Aircraft Observations of Ice Crystal Evolution in an Altostratus Cloud

Abstract

Observations from a Lagrangian spiral descent within altostratus cloud associated with a cold front were used to study the evolution of ice particle spectra by following populations of ice crystals as they fell through the cloud. The flight track was corrected for wind effects and was divided into distinct regions for spatial comparison of ice particle spectra. Analysis of size spectra for particles larger than 800 ?m revealed heterogeneity on horizontal scales of 5 km in average particle diameter and concentration. In the temperature range ?40° to ?20°C the ice crystal evolution was dominated by diffusional growth, although observations of the evolution of the bimodal size spectra suggested that aggregation was occurring. Between ?20° and ?10°C aggregation dominated evolution.