Studies on the Impacts of 3D-VAR Assimilation of Satellite Observations on the Simulation of Monsoon Depressions over India - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 641

improvement in assimilation is mostly due to ingestion of SSM/I TPW data ( Rakesh
et al. 2009b ). The results of the study also indicated that the largest improvement in
the MM5/WRF rainfall prediction was associated with the SSM/I TPW assimilation.
The assimilation of only the SSM/I wind speed in MM5/WRF model, however
resulted in some degradation in the simulation of the humidity and rainfall fields
( Rakesh et al. 2009b ).
Singh et al. ( 2011a ) have recently investigated the impact of ATOVS radiance
on the analysis and forecasts of WRF model over the Indian region during the
2008 summer monsoon and found a positive impact of the assimilated ATOVS
radiance on both the analysis as well as the short-range forecasts. The above study,
in addition to the control (no assimilation) run, also utilized satellite radiances from
AMSU-A, AMSU-B and HIRS sensors. Singh et al. ( 2011b ) have also compared
the performances of Kalpana and HIRS water vapor radiances in the WRF 3D-Var
assimilation system for the period 10-20 July 2008 over the Indian region and found
that the assimilation of Kalpana radiances provided significant improvements to the
results as compared to the assimilation of HIRS radiances. Singh et al. ( 2011c )
investigated the impact of assimilating Oceansat2 surface wind vectors for the
month of July 2010 over the Indian region. The results of the above study indicated
that the assimilation of Oceansat2 surface wind vectors led to small, but positive,
impact on the forecast (particularly later hours of forecast) of mid-tropospheric
moisture, temperature, and upper tropospheric winds. Also, the assimilation of
Oceansat2 surface wind vectors improved the precipitation forecast (as compared
to the control run) for moderate to heavy rainfall thresholds when validated against
TRMM rainfall.

26.4 Case Studies of the Impact of Satellite Data

Assimilation Using WRF-3D-VAR System on Three
Monsoon Depressions That Formed Over India

This section investigates the impact of assimilation of the vertical profiles of air
temperature, and humidity from the MODIS and the ATOVS, and total precipitable
water (TPW) from the SSM/I sensors using the WRF model. Three cases of
monsoon depressions have been considered for the above investigation in the present
study: (1) a monsoon depression that formed over the Bay of Bengal, between
September 19-22, 2006, (2) a monsoon depression that formed over the Bay of
Bengal, between September 02-05, 2006, and (3) a monsoon depression that formed
over the Bay of Bengal, between June 18-22, 2007. The following sections represent
descriptions and features of the monsoon depressions investigated, the experimental
design, together with the results and discussions as well as conclusions of the
study.