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992 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 A similar sensor was developed for ground use and is described by Tattleman (1982). Over the course of the years the ground-based icing detector went through several significant modifications, which resulted in the unit used in SCPP, the Rosemount 872B Icing detector. The detector is aerodynamically designed and detects icing from any wind quadrant. The instrument operates on the principle of magnetostrictive oscillation. The sensing element vibrates at 40 000 Hz in no-ice con- ditions. The frequency decreases as ice forms on the sensing element. When the frequency decreases below 39 867 Hz, an ice signal output and deicing heaters are activated for 90 sec. The ice detectors were calibrated at the factory to emit an icing signal when 0.4-0.6 mm of ice has accumulated on the sensing element. One occurrence of the icing signal output defines one trip. For more details about this device, see Tattleman (1982). Data from two icing stations will be discussed in this paper. The first was sited atop the Sierra Nevada crest at Squaw Peak (SQP in Fig. 2). The other was located 30 km northwest of SQP at Signal Peak (SIG). The icing stations were linked to a Handar 540 data col- lection platform, where temperature, relative humidity, wind speed, and wind direction were recorded every 5 min. Once an hour, the data were transmitted by sat- ellite and down-linked to a local computer, where they could be displayed. 3. General storm patterns influencing the Sierra Ne- vada Synoptic patterns that brought weather to the central Sierra Nevada were classified broadly into two groups based on the prevailing flow: those that had a predom- inant zonal component and those that had a predom- inant meridional component. Within each general group, storms were further organized according to gen- eral synoptic scale characteristics and the state of evo- lution of each storm as it moved into the Sierra Nevada. The categorization presented here includes all storms that moved across the Sierra Nevada during the four field seasons beginning in 1983/84. Each category in- cluded storms with similar synoptic characteristics, such as storm trajectory, wind flow, and cloud fields. Storm categorization was based on cloud patterns from satellite images and 500 mb flow fields. Fifty-six ofthe 63 storms that affected the project area were found to fit into one of the five general categories discussed below. a. Storms with zonal flow characteristics Storms originating in zonal flow over the eastern Pacific undergo continuous changes as they develop, occlude, and dissipate. The stage of evolution of the storm often dictates the characteristics of supercooled water observed in the storm during its passage over the Sierra Nevada. 1) DEVELOPING STORM EMBEDDED IN STRONG WESTERLY OR SOUTHWESTERLY FLOW Figure 3a shows a typical flow field, cloud pattern, and trajectory of the vorticity maximum associated with the 500 mb trough in a developing storm embed- ded in strong zonal flow. This type of storm system produced heavy, sustained precipitation in the Sierra Nevada and was often associated with flooding. Typ- ically, the surface circulation center was located be- tween 450 and 50 ON latitude. Secondary wave for- mation along the front was common. As the wave de- veloped, a strong circulation center sometimes formed. The latitude of the circulation influenced the typical temperatures encountered during the storm and whether warm, cold, and/or occluded frontal structure was observed in the vicinity of the project area. These storms generally had several bands of heavier precip- itation, and their orientation and evolution were gen- erally similar to those described by Parsons and Hobbs ( 1983). 2) MODERATE AMPLITUDE SHORT WAVE ASSO- CIA TED WITH AN OCCLUDED STORM During this type of storm, the belt of strong westerly flow was generally north of the state of California (Fig. 3b). The center of circulation at the surface was gen- erally near the coast and north of 45 ON latitude, but was occasionally as far south as 420N. The low-level wind flow was southwesterly over the project area, shifting to westerly at frontal passage. Within the pro- ject area, precipitation occurred primarily in associa- tion with the passage of a cold front. The surface front was often indistinct by the time it reached the Sierra Nevada foothills. However, an upper-level front was often present and generally quite distinct, coi-respond- ing to the transition region from the anticyclonic to the cyclonic side of the upper troposphere jet as de- scribed by Hobbs (1978). The upper-level front was often evident on satellite photos as a distinct cirrus boundary (Fig. 3b), an important element in the split- front concept developed by Browning and Monk (1982). 3) SPLIT FLOW IN THE MIDDLE TROPOSPHERE AS- SOCIATED WITH A DISSIPATING STORM This storm type was associated with the latter stages in the evolution of a cyclonic storm over the Pacific. During these situations, a moderate amplitude ridge was often present over the western United States (Fig. 3c). The center of circulation in these storms was gen- erally located near 500 latitude. The storms were oc- cluded and weakening in intensity. A weak cold front extended southward from the low and moved into a