Role of liquid phase in the development of ice phase in monsoon clouds: Aircraft observations and numerical simulations

The effect of warm microphysical properties on ice processes is investigated in developing monsoon (MON) and premonsoon (PRE) cumulus clouds growing under different thermodynamic and aerosol conditions. We used airborne observations of cloud microphysical properties during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) and idealized large eddy simulation (LES) with the Weather Research & Forecasting (WRF) model coupled with the spectral-bin microphysics scheme. Airborne observations showed that ice microphysical properties including ice number concentration and ice water content (IWC) strongly depend on cloud base height, boundary layer water vapor mixing ratio, and cloud drop effective radius at the freezing level. The MON clouds in both high and low aerosol concentrations, characterized by a moister boundary layer and warmer cloud bases compared with PRE clouds, have broader drop size distributions (DSD) above the freezing level and produce more ice particles. Idealized WRF-LES simulations of MON and PRE clouds support the observed results with larger IWC in both polluted and clean MON clouds when compared to polluted PRE clouds, and the moisture differences play a more important role in altering the ice microphysical characteristics than aerosol number concentration. Cloud droplet and rain drop properties in both PRE and MON clouds greatly affect ice processes and change ice particle number concentration and types, through impacting drop freezing and riming. The larger IWC in the simulated MON clouds compared with PRE clouds is mainly due to enhanced deposition and riming as a result of the increased boundary layer moisture.