1. Data Set Information
This dataset contains the TIROS Operational Vertical Sounder (TOVS) level 3 geophysical parameters derived using the classification method of Chedin and Scott (1985) and processed by the Atmopheric Radiation Analysis (ARA) group of the Laboratoire de Meteorologie Dynamique (LMD) du CNRS (France). This method, which isa priori data-dependent but model-independent, was designated as the Path B scheme by the TOVS Pathfinder Science Working Group. Two of the three sounding instruments aboard the NOAA-xx series of Polar Orbiting Satellites are used to produce global, 1 degree by 1 degree fields of the 3-dimensional temperature-moisture structure of the atmosphere, namely, the 20 channel High resolution Infrared Radiation Sounder 2 (HIRS2) and the 4 channel Microwave Sounding Unit (MSU). In addition to profiles of temperature and moisture, the HIRS2/MSU radiances are used by the Path B methodology to derive surface skin temperature, surface microwave emissivity, cloud fraction, and cloudtop height and temperature.
The primary goal of Pathfinder was the production of consistently processed, long-term data sets for use in global change studies. TOVS Pathfinder is one of several Pathfinder studies initiated by NOAA and NASA (the others being AVHRR, GOES, and SSM/I Pathfinders). As a first step in the processing, a common benchmark period was selected to facilitate the analysis and intercomparison of geophysical fields derived from the various Pathfinders. The benchmark period covers the time period April 1987 through November 1988. For the TOVS Pathfinder case, three distinct methodologies were implemented by different groups, termed the Path A, Path B and Path C methodologies. The Path A (Susskind et al, 1997) technique involves the use of both a priori data and the output from a forecast/analysis model, the Path B technique (Chedin et al., 1985) relies on a library of archived atmospheric situations and the Path C technique (Spencer and Christy, 1992a, 1992b, 1993, Goldberg and Flemming, 1995) requires no a priori or model output information but is limited to estimating only coarse layer mean temperatures.
The basic premise behind the Path B technique involves deriving geophysical parameters from TOVS radiance data in a manner which is independent of the output from any hydrodynamic model forecast or analysis. It is based upon the Improved Initialization Inversion (3I) algorithm of Chedin et al.(1985), which relies on a pattern recognition approach to relate cloud-corrected radiances to geophysical parameters. The inversion of the radiative transfer equation comprises two steps, which each take satellite measurements as their data sources. The first step performs a library search to obtain a best initial profile out of a large selection of archived atmospheric situations classified in groups according to latitude and season.This library of atmospheres, called the Thermodynamic Initial Guess Retrieval (TIGR) data set, consists of 1800 situations selected by statistical methods out of 150,000 samples. Transmittances, radiances, and weighting functions for all sounding channels have been precalculated for each profile using the 4A line-by-line model of Scott and Chedin (1981).Calculations have been performed for multiple viewing angles, surface pressures, and surface types (land or sea). The optimal initial profile is that profile which minimizes the overall deviation of the cloud-corrected, observed brightness temperatures with the precomputed brightness temperatures stored in the TIGR database.
The second step performs the inversion of the radiative transfer equation by means of a Bayesian-type direct estimation method (maximum a posteriori probability) which aims at minimizing the deviation between the observations and the initial profile. See Chedin et al. (1985) for further details on the inversion methodology and the technique used to correct the observed radiances for the effects of clouds.
TOVS (before AIRS) was the only long-term source of high resolution global information pertaining to the temperature and moisture structure of the atmosphere. Because similar HIRS/2 and MSU instrumentation has flown on operational satellites from 1979 to the present, data from these instruments can make an important contribution to the understanding of the variability of atmospheric and surface parameters as well as the correlations between spatial variations of atmospheric and surface quantities. In addition, the data can potentially be used to identify and monitor trends in temperature, moisture, clouds, radiation, and precipitation, provided satellite drifts and cross-calibration of different satelites are taken into account. The full TOVS Pathfinder data set, of which the NOAA-10 benchmark period is a subset, is processed in a consistent manner and as such will be useful for all of the applications listed above.
The full directory tree on the FTP server reflects which satellites were used in the Path-B processing, and thus have the patterns:
TOVSB [D,5,M] [NG,ND]
[D,5,M] - stands for Daily, 5-day (pentad), and Monthly
[NG,ND] - which satellite, correspondingly: TIROS-N, NOAA-G,D (see history)