Validation of Upper Tropospheric Humidity Inferred from the Water Vapor Channel of Geostationary Satellites for the Summer Season in the East Asian Region

Upper Tropospheric Humidity (UTH), the mean relative humidity for the layer from 500 to 200 hPa, can be retrieved from data of the 6.7 mu m water vapor channel of meteorological satellites. The present study aims to assess the validity of the UTH retrieval algorithm, which will be incorporated into the data processing system of Communication, Ocean and Meteorological Satellite (COMS), for the summer season in the East Asian region. We analyze the algorithm parameters and the coefficients for the water vapor channels of Geostationary Meteorological Satellite-5 (GMS-5) and Geostationary Operational Environmental Satellite-9 (GOES-9) using the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis data. The coefficients applicable to data of the COMS water vapor channel are also determined. As COMS is not operating yet, the brightness temperatures of GMS-5 and GOES-9 have been used for the validation of the algorithm. UTH values calculated from radiosonde profiles of upper-air stations in the region are compared with UTHs retrieved from collocated satellite data. When the ECMWF-based coefficients are used, the root-mean-square difference between the retrieved and the measured UTHs is about 5.3 and 7.5%, respectively, for GOES-9 and GMS-5. However, the retrieved UTHs from GMS-5 data tend to be larger than those of radiosonde data. It has been noticed that brightness temperatures of GMS-5 are different from those calculated by the radiative transfer model. Thus, we have determined the algorithm coefficients using GMS-5 data and UTHs of radiosonde profiles. This improves the performance of the algorithm considerably; the root-mean-square difference decreases to about 4.3%. The results suggest that determining the algorithm coefficients from satellite data and in situ measurements of UTH is a practical measure of the UTH algorithm, if brightness temperatures of the water vapor channel behave differently, like GMS-5, from those of theoretical calculations.