Seasonal and Interannual Trends of Sierra Nevada Clouds and Precipitation

Abstract

Seasonal and interannual variations in Sierra Nevada winter storms are discussed with reference to precipitation augmentation. Seasonal variations occur with respect to freezing level, storm type, vertical cloud distribution, mesoscale precipitation systems, snowmelt and runoff. Statistical results from a previous operational program by Mooney and Lunn suggest that ?cold westerly? storms yield increased precipitation from cloud seeding. Case studies from the Sierra Cooperative Pilot Project have shown that postfrontal conditions, which closely correspond to cold westerly storms, are characterized by high supercooled liquid water contents. Eight years of data from the American River Basin have been analyzed here which show that cold westerly storms 1) are more frequent in late winter and spring than earlier in the precipitation season; 2) contribute greater precipitation in seasons of normal and below-normal precipitation than in above-normal seasons. Hydrological factors make these storms attractive targets for precipitation augmentation.