On the Freezing Time of Supercooled Drops in Developing Convective Clouds

Abstract. In this study, the particle size distributions (PSDs) measured in fresh developing maritime convective clouds sampled during the Ice in Clouds-Tropical (ICE-T) project are shown and compared with the PSDs modeled using a parcel model containing a spectral bin microphysics scheme. The observations suggest that the first ice in convective clouds is small. To interpret the observed ice PSDs, the freezing times and temperatures of supercooled drops are analyzed. The results indicate that the freezing time is longer for large drops than it is for small drops. Due to instrumental limitations, freezing drops cannot be identified until they exhibit obvious shape deformation. If the updraft is strong enough, large freezing drops can be carried upwards to a lower temperature than their nucleation temperature before obvious shape deformation occurs. In models, drop freezing is assumed to be instantaneous, which is not realistic; thus, the model yields a broader first ice PSD than is observed. This study allows us to interpret the observed ice PSDs in fresh developing convective clouds from the perspective of the freezing time of supercooled drops and notes the deficiency of instantaneous drop freezing in models. To better understand the mechanisms of drop freezing and ice initiation in convective clouds, more laboratory experiments and in situ measurements are needed in the future.