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General Information

Although output products from the TOVS Pathfinder Path A and Path B data sets are based upon measurements from the same instruments (HIRS/2 and MSU), have identical spatial and temporal resolutions, and are structuredusing the same Hierarchical Data Format (HDF) file specifications, fundamental differences exist in the methodologies used to determine the geophysical parameters.

The primary difference between the two methods hinges upon the derivation of the first guess fields to be used as input to the retrieval code. In short, the Path A methodology relies upon both a-priori and hydrodynamic model information while the Path B approach makes use only of a-priori information as described below. In addition, there are differences in how the observed radiances are corrected for the effects of clouds, as well as how systematic errors are treated from one satellite to another. Finally, there are some diferences in the interpretation of some of the retrieved parameters (e.g., temperature at a particular level for Path A versus layer mean temperatures for Path B).


Path A

The basic methodology used to retrieve geophysical parameters from the TOVS observations involves use of an interactive forecast-retrieval-analysis scheme in which the first guess information for the retrievals (surface pressure, temperature profile, and moisture profile) comes from a 6 hour forecast generated by a general circulation model. This information, together with the observed radiances, is used to generate retrievals for the 6 hour period centered on the forecast time. An analysis is then performed using these retrievals and all available in situ data for that time period. This is then used as input to the forecast model and the cycle is repeated for the next 6 hour period. The overall TOVS Path A processing flow can be illustrated by the following schematic:

process flow diagram

NOTES:The diagram mistakenly implies that Level 1B SSU data is used, but actually it is not.Also, SBCP is the Systematic Bias Correction Program.

Path B

The first step performs a library search for a best initial guess out of a large selection of atmospheric situations that were archived in advance and classified in groups. This library of atmospheres, termed TIGR (Thermodynamic Initial Guess Retrieval), comprises approximately 1800 representative atmospheric profiles statistically screened from a global set of 150,000 radiosoundings. From the initial profile obtained in this manner, the second step performs the inversion of the radiative transfer equation by means of a Bayesian-type direct estimation method. The overall TOVS Path B processing flow can be illustrated by the following schematic:

process flow diagram


Different Parameters

The designation A,B,or Both in the column labeled Path indicates whether the specified parameter is available in either Path A, Path B or both products. It is important to realize that there are distinctions between the products, even though some of the same parameters are reported in both with the same names.

Path A Parameters


Because the Path A method is dependent upon the output of a hydrodynamic model, retrieved geophysical parameters such as temperature and humidity can be derived at specific levels of the atmosphere, as well as within pressure layers corresponding to the vertical resolving power of the instruments. The Path A method also outputs useful secondary products such as total ozone and outgoing longwave radiation.

Path B Parameters


Since the Path B methodology does not make use of the detailed vertical structure from a general circulation model, temperature and humidity parameters are derived as layer mean quantities throughout the atmosphere. Mean virtual temperature is explicitly included as part of the output products, which in the Path A case can be computed using the specific humidity and temperature values reported at individual pressurelevels. Finally, auxiliary information such as surface emissivity and airmass type are included in the Path B output since these are important quantities characterizing the initial guess fields.

Two-Dimensional Parameters

TSURFBothKSurface skin temperature
FCLDBoth0-1Total cloud fraction
PCLDBothmbCloud top pressure for FCLD
TCLDBothKCloud top temperature for FCLD
ZANGLEBothdegHIRS/2 satellite zenith angle
TIMEBothhrsGMT time of observation
QFLAGAN/AQuality flag (0 is best, 4 is marginal)
FLAGSBN/AProfile rejection criteria
TOZAD.U.Total ozone index
OLRAW/m^2Outgoing longwave radiation
LCRFAW/m^2Longwave cloud radiative forcing
PRECIPAmm/dayPrecipitation estimate
EMISSB0-150 GHz surface emissivity
AIRMASSBN/AAir mass class used during processing
PSURFAmbForecast surface pressure

Three-Dimensional Parameters

VariablePathUnitsDescriptionLayers or Levels (Path A only)
TEMPAKTemperature1000,850,700,500,400,300,200,100,70,50,30 mb
CLTEMPBothKCourse layer temperaturesurface to 500 mb, 500 to 300 mb, 300 to 100 mb, 100 to 30 mb
MTEMPBKMean layer temperature1000 mb to 850 mb,850 to 700 mb, 700 to 500 mb, 500 to 300 mb, 300 to 100 mb, 100 to 70 mb, 70 to 50 mb, 50 to 30 mb, 30 to 10 mb
VTEMPBKMean layer virtual temperature1000 mb to 850 mb,850 to 700 mb, 700 to 500 mb, 500 to 300 mb, 300 to 100 mb, 100 to 70 mb, 70 to 50 mb, 50 to 30 mb, 30 to 10 mb
PRWATBothcmPrecipitable waterabove surface, above 850 mb, above 700 mb, above 500 mb, above 300 mb
FCLDPBoth0-1Cloud fraction above 180 mb, between 310 and 180 mb,440 and 310 mb, 560 and 440 mb, 680 and 560 mb, 800 and 680 mb, below 800 mb
SPHUMAg/kgSpecific humidity 1000 mb, 850 mb, 700 mb, 500 mb, 300 mb
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Last updated: Nov 23, 2009 02:26 PM ET