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The objective of this study is to investigate the impact of a surface data assimilation (SDA) technique, together with the
traditional four-dimensional data assimilation (FDDA), on the simulation of a monsoon depression that formed over India during
the field phase of the 1999 Bay of Bengal Monsoon Experiment (BOBMEX). The SDA uses the analyzed surface data to continuously
assimilate the surface layer temperature as well as the water vapor mixing ratio in the mesoscale model. The depression for
the greater part of this study was offshore and since successful application of the SDA would require surface information,
a method of estimating surface temperature and surface humidity using NOAA-TOVS satellites was used. Three sets of numerical
experiments were performed using a coupled mesoscale model. The first set, called CONTROL, uses the NCEP (National Center
for Environmental Prediction) reanalysis for the initial and lateral boundary conditions in the MM5 simulation. The second
and the third sets implemented the SDA of temperature and moisture together with the traditional FDDA scheme available in
the MM5 model. The second set of MM5 simulation implemented the SDA scheme only over the land areas, and the third set extended
the SDA technique over land as well as sea. Both the second and third sets of the MM5 simulation used the NOAA-TOVS and QuikSCAT
satellite and conventional upper air and surface meteorological data to provide an improved analysis. The results of the three
sets of MM5 simulations are compared with one another and with the analysis and the BOBMEX 1999 buoy, ship, and radiosonde
observations. The predicted sea level pressure of both the model runs with assimilation resembles the analysis closely and
also captures the large-scale structure of the monsoon depression well. The central sea level pressures of the depression
for both the model runs with assimilation were 2–4 hPa lower than the CONTROL. The results of both the model runs with assimilation
indicate a larger spatial area as well as increased rainfall amounts over the coastal regions after landfall compared with
the CONTROL. The impact of FDDA and SDA, the latter over land, resulted in reduced errors of the following: 1.45 K in temperature,
0.39 m s−1 in wind speed, and 14° in wind direction compared with the BOBMEX buoy observation, and 1.43 m s−1 in wind speed, 43° in wind direction, and 0.75% in relative humidity compared with the CONTROL. The impact of SDA over land
and sea compared with SDA over land only showed a further marginal reduction of errors: 0.23 K in air temperature (BOBMEX
buoy) and 1.33 m s−1 in wind speed simulations. 相似文献
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A low pressure system that formed on 21 September 2006 over eastern India/Bay of Bengal intensified into a monsoon depression
resulting in copious rainfall over north-eastern and central parts of India. Four numerical experiments are performed to examine
the performance of assimilation schemes in simulating this monsoon depression using the Fifth Generation Mesoscale Model (MM5).
Forecasts from a base simulation (with no data assimilation), a four-dimensional data assimilation (FDDA) system, a simple
surface data assimilation (SDA) system coupled with FDDA, and a flux-adjusting SDA system (FASDAS) coupled with FDDA are compared
with each other and with observations. The model is initialized with Global Forecast System (GFS) forecast fields starting
from 19 September 2006, with assimilation being done for the first 24 hours using conventional observations, sounding and
surface data of temperature and moisture from Advanced TIROS Operational Vertical Sounder satellite and surface wind data
over the ocean from QuikSCAT. Forecasts are then made from these assimilated states. In general, results indicate that the
FASDAS forecast provides more realistic prognostic fields as compared to the other three forecasts. When compared with other
forecasts, results indicate that the FASDAS forecast yielded lower root-mean-square (r.m.s.) errors for the pressure field
and improved simulations of surface/near-surface temperature, moisture, sensible and latent heat fluxes, and potential vorticity.
Heat and moisture budget analyses to assess the simulation of convection revealed that the two forecasts with the surface
data assimilation (SDA and FASDAS) are superior to the base and FDDA forecasts. An important conclusion is that, even though
monsoon depressions are large synoptic systems, mesoscale features including rainfall are affected by surface processes. Enhanced
representation of land-surface processes provides a significant improvement in the model performance even under active monsoon
conditions where the synoptic forcings are expected to be dominant. 相似文献
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