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 662

26.4.2.1 Numerical Experiment

The model settings and the domain used are the same as in Sect. 26.4.1 .The
NCEP-GFS forecast data available at a horizontal resolution of 1 ı 1 ı and a

time resolution of 6 h have been used to develop the initial and lateral boundary
conditions. Four numerical experiments are performed as in Sect. 26.4.1 to study
the impact of temperature, humidity and total precipitable water in the simulated
structure of monsoon depression. All the simulations are started with the same
initial conditions on 01 September 2006 18 UTC; however, the observations are
assimilated at different times depending on the availability of the satellite data
over the domain. For the CTRL experiment, the model integrations are performed
till 05 September 2006 12 UTC. In the ATOVS run, the ATOVS temperature and
humidity profiles are ingested into the model using 3D-VAR, initially, after 6 h
of forecast, i.e., on 02 September 2006 00 UTC and subsequently ingested up to
03 September 2006, 00 UTC in a 12 hourly interval. The WRF model is then
subsequently integrated for the next 60 h in free forecast mode without any further
assimilation of the satellite data. In the “MODIS run”, the MODIS temperature and
humidity observations have been assimilated in 12 h intervals from 01 September
2006 18 UTC to 02 September 2006 18 UTC. Subsequently, the model is run in
a free forecast mode up to 05 September 2006 12 UTC. As in Sect. 26.4.1 ,the
MODIS data is subjected to “thinning”. The “SSM/I run” is similar to the ATOVS
experiment except that it incorporates SSM/I total precipitable water instead of
ATOVS temperature and humidity observations. The CTRL run is subjected to data
cycling without any assimilation of observations. The domain of the study is same
as that of Sect. 26.4.1 . The results presented are from 12 km resolution domain only.
All the satellite observations have shown impact on the initial condition, with the
ATOVS observation having maximum significance while the SSM/I observations
have the lowest significance.

26.4.2.2 Results and Discussion

Initial Conditions

The analysis increment of wind speed at 850 hPa level for the MODIS, ATOVS
and SSM/I experiments are presented in Fig. 26.12 for the 2-5 September 2006
depression. It can be seen that the assimilation of the satellite observations has
introduced clear increments in the 850 hPa wind speed. The assimilation of MODIS
temperature and humidity observations has introduced negative increments over
the Arabian Sea together with a small pocket of positive increments to the south
of the peninsular India. However, the increment values of 850 hPa wind speed in
the MODIS run is almost zero over the Bay of Bengal region. The assimilation
of ATOVS temperature and humidity observations has introduced larger spatial
distribution of positive values over the Arabian Sea, the land surface and the Bay