Satellite Determinations of the Relationship between Total Longwave Radiation Flux and Infrared Window Radiance

Abstract

Nimbus-7 satellite observations are used to determine the relationship between the total longwave radiation flux and the radiance in the 10-12 ?m infrared window. The total longwave fluxes are obtained from the earth radiation budget (ERB) narrow-field-of-view (NFOV) observations of total radiance; the IR window radiances are those measured by the Temperature Humidity Infrared Radiometer (THIR). Regression equations are obtained relating the total flux equivalent brightness temperatures to the radiance equivalent brightness temperature of the IR window. These empirical equations are compared to similar regression equations based on radiative transfer calculations for a large sample of atmospheric soundings. The latter theoretical equations are used by NOAA in the processing of IR window observations from operational polar orbiting satellites to obtain total longwave flux estimates. The observational results indicate that there is a very high correlation between the flux equivalent brightness temperature and the IR window radiance equivalent brightness temperature, and that the former can indeed be determined from measurements of the latter, thus validating the general NOAA approach. Tests on independent data suggest that rms flux errors of ?11 w m?2 are to be expected for single applications of the empirical equations. The theoretical equations used by NOAA have an average positive bias of ?13 wm?2 or a relative bias of ?6% with respect to the ERB NFOV observations; the relative bias disappears at high flux values and increases with decreasing flux. A preliminary attempt to determine the cause of the discrepancy between the empirical and theoretical results indicates that a major factor may be the unrepresentativeness of the atmospheric soundings used in developing the theoretical regression coefficients.