Retrieval of upper stratospheric and mesospheric temperature profiles from Millimeter‐Wave Atmospheric Sounder data

Abstract

The Millimeter‐Wave Atmospheric Sounder (MAS) is a shuttle‐based instrument, observing the atmosphere in limb sounding geometry. Temperature information is derived from three oxygen lines near 60GHz. Temperature profiles in the altitude range from 30 to 90 km can be measured. The data evaluation is done with the aid of an atmospheric radiative transfer model. Usually, the radiation is assumed to be unpolarized, so that spectral power is a scalar quantity. This assumption is not valid for oxygen lines that show Zeeman splitting in the Earth's magnetic field. A more general form of the radiative transfer in which spectral power is represented by a vector has to be used in this case. The obtained information depends on the magnetic field. Using the MAS instrument characteristics, we investigate the possible temperature information that can be obtained from observation of polarized O2 emission lines with a spaceborne instrument. Synthetic retrievals show that the obtained accuracy of the retrieved temperature profile is below 3 K from 30 to 55 km and below 6 K for altitudes up to 90 km. The resolution is 4 km for altitudes of 30 to 50 km and 10 km above. The magnetic parameters influence the accuracy of the obtained profile by as much as 2 K. The obtained MAS temperature profiles were validated by a comparison to data from the Upper Atmosphere Research Satellite (UARS). Three UARS instruments were chosen: Microwave Limb Sounder, Improved Stratospheric and Mesospheric Sounder, and the Cryogenic Limb Array Etalon Spectrometer. All comparisons show similar features for the stratospheric and mesospheric temperatures.