Abstract:
This dataset contains the TIROS Operational Vertical Sounder (TOVS)
level 3 geophysical parameters derived using the physical retrieval
method of Susskind et al. (1984) and processed by the Satellite Data
Utilization Office of the Goddard Laboratory for Atmospheres at
NASA/GSFC. This method, which is
... hydrodynamic model- and a priori
data-dependent, is designated as the so-called Path A scheme by the
TOVS Pathfinder Science Working Group. The 20 channel High resolution
Infrared Radiation Sounder 2 (HIRS2) and the 4 channel Microwave
Sounding Unit (MSU) aboard the NOAA-xx series of Polar Orbiting
Satellites are used to produce global fields of the 3-dimensional
temperature-moisture structure of the atmosphere. In addition to
profiles of temperature and moisture, the HIRS2/MSU data are used to
derive important quantities such as land and sea surface temperature,
outgoing longwave radiation, cloud fraction, cloudtop height, total
ozone overburden and precipitation estimates.
The Path A system steps through an interactive
forecast-retrieval-analysis cycle. In each 6 hour synoptic period, a
2nd order General Circulation Model (Takacs et al., 1994) is used to
generate the 6 hour forecast fields of temperature and humidity. These
global fields are used as the first guess for all soundings occuring
within a 6 hour time window centered upon the forecast time. These
retrievals are then assimilated with all available insitu measurements
(such as radiosonde and ship reports) in the 6 hour interval using an
Optimal Interpolation (OI) analysis scheme developed by the Data
Assimilation Office of the Goddard Laboratory for Atmospheres. This
analysis is then used to specify the initial conditions for the next 6
hour forecast, thus completing the cycle.
The retrieval algorithm itself is a physical method based on the
iterative relaxation technique originally proposed by Chahine
(1968). The basic approach consists of modifying the temperature
profile from the previous iteration by an amount proportional to the
difference between the observed brightness temperatures and the
brightness temperatures computed from the trial parameters using the
full radiative transfer equation applied at the observed satellite
zenith angle. For the case of the temperature profile, the updated
layer mean temperatures are given as a linear combination of
multichannel brightness temperature differences with the coefficients
given by the channel weighting functions. Constraints are imposed upon
the solution in order to ensure stability and convergence of the
iterative process. For more details see Susskind et al (1984).
There are 6 level 3 data product files, each of which is in the HDF
format and each representative of a different averaging time
period. All files contain the same number of geophysical parameter
arrays stored as HDF Scientific Data Sets (SDSs). The time periods
include daily, 5 day (pentad) and monthly, with the AM and PM portions
of the orbits treated separately. All data are mapped to a 1 degree
longitude by 1 degree latitude global grid. 
