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 665

intensified into a depression with a minimum central pressure of 998 hPa. The
system experienced landfall before 5 September 2006 and started weakening
subsequently after landfall. The MSLP plots of the NCEP-FNL therefore are
consistent with that of the observations. Figure 26.14 a-p depicts the MSLP
patterns on 03 September 2006 00 UTC and subsequent predicted values at 24,
48 and 60 h of forecast for the CTRL experiment (Fig. 26.14 a-d), the ATOVS
experiment (Fig. 26.14 e-h), the SSM/I (Fig. 26.14 i-l) experiment and the MODIS
(Fig. 26.14 m-p) experiment for the 2-5 September 2006 depression. From the
spatial distribution of MSLP patterns it can be seen that the CTRL and the ATOVS
experiments have failed to simulate accurately the evolution of the depression over
the Bay of Bengal. After 48 h of forecast the CTRL run simulates a low pressure
region over the south east of the Bay of Bengal, a feature absent in the observation
records. A pattern similar to the CTRL run with a lower intensity of MSLP is seen
in the results of the ATOVS experiment too. The above-mentioned erroneous low
pressure region is not seen in the SSM/I and the MODIS experiments. However, the
SSM/I run, simulates the location of the depression centre at the 60 h of forecast
near Hyderabad which is much south of the observed position of the monsoon
depression at that time. The experiment which assimilated MODIS observation
has, however, stimulated the exact location of the depression with slightly higher
intensity of MSLP than that of the observed.
Wind Speed
The lower tropospheric wind vector simulated by all the four experiments at 850 hPa
are shown in Fig. 26.15 for the 2-5 September 2006 depression, which includes the
CTRL run (Fig. 26.15 a-d), ATOVS experiment (Fig. 26.15 e-h), SSM/I experiment
(Fig. 26.15 i-l) and MODIS experiment (Fig. 26.15 m-p).
The CTRL, ATOVS and the SSM/I experiments simulate almost the same
patterns of wind vector field with a strong south-westerly flow over the extreme
south of the Indian peninsula. The south-westerly flow is found to be slightly intense
for the SSM/I run. However, the 850 hPa cyclonic circulation usually associated with
the monsoon depression, is found to be absent in the case of the CTRL, ATOVS and
SSM/I runs. The MODIS experiment simulates a weak cyclonic vortex over the
Head Bay region and over the inland regions at 24, 48 and 60 h of forecast. The
notable feature about the MODIS experiment is the absence of the south-westerly
flow as seen in the other experiments. Hence it can be concluded that the CTRL
ATOVS, SSM/I experiments failed to simulate adequately the cyclonic circulation
associated with the monsoon depression over the Head Bay region. The reason for
the above failure may be partly due to the erroneous simulation of a low pressure
system over south-east of Bay of Bengal and the presence of a stronger south-
westerly flow over the extreme south of the Indian peninsula.