting ~his storm system is located farther north at 62 N, 1500W. There is a strong jet maximum oriented east- west from 350N, 1400W to 350N, 17SoW. However, Figure 8 shows a much different picture at 30 kPa. The high latitude low center shifted southeastward 70 to 550N, 1470W and the jet maximum decreased in east-west extent and intensity but located itself near 400N, 1400W to give support to the forming of a short-wave trough at lower levels (see Figures 3 and 6). The positioning of the 30 kPa jet jus~ offshore produced divergence aloft near to proJect area while causing convergence in the low levels of the atmosphere. This convergence was strong below 70 kPa and produced a low level jet which is discussed in the next section. 3. CROSS SECTION ANALYSIS An examination of several time cross sections of this system as it passed through the area point up some interesting aspects of this storm. Figures 9 and 10 are time cross sections of wind direction and speed for a period from 0000 GMT, 14 December to 0000 GMT, 16 December, 1977. All data are based on soundings taken at Sheridan or Oakland, California. An examination of these figures indicate some pronounced low and mid-level wind features associated with various portions of this storm. The first three soundings (0000, 0600, 1200 GMT, 14 December) show the characteristic veering of winds with height in advance of the main trough approaching the area. The wind speeds increase slowly aloft, with a jet developing near 66 kPa by 1200 GMT, 14 December. This jet just pre- ceded the onset of precipitation in the SCPP project area and a convective band which passed just north of the area after the 1200 GMT sounding. The direction of this jet was from about 2550. This direction has a large perpen- dicular component to the barrier which enhanced the orographic nature of the precipitation. Rain and snow fell throughout much of 14 December and persisted through the morning of 15 December in most foothill and mountain locations. The time cross section of wind direction shows the backing of winds to more southerly directions until about 0300 GMT on 15 December. Thereafter veering occurred in the winds in advance of the frontal passage, around 1200 GMT, 15 December 152