ECMWF analyses of humidity: comparisons to POLDER estimates over land

The POLarization and Directionality of Earth Reflectances (POLDER) mission provided satellite observations of Total Column Water Vapour (TCWV) over land, in clear sky conditions, between November 1996 and June 1997. For this study, POLDER TCWV estimates are corrected for the calibration and surface biases pointed out previously by validation work. ECMWF analyses of humidity are then compared to POLDER fields on a daily basis and on monthly average. They appear in good agreement with POLDER over most Northern Hemisphere continents except the Asian plateau showing significant discrepancies in June. The major differences appear in tropical regions where few radiosondes are available, especially over the eastern part of Africa with persistent excess of moisture between March and May and over South America with more variable situations. Differences appear mainly off the maximum intensity of the Inter Tropical Convergence Zone (ITCZ) showing a problem of the forecasting system in positioning the convective activity when it is not forced by appropriate observations. The position and intensity of discrepancies vary along the year depending on the seasonal variations of the ITCZ and on how dense the radiosonde network is in convective areas.