Geoscience Reference
In-Depth Information
3.2 Cloud Analysis
Cloud Analysis procedure is used to assimilate reflectivity data in WRF model.
Reflectivity data is used to adjust moisture and temperature in precipitation
regions. The radar reflectivity and thermodynamics are used to solve for the
hydrometeor mixing ratios. The analyzed temperature is used for diagnosis of
precipitation area. Direct replacement of the background hydrometeors is done
in areas where observed reflectivity is greater than a prescribed threshold
(typically 10 dBZ). Precipitation is removed from the background in areas
having reflectivity less than the precipitation threshold (within the radar volume
coverage).
4. Design of Experiment
Three set of experiments are performed using ADAS system to investigate the
impact of radar data on simulation of cyclone Aila. Among these three
experiments, the first experiment (DWR1) only assimilates radar reflectivity
data; the second experiment (DWR2) only assimilates radar radial velocity
data; and both the reflectivity and wind data are assimilated in the third
experiment (DWR2). In all three experiments, data assimilation is cycled every
30 minutes within 3-hour assimilation window from 0600 UTC to 0900 UTC
25 May 2009. Since the radar reflectivity and the radial velocity have been
pre-processed and quality control by WDSS-II software installed at NWP
Division, IMD, New Delhi, no additional quality control is done before the
data assimilation. Inside the ADAS the quality of radial velocity and reflectivity
is further checked by default quality control process.
5. Result and Discussion
Figure 3 depicts the comparison of sea level pressure (SLP) at the end of data
assimilation experiments (0900 UTC 25 May) and sea level pressure from
control (CTRL) experiment. The observed minimum SLP for Aila was 968
hPa and it was noticed that all the experiments underestimate the intensity of
cyclone. The CTRL run produces a system with the minimum central SLP of
992.2 hPa. Slight change in intensity of the system is seen with the assimilation
of radar reflectivity data in DWR1 experiment. Simulated minimum central
SLP in this DWR1 experiment is 991.3 hPa. Simulated minimum central SLP
is 990.5 hPa when the radar wind data are assimilated in DWR2 experiment.
When both radar reflectivity and wind data are assimilated, the experiment
DWR3 produces minimum SLP as 990.1 hPa, which is 0.4 hPa less than DWR2
experiment. Apart from that only one closed isobar is seen in control run while
three closed isobar are seen in data assimilation experiments. Pressure deficient
in control run is 1.8 hPa while in assimilation experiments it is of the order of
(~6 hPa). This indicates that data assimilation experiments are successful but
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