The SPARC Water Vapor Assessment II: assessment of satellite measurements of upper tropospheric humidity Atmospheric Measurement Techniques

Abstract. Nineteen limb-viewing data sets (occultation, passive thermal, and UV scattering) and two nadirupper tropospheric humidity (UTH) data sets are intercompared and also compared to frost-point hygrometerballoon sondes. The upper troposphere considered here covers the pressure range from 300–100 hPa.UTH is a challenging measurement, because concentrationsvary between 2–1000 ppmv (parts per million by volume), with sharp changes in verticalgradients near the tropopause. Cloudiness in this regionalso makes the measurement challenging. The atmospheric temperature is also highlyvariable ranging from 180–250 K. The assessment of satellite-measured UTHis based on coincident comparisons with balloon frost-point hygrometersondes, multi-month mapped comparisons, zonal mean time series comparisons, andcoincident satellite-to-satellite comparisons. While the satellite fields showsimilar features in maps and time series, quantitatively they can differ by afactor of 2 in concentration, with strong dependencies on the amount of UTH.Additionally, time-lag response-corrected Vaisala RS92 radiosondes arecompared to satellites and the frost-point hygrometer measurements. In summary,most satellite data sets reviewed here show on average ∼30 % agreementamongst themselves and frost-point data but with an additional ∼30 %variability about the mean bias. The Vaisala RS92 sonde, even with a time-lagcorrection, shows poor behavior for pressures less than 200 hPa.