Wednesday, 1 August 2001: 4:40 PM
Physical and Computational Considerations for the Use of GPS Occultation Observations in NWP
The development of small, high-performance instruments to receive Global
Positioning System (GPS) signals has created
an opportunity for active remote sounding of the Earth's
atmosphere by radio occultation techniques. A prototype
demonstration of this capability has been provided by
the GPS Meteorology (GPS/MET) experiment, following
the first launch of a small Low Earth Orbit (LEO) satellite, MicroLab-1,
in April 1995.
With known observation geometry specified by the satellites' positions
and velocities, the derivative of the phase excess (the Doppler shift
excess) characterizes the atmospheric and ionospheric effect on the
Doppler frequency shift and can be used to derive
the radio occultation bending angle
assuming spherical symmetry of the atmospheric refractivity.
Ionospheric effects on refraction angles
can be eliminated or considerably removed by using two frequencies.
The refraction angle profile can
then be inverted to obtain the refractivity profile using the Abel transform
by invoking the spherical symmetry assumption for the second time.
Since atmospheric refractivity depends on pressure,
temperature, and specific humidity
in a neutral atmosphere, independent knowledge of one of the three
quantities allows the other two
variables to be retrieved from the GPS refractivity profile.
For example, when water vapor contributions to the refractivity are
small, a refractivity profile and the hydrostatic
equation uniquely define pressure and temperature profiles.
The GPS measurements are not affected by clouds or precipitation and are
of high vertical resolution. Instrument calibration is not required and
observation errors are statistically independent of the other
types of measurements.
With these attractive features of GPS measurements,
and the promising results of the preliminary GPS-retrieval products,
several international projects include plans to launch
many more LEO satellites equipped with GPS-receivers.
How will this new type of data
affect global analyses and possibly change the need for other types of
observational data? Before one can answer this question, a method
that can extract useful information from these measurements needs to be developed.
This talk discusses several physical and computational considerations for the use of
GPS occultation observations in numerical weather prediction. We aim at developing
a GPS data assimilation method which is accurate, computationally efficient,
and feasible for an operational application.