Microphysics and dynamics of clouds associated with mesoscale rainbands in extratropical cyclones

Abstract

AbstractMeasurements obtained with research aircraft in clouds associated with various types of mesoscale rainbands in extratropical cyclones have been examined together with radar data and other detailed measurements. The analyses have yielded information on the structure and microphysical characteristics of the clouds and the nature of the vertical air motions in the clouds associated with each type of mesoscale rainband, and schematic models of the different mesoscale rainbands have been constructed.In clouds accompanying warm frontal rainbands, ice particles formed above the warm front, probably in shallow convective cells, and fell into the stable air mass below, where they grew by aggregation and helped glaciate stratiform clouds.Warm sector rainbands resembled squall lines. A younger, convectively active rainband occurred just ahead of an older, less convectively active one. The younger rainband contained both cloud liquid water and ice particles, and ice particles were growing by riming. The older rainband was nearly glaciated, with ice particles growing by aggregation.Narrow cold frontal rainbands were located along the convergence lines at the advancing edges of cold frontal zones. Each was associated with a convective updraught, 1 to 5 km wide and 1 to 3ms−1 in magnitude, and a similarly narrow downdraught. The core of the updraught contained young, developing cloud, with much higher cloud liquid water content and lower ice particle concentration than in its immediate surroundings. Ice particles grew by aggregation and were heavily rimed. Wide cold frontal rainbands (several tens of kilometres in width) occurred when lifting above a cold front was enhanced to several tens of centimetres per second. Clouds in this region consisted of supercooled water and ice and contained embedded convective elements. Below the cold front, high concentrations of ice particles grew by aggregation.Prefrontal surges of cool and dry air occurred above the warm front and ahead of the cold front in occluded cyclones. A surge rainband, resembling a wide cold frontal rainband, occurred at the leading edge of the surge and was followed by a field of convective elements. These elements, sometimes arranged in lines, were in various stages of development.Postfrontal rainbands were lines of convective clouds located well behind a cold front. They may have been associated with secondary fronts and sometimes appeared to behave as organized convective systems.