Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 5 2008 10.5194/acpd-8-17907-2008 http://www.atmos-chem-phys-discuss.net/8/17907/2008/ http://www.atmos-chem-phys-discuss.net/8/17907/2008/acpd-8-17907-2008.html http://www.atmos-chem-phys-discuss.net/8/17907/2008/acpd-8-17907-2008.pdf 17907 17937 2008-10-01 Evaluation of upper tropospheric humidity forecasts from ECMWF using AIRS and CALIPSO data N. Lamquin nicolas.lamquin@lmd.polytechnique.fr K. Gierens C. J. Stubenrauch R. Chatterjee Laboratoire de Météorologie Dynamique, Ecole Polytechnique, Palaiseau, France Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany National Institute of Technology, Tiruchchirappalli, Tamil, Nadu, India An evaluation of the upper tropospheric humidity from the European Centre of Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) is presented. We first make an analysis of the spinup behaviour of ice supersaturation in weather forecasts. It shows that a spinup period of at least 12 h is necessary before using forecast humidity data from the upper troposphere. We compare the forecasted upper tropospheric humidity with coincident relative humidity fields retrieved from the Atmospheric InfraRed Sounder (AIRS) and with cloud vertical profiles from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). The analysis is made over one year, from October 2006 to September 2007, and we discuss how relative humidity and cloud features appear both in the IFS and in the observations. In a last part, we investigate the presence of ice supersaturation within low vertical resolution pressure layers by comparing the IFS outputs for high-resolution and low-resolution humidity profiles and by simulating the interpolation of humidity over radiosonde data. A new correction method is proposed and tested with these data. 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