The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. I: A Case Study of a Cold Front

Abstract

Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations. The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between ?3?6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was probably responsible for the heavy precipitation on the cold front being organized, on the small mesoscale, into ellipsoidal areas with similar orientations. The precipitation efficiencies in the warm-sector and narrow cold-frontal rainbands were ?40?50% and ?30?50%, respectively. One of the wide cold-frontal rainbands, in which there was a steady production of ice panicles in the main updraft, had a precipitation efficiency of at least 80%, whereas another wide cold-frontal band, in which some precipitation evaporated before reaching the surface, had a precipitation efficiency of ?20%. Ice particles from shallow convective cells aloft played important roles in the production of precipitation in the wide cold-frontal rainbands and in some regions of the warm-sector rainband. These ?seed? ice particles grew by aggregation and by the deposition of vapor as they fell through lower level ?feeder? clouds. About 20% of the mass of the precipitation reaching the ground in the wide cold-frontal rainbands originated in the upper level ?seeder? zones and ?80% in the ?feeder? zones.