An Attempt to Improve Kessler-Type Parameterization of Warm Cloud Microphysical Conversion Processes Using CloudSat Observations

Improvements to the Kessler-type parameterization of warm cloud microphysical conversion processes (also called autoconversion) are proposed based on a large number of CloudSat observations between June 2006 and April 2011 over Asian land areas. The emphasis is given to the vertical distribution of liquid water content (LWC), particularly, the threshold values of LWC for autoconversion. The results warrant a new approach to the numerical parameterization of autoconversion in warm clouds. One feature of this new approach is that the autoconversion threshold, which has been treated as a constant in previous parameter-ization schemes, is diagnosed as a function of altitude by using a relationship between LWC and height (H) derived from CloudSat observations: LWCdig = -500.0 ln(H/9492.2). Under this framework, the threshold LWC decreases with increasing H, allowing autoconversion to occur in clouds with low LWC (approximately 0.3 g m-3) at levels above 5.5 km. Autoconversion rates calculated based on the new parameterization are compared to those calculated based on several commonly used parameterization schemes over a range of LWCs from 0.01 to 1.0 g m-3 The new scheme provides reasonable simulations of autoconversion at various vertical levels.