On the freezing time of supercooled drops in developing convective clouds over tropical ocean

Ice generation and evolution in tropical maritime convective clouds are still poorly understood and challenging to model. Aircraft measurements during the Ice in Clouds-Tropical (ICE-T) project suggest that the observed ice particles in intense convective clouds are primarily small at relatively warm temperature (between −7 °C and −10 °C), and large frozen drops are detected at a temperature colder than −10 °C. However, the ice particle size distributions (PSDs) between −7 °C and −10 °C modelled using a parcel model with spectral bin microphysics scheme are much broader than the observation. To interpret the difference in the temperature-dependent ice PSD evolution between the model simulations and the observations, the freezing times and temperatures of supercooled drops are modelled and analyzed. The results indicate that the freezing time (from the initial nucleation to fully frozen) must be considered; it is not instantaneous, and is longer for large drops than for small drops. In strong updrafts, such as that sampled by the Learjet during ICE-T, large freezing drops may be carried upwards to a temperature lower than their nucleation temperature before they are fully frozen. This offers a feasible explanation for the temperature-dependent ice particle size evolution in strong updrafts observed during ICE-T. In models, drop freezing is normally assumed to be instantaneous, which is not realistic; the models yields broader ice PSDs between −7 °C and −10 °C than is observed. The results highlight the importance to consider the freezing time of supercooled drops in interpreting the observed and modelled ice PSDs in growing turrets and in modelling ice generation in cloud resolving models. To better understand the time-dependent drop freezing and its impact on the microphysics and dynamics of convective clouds, more field measurements and laboratory experiments, as well as modelling studies are needed.