Estimation of Stratospheric-Mesospheric Density Fields from Satellite Radiance Data

Abstract

Knowledge of the air density at altitudes above 30 km is needed for such applications as the calculation of space shuttle reentry heating. A method is described for deriving hemispheric (or global) horizontal density fields at 40?60 km directly from radiance maps based on infrared measurements by such instruments as the Nimbus 4 Satellite Infrared Spectrometer and the Selective Chopper Radiometer. Direct regression of air density with the radiance measured in individual channels of these instruments is investigated. From hydrostatic considerations, maximum density-radiance correlation is expected to occur at about 2.5 scale-heights above the level of maximum temperature-radiance correlation; the latter is found near the peak of the transmittance weighting function for each channel. This expectation is substantially verified with the aid of a statistical sample of rocketsonde temperature and density profiles and radiances computed with the appropriate transmittance data. Regression equations are developed for specifying the density with a standard error within 5?7% of the observed density. For the period of a major stratospheric warming in January?February 1973, sample density maps at 50 km are shown, derived from radiance measurements of the NOAA-2 Vertical Temperature Profile Radiometer. These indicate a density increase by more than 50% near the North Pole, from late January to early February.