  An unresolved problem of present generation coupled climate models is the
realistic distribution of rainfall over Indian monsoon region, which is also
related to the persistent dry bias over Indian land mass. Therefore,
quantitative prediction of the intensity of rainfall events has remained a
challenge for the state-of-the-art global coupled models. Guided by the
observation, it is hypothesized that insufficient growth of cloud droplets and
processes responsible for the cloud to rain water conversion are key components
to distinguish between shallow to convective clouds. The new diffusional growth
rates and relative dispersion based autoconversion from the Eulerian-Lagrangian
particleby-particle based small-scale model provide a pathway to revisit the
parameterizations in climate models for monsoon clouds. The realistic
information of cloud drop size distribution is incorporated in the
microphysical parameterization scheme of climate model. Two sensitivity
simulations are conducted using coupled forecast system (CFSv2) model. When our
physically based small-scale derived modified parameterization is used, a
coupled climate model simulates the probability distribution (PDF) of rainfall
and accompanying specific humidity, liquid water content, and outgoing
long-wave radiation (OLR) with increasing accuracy. The improved simulation of
rainfall PDF appears to have been aided by much improved simulation of OLR and
resulted better simulation of the ISM rainfall.
