Relation between METEOSAT Water Vapor Radiance Fields and Large Scale Tropical Circulation Features

Abstract

Mean monthly images from the water vapor channel of METEOSAT characteristically contain large-scale spatial structures, especially in tropical regions. The aim of this paper is to establish connections between these structures and large-scale circulation features. For this purpose, statistical relationships between radiances and some meteorological parameters provided by ECMWF analyses are studied. Temporal correlations are computed for two sizes of regions, in order to compare temporal changes associated with both large-scale circulations and smaller scale systems. The correlations obtained are poor, suggesting that the chosen parameters are not well related at short time scales. Temporal averages appear more suitable for these comparisons. As expected, the mean relative humidity yields the best correlation with the mean water vapor radiances. A (weaker) relationship exists also with mean dynamic fields: large water vapor radiances are almost always related to subsidence in the middle troposphere, divergence near the surface, and convergence in the upper troposphere. However, there is regional variability in the results., one explanation may be different contributions from horizontal advecion and vertical motions to the humidity of the middle troposphere.