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 637

26.2.3 Various Satellite Observations

26.2.3.1 QuikSCAT

The National Aeronautical and Space Administration (NASA) launched the Quick
Scatterometer (QuikSCAT) in June 1999 at an altitude of 800 km with a swath
width of 1,800 km and having an orbital period of 101 min in a polar orbiting
configuration. The sensor (Seawinds) aboard the QuikSCAT is a 13.4 GHz Ku-band
conical-scanning microwave radiometer which measures the ocean surface wind
vector from the relationship between the sea surface roughness and the back scat-
tered radar signal. The accuracy of the retrieved ocean surface wind from QuikSCAT
is about ms 1 in wind speed and 20 ı 2 in wind direction for winds of magnitude

ms 1 ( Shirtliffe 1999 ). It is known that rainfall can affect the accuracy of
the scatterometer sea surface wind measurements ( Weissman et al. 2002 ; Hoffman
and Leidner 2005 ). While in the scatterometer, light winds can get overestimated
by excess back scatter from the rain, strong winds can be underestimated due to
rainfall attenuation. The QuikSCAT Operational Standard Data Products (L2B) are
being processed and distributed by NASA Jet Propulsion Laboratory (JPL) Physical
Oceanography Distributed Active Archive Center (PO DAAC). The sea surface
wind vectors retrieved from QuikSCAT have been validated with wind data from
ocean buoys and were found to be in good agreement with the buoy data ( Ebuchi
et al. 2002 ) with the root mean square (rms) differences of about

3 - 20

23 ı
for the wind speed and wind direction, respectively. The horizontal spatial resolution
of the QuikSCAT data wind vector data is 25 km while the reference height of the
surface wind vector from QuikSCAT is 10 m.

ms 1 and 1:01

26.2.3.2 Spectral Sensor Microwave Imager (SSM/I)

The SSM/I sensor was first launched aboard the polar orbiting Defense Mete-
orological Satellite Program (DMSP) of the Unites States Navy in June 1987.
The SSM/I sensor is a conical scanning, linearly polarized, four-frequency and
seven channel passive microwave radiometer. The SSM/I sensor aboard the DMSP
satellite has an orbital period of 102 min. The SSM/I sensor also has an incidence
angle of 53 ı , a swath width of 1,400 km, a mean altitude of 830 km and a horizontal

spatial resolution of 25 km. Detailed information about the SSM/I sensor is given
in Hollinger ( 1989 ). The retrieved total precipitable water (TPW) observation from
the SSM/I satellite is used as observations in our recent study whose results will be
presented in the following section. Also, version-5 multistage regression algorithm
is used for the retrieval of SSM/I data products.

26.2.3.3 Advanced TIROS Operational Vertical Sounder (ATOVS)

The Advanced (Television and Infrared Observational Satellite) TIROS Operational
Vertical Sounder (ATOVS) is a sounding instrument package, first flown on the