System Noise in the NESDIS TOVS Forward Model. Part I: Specification

Abstract

The National Environmental Satellite Data and Information Service (NESDIS) collocation data archive of satellite and radiosonde measurements is used to investigate errors in in situ radiosonde data and a NESDIS-like radiative transfer forward model. It is shown that the radiative transfer model errors have a strong airmass dependence and that these errors are not primarily due to nonrepresentativeness or radiosonde errors. However, errors in the in situ data do exist. For example, the National Meteorological Center radiosonde radiation adjustment algorithm in use during the period of data collection (1989?90) does not appear to provide adjustments of uniform quality across radiosonde sounding systems. The total system noise appropriate for use in retrieval algorithms is shown to vary from values close to the radiometer noise equivalent temperature difference (NE?T) specifications for stratospheric channels to several times the NE?T values for lower-tropospheric channels. Because of the significant discrepancies between measured and modeled radiances for the two most opaque water vapor sounding channels of the TIROS Operational Vertical Sounder, their use in a physical retrieval algorithm is considered problematic. Evidence for errors in NESDIS cloud-cleared (equivalent clear column) radiance temperature estimates is presented.