A clear-sky radiance archive from Meteosat "water vapor" observations

A long- term archive of clear- sky Meteosat "water vapor'' observations, covering the July 1983 to February 1997 period with a 3 hourly time step and a spatial resolution of 0.625 degrees, is presented. Cloud clearing is performed using a scene selection procedure based on the International Satellite Cloud Climatology Project DX product. In this procedure low cloud scenes are kept because of the negligible contribution of the low atmospheric layer in this spectral band. Cloud contamination is shown to have little influence on the clear- sky radiance ( CSR) field and is mainly confined to the continental Intertropical Convergence Zone with values less than 0.5 K. This scene selection yields to a significantly enhanced sampling with respect to pure clear- sky in the subtropical high regions. Homogenization of the 14 year database is performed in accordance with existing technique. A comparison to the operational radiosondes archive indicates a small bias of 0.3 K that is stable throughout the period. A first analysis of the CSR variability reveals that the intraseasonal variance over the subtropical dry regions has a strong seasonal cycle in the Northern Hemisphere that is not observed in the Southern Hemisphere. Such a data set completes the ones currently available to document the water vapor variability of the troposphere from climatic down to regional and daily scales.