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...