GSI/WRF Regional Data Assimilation System and Its Application in theWeather Forecasts over Southwest Asia - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 618

25.6.1 Experiment Design

In this study, the GSI analysis system is integrated with the WRF-ARW mesoscale
system, and the Advanced TIROS-N (Television and Infrared Observation Satellite)
Operational Vertical Sounder (ATOVS) radiance observations are employed. The
ATOVS datasets supplied by National Environmental Satellite, Data, and Informa-
tion Service (NESDIS) are composed of radiances from the Advanced Microwave
Sounding Unit (AMSU) and the High-Resolution Infrared Sounder (HIRS)/3. Two
separate radiometers (AMSU-A and AMSU-B) compose the AMSU platform.
The AMSU-A is a cross-track, stepped-line scanning total power radiometer. The
instrument has an instantaneous field-of-view of 3:3 ı at the half-power points

providing a nominal spatial resolution at nadir of 48 km. The AMSU-B is a cross-
track, continuous line scanning, total power radiometer and has an instantaneous
field-of-view of 1:1 ı (at the half-power points). Spatial resolution at nadir is

nominally 16 km. The antenna provides a cross-track scan, scanning ˙ 48:95 ı from
nadir with a total of 90 earth fields-of-view per scan line.
The AMSU-A and AMSU-B radiance data used here have undergone substantial
preprocessing by NESDIS to remove various biases before being made available.
The data have been statistically limb corrected (adjusted to nadir) and surface
emissivity corrected in the microwave channels. Figure 25.12 shows an example
of the scan position of the two microwave sensors of NOAA-15 and -16 during
the study period. It is clear that NOAA-16 data covers most Southwest Asia and
AMSU-B has a higher density of observations than AMSU-A.
Derber and Wu ( 1998 ) pointed out that the presence of a single data point
containing large errors can result in substantial degradation of the analysis and
subsequent forecast. For this reason, a simple quality control has been developed. To
achieve similar radiances between instruments, the observed brightness temperature
data have been modified empirically with different adjustment procedures for each
instrument. In the GSI analysis system, this check includes two steps. First, a
location check (including removal of observations outside the domain) and thinning
procedure (excluding location/time duplicates and incomplete observations) is
performed to ensure vertical consistency of upper-air profiles. Secondly, numerous
quality control (QC) checks are redone based on various quality parameters after
the model brightness temperatures are obtained from the radiative transfer model.
The quality parameters are formulated in terms of the expected observational error
variance as a function of the channels and have been adjusted for their position
across the track of the scan, whether it is over land, sea, snow, sea ice, or a transition
region, for elevation, the difference between the model and the real topography, and
the latitude. In Fig. 25.13 , the statistics show that the number of observations used
in the GSI regional data assimilation system is quite different. AMSU-B has many
more observations than the two AMSU-A platforms. For NOAA-15 (Fig. 25.13 ),
the maximum number of AMSU-B observations for all 30 days range from 50,000
to 150,000 pixels, and for AMSU-A, the number is only around 40,000 pixels. For
NOAA-16, the number of AMSU-B observations exceeded 150,000 pixels, while