The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. II: Warm-Frontal Clouds

Abstract

The air motions and growth of precipitation in warm-frontal clouds containing mesoscale rainbands have been studied through Doppler radar, aircraft, rawinsonde and surface measurements. Precipitation growth in deep warm-frontal ice clouds occurred through a ?seeder-feeder? process. About 20% of the total mass of precipitation formed in a ?seeder? zone (above 5 km) and 80% of the precipitation formed in a ?feeder? zone (below 5 km). Wavelike rainbands originated from ice particles which fell from linear arrays of convective generating cells in a seeder zone. Riming growth was important in these cells where updrafts reached 60 cm s?1, but deposition and aggregation dominated at lower levels where the updraft velocities were ≤15 cm s?1. Some form of ice multiplication process appeared to be active in the feeder zone. The growth of precipitation in a warm-frontal rainband embedded in shallow liquid water clouds took place through the coalescence of water drops. The updraft velocities in this rainband reached ?20 cm s?1 and were due to the low-level convergence of air in a stable region beneath the warm-frontal surface.