Microwave atmospheric sounding

Microwave atmospheric sounding is the passive-microwave method used to retrieve atmospheric temperature and humidity vertical profiles from oxygen and water-vapour absorption-band emission. It is for atmospheric profiling when the question is vertical structure of the atmosphere, including data-assimilation inputs for numerical weather prediction, rather than surface imaging or precipitation mapping.^[1][2]

The method depends on cross-track or nadir microwave sounders with oxygen-band channels for temperature structure and water-vapour channels around 183.31 GHz for humidity profiles. ATMS is a 22-channel cross-track sounder using 54 GHz and 183 GHz bands, while MWS extends the pattern to 24 channels from 23.8 to 229 GHz, including oxygen-band and 183.31 GHz channels.^[3][4] This makes the methodology distinct from passive microwave imaging radiometry: the defining feature is absorption-band sounding for profiles, not broad surface-brightness imaging.^[3][4]

Its operating advantage is day-night profile retrieval in many cloudy conditions. ATMS provides atmospheric temperature and moisture information and supports three-dimensional atmospheric profiles through clouds, and MWS is specified for temperature and humidity profile information in clear and cloudy regions.^[1][2] The method still has coarse spatial and vertical resolution compared with imaging missions, and retrieval quality is constrained by precipitation and cloud-ice contamination.

Demonstrated implementations include the AMSU-A temperature sounder, AMSU-B and MHS humidity sounders, ATMS, MWS, FY-3 MWTS and MWHS variants, SAPHIR, and SSMIS-class combined imager-sounder instruments.^{[5][6][7][8][9][10]} SAPHIR narrows the pattern to tropospheric water-vapour profiling with six 183 GHz channels, while SSMIS combines imager and sounder functions with temperature and humidity channels in the 54 GHz and 183 GHz bands.^[9][11][10]