The Influence of Aerosol Hygroscopicity on Clouds and Precipitation over
Western Ghats, India
Abstract
We examine the role of aerosol hygroscopicity (κ) affects clouds and
precipitation formation over the Western Ghats (WG) in India using
various numerical model simulations (i.e., particle-by-particle based
small-scale, high resolution mesoscale model). For the diffusional
growth of cloud droplets, the size dependent hygroscopicity is used in
κ- Köhler equation of direct numerical simulation. The results of the
small-scale model reveal that the distribution of cloud drop size varies
from the initial mixing state to well mix state due to variation in κ.
The value of κ is obtained from HTDMA instruments at High Altitude Cloud
Physics Laboratory, India. The idealized and real simulations using WRF
model with aerosol-aware Thompson microphysics scheme are conducted by
changing κ values. Depending on the type of clouds (shallow or deep),
different κ values determine the mass, number and precipitation of cloud
and rain droplets. Low hygroscopicity (organics) simulates more and
smaller drops, as well as uplifts below freezing level, resulting in
more ice phase hydrometeors. Organic aerosols have a significant impact
on the formation of more snow and graupel hydrometeors. As compared to
high κ, low hygroscopicity weakens updrafts at the intermediate level
and strengthens them at the upper level in the deep cloud region. The
intensity of precipitation varies due to low and high κ. The findings
indicate that aerosol composition has a significant impact on the
activation of cloud condensation nuclei. This study suggests that
aerosol hygroscopicity is essential in weather prediction models in
order to integrate aerosol chemical compositions.