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Mark F. Heggli and Robert M. Rauber

Abstract

A comprehensive examination of the evolution and vertical distribution of supercooled liquid water in Sierra Nevada winter storms was completed as part of the Sierra Cooperative Pilot Project. Measurements of supercooled water in 63 storms, which affected the area from the 1983/84 through the 1986/87 winter field seasons, were made with a dual-channel microwave radiometer located near the Sierra Nevada crestline.

The analyses were carried out in two parts. In the first part, winter storms were grouped into two general categories according to the prevailing flow and resultant storm trajectory. The two storm types consisted of those propagating in zonal flow and those in meridional flow. A complete description of the storm typing is provided. Storms in zonal flow were either developing, occluding, or dissipating during the time they affected the Sierra Nevada. The stage of evolution of the storm dictated the characteristics of supercooled water observed during the storm's passage over the Sierra Nevada. The presence of supercooled water in meridional storms was influenced mainly by the trajectory of the storm.

Results indicated that zonal storms had the most sustained periods of supecooled water in the postfrontal region, while sustained supercooled water measurements in meridional storms were usually greatest within the prefrontal portions of the storm. These periods of supercooled water were associated primarily with orographic clouds. Studies of the vertical supercooled water distribution showed that saturation often existed within 1 km of the local terrain when supercooled water was measured by the radiometer. Fifty percent of the rawinsondes launched through supercooled water-bearing clouds measured water-saturated layers deeper than 0.5 km. Saturated layers were found to exist most often between −8° and and −10°C, although they were often present over a much wider temperature range.

Conditions favorable for ice crystal growth were common, which supports the pursuit of precipitation enhancement as a means of supplementing regional water supplies.

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Mark F. Heggli, Larry Vardiman, Ronald E. Stewart, and Arlen Huggins

Abstract

Cloud physics data measured by aircraft during two successive winter field seasons (1978–79 and 1979–80) of the Sierra Cooperative Pilot Project operating over the Sierra Nevada Range have been examined in order to determine the distributions of supercooled liquid water and ice crystals. Results indicate that convective clouds provide the greatest likelihood of significant supercooled water. The Sierra barrier appears to optimize these conditions 40 to 90 km upwind of the crest within pockets of horizontal extent up to 64 km, although these conditions were greatly reduced at temperatures less than −10°C. The dominance of liquid water content over ice crystal concentration was maximized 7–10 h after the 700 mb trough passage. Area-wide and banded clouds, which make up the remaining precipitation events, showed only small amounts of supercooled water and general abundance of ice crystals. The largest liquid water contents were observed at the greatest temperatures, usually 0° to −5°C. Such climatological information suggests that a weather modification program to enhance snowfall should concentrate primarily on the convective clouds.

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