  The quantitative prediction of the intensity of rainfall events (light or
heavy) has remained a challenge in Numerical Weather Prediction (NWP) models.
For the first time the mean coefficient of diffusional growth rates are
calculated using an Eulerian-Lagrangian particle-based small-scale model on in
situ airborne measurement data of Cloud Aerosol Interaction and Precipitation
Enhancement Experiment (CAIPEEX) during monsoon over Indian sub-continent. The
results show that diffusional growth rates varies in the range of 0.00025 -
0.0015(cm/s). The generic problem of the overestimation of light rain in NWP
models might be related with the choice of cm in the model. It is also shown
from DNS experiment using Eulerian-Lagrangian particle-based small-scale model
that the relative dispersion is constrained with average values in the range of
~ 0.2 - 0.37 (~ 0.1- 0.26) in less humid (more humid) conditions. This is in
agreement with in situ airborne observation (dispersion ~ 0.36) and previous
study over Indian sub-continent. The linear relationship between relative
dispersion and cloud droplet number concentration (NC) is obtained from this
study using CAIPEEX observation over Indian subcontinent. The dispersion based
autoconversion-scheme for Indian region must be useful for the Indian summer
monsoon precipitation calculation in the general circulation model. The present
study also provide valuable guidance for the parameterization of effective
radius, important for radiation scheme.
