The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. IV: Vertical Air Motions and Microphysical Structures of Prefrontal Surge Clouds and Cold-Frontal Clouds

Abstract

The vertical air motions and microphysical structures of the clouds associated with two mesoscale precipitation systems in the Pacific Northwest are examined using rawinsonde, aircraft and vertically pointing Doppler radar data. A rainband associated with a prefrontal surge of cold air aloft was found to consist of deep (3?4 km) convective cells. Natural seeding by cirrostratus cloud spread ice crystals throughout the rainband. All of the precipitation growth observed took place through ice-phase processes. Much of the moisture necessary for precipitation growth entered the rainband at low levels in the form of vapor and condensate associated with widespread stratiform cloud. A weakly organized cold-frontal precipitation area was found to consist of snow trails originating from shallow (1?2 km) convective cells in a ?seeder? zone above 5.5 km altitude. Below this level the trails swept through a ?feeder? zone which consisted of stratiform cloud. Downdrafts observed in one region of the feeder zone, and weak updrafts observed in another, resulted in precipitation growth in this zone being much more limited than that commonly observed in the feeder zones of cold-frontal precipitation systems.