On the parameterization of cloud droplet effective radius using CAIPEEX aircraft observations for warm clouds in India

Aircraft measurements of microphysical parameters in cumulus clouds taken during the Phase I of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX-1) project over the Indian region are used in this study. This data is found useful for verifying and tuning the schemes used in large scale models to parameterize the cloud effective radius (a representative parameter of cloud droplet spectra) using the cloud liquid water content and the cloud droplet number concentration. This analysis used 50-m altitude bin averaged vertical profiles of the microphysical parameters measured in clouds during pre-monsoon, transition-to-monsoon, and active monsoon situations sampled at various locations during June to August 2009. These monsoon cumulus clouds are found to be significantly diluted due to entrainment mixing. A linear relationship between the cloud droplet number concentration and adiabatic fraction (a measure of mixing between the cloud and its environment) is found for the each flight averaged data. The least squares best fit of this linear relationship is suggested as a simple formulation that will be useful for large scale models to predict cloud droplet number concentration, when only cloud liquid water content is diagnosed. The use of this simple formulation is shown to improve the skill of parameterized cloud effective radius for monsoon clouds.