Reproducibility of Indian summer monsoon rainfall in convection
permitting Weather Research Forecasting model
Abstract
Dynamical downscaling of Indian summer monsoon rainfall (ISMR) by using
regional climate models (RCMs) portrays the inability of the RCMs in
simulating the ISMR, and certain systematic biases appear in the
seasonal monsoon rainfall climatology. The inconsistency in RCMs
simulation of ISMR can be due to the improper representation of
convection by convective and/or microphysical parameterization schemes
in different RCMs. In this study, we conducted convection permitting
simulations in WRFv3.8.1 and compared with parameterized simulations, to
understand the difference of reproducibilities of time-space patterns in
the ISMR. Our experimental set-up consists of two sets of simulations
with parameterized and explicit convection on a grid resolution of 25
km. The simulations are conducted for three different monsoon seasons:
flood, drought, and normal years, to ascertain robustness in the
analysis of the model output. These simulations are forced by using
ERA-Interim reanalysis as the lateral boundary and large-scale forcing
input. The mean large-scale circulation, the spatial distribution of
rainfall, seasonal northward propagation of rain bands, and
magnitude-phase of the Indian summer monsoon rainfall are verified
against the JRA55 reanalysis and India Meteorological Department gridded
rainfall datasets. The results show that regional simulations with
explicit convection have benefited in the simulation of ISMR features.
Simulated seasonal mean rainfall in parameterized convection shows
positive bias over Gangetic plains and the Western Ghats. The same bias
reduced in explicit simulations and seasonal mean ISMR behaves
realistically concerning IMD observations. The added value in the
simulation of ISMR in explicit experiments is found to be consistent
during the flood, drought, and normal monsoon seasons. Further
evaluation of the results reveals that over Indian region, explicit
convection simulations of Indian summer monsoon are more realistic than
parameterized convection simulations. Therefore, the current study tried
to show up the uncertainties in ISMR simulation associated with
parameterizations, and explicit convection experiments highlight the
reduction of these uncertainties.