Precipitation Development in Stratiform Ice Clouds: A Microphysical and Dynamical Study

Aircraft microphysical measurements in ice clouds associated with warm frontal overrunning systems, warm frontal occlusions, closed lows aloft and the jet stream were combined with Doppler radar measurements in four case studies. Good agreement was obtained between aircraft calculations of the radar reflectivity factor and air velocity, and radar measurements and calculations of these parameters. Vertical velocities typically ranged from 10 cm s−1 in warm frontal overrunning systems to in excess of 50 cm s−1 in clouds associated with a closed low aloft, longitudinal rolls and isolated convective cells. Ice crystal seeding in trails emanating from longitudinal rolls were measured to extend over 7 km in the vertical and over 100 km in horizontal distances. Several general results were deduced from the aircraft measurements. Vertical velocities generally in excess of 50 cm s−1 at temperatures lower than –5°C were found to be necessary for liquid water occurrence in deep stratiform ice clouds. Water saturation was not necessary for nucleation to occur. The ice water content and ice crystal concentration, parameterized in terms of the vertical air velocity and temperature, were found to be directly dependent on the vertical velocity. Ice crystal concentrations were found to he 2–4 orders of magnitude higher than ice nuclei concentrations at temperatures warmer than –15°C.