UARS Microwave Limb Sounder upper tropospheric humidity measurement - Method and validation

The methodology and validation of upper tropospheric humidity (UTH) measurements by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are described. The MLS instrument was not specifically designed to measure UTH, but its measurement is achieved by modeling the radiance signal from a window channel as the instrument field of view is scanned through the troposphere. UTH is retrieved as a piecewise-linear function in relative humidity with respect to ice (percent RHi) with breakpoints at 147, 215, 316, and 464 hPa. MLS version 4.9 is believed to be the most accurate version to date; its estimated global-averaged accuracy(precision) is 23(21), 22(10), 22(8), and 50(19) percent RHi at 147, 215, 316, and 464, hPa, respectively. The validation of UTH is established by its consistency with (1) climatological fields, (2) coincident comparisons with in situ Vaisala RS80 balloon sondes, frostpoint hygrometers and Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft measurements, (3) cirrus observations, and (4) meteorological dynamics and assimilation fields. MLS climatological UTH is morphologically consistent with Stratospheric Aerosol and Gas Experiment II and Vaisala zonal climatologies. Comparisons with coincident in situ data show a wide range of agreement. (Author)