Moistening processes in the tropical upper troposphere observed from Meteosat measurements

In order to investigate processes of moistening the upper troposphere (UT), we examined Lagrangian evolution of deep convection index, cloudiness, UTH tendency, and condensation/evaporation rate of the convective cloud system observed by Meteosat‐8 infrared measurements over tropical Africa and the Atlantic Ocean. Condensation/evaporation rates were inferred from the imbalance between cloud expansion rate and wind divergence which were directly determined from the Meteosat‐8 measurements. It was found that wind divergence corresponds much more with UTH tendency, in comparison to the poor correlation found between evaporation rate of cloud condensates and UTH tendency. Because water vapor concentrations in the UT are nearly an order of magnitude larger than ice water concentrations, the transport of moist air associated with the formation and vertical growth of clouds and the subsequent upper‐tropospheric divergence should be primarily responsible for moistening the UT. On the other hand, the role of evaporation of detrained hydrometeors from the convection center appears to be secondary, as indicated by less coherent relationship between UTH tendency and evaporation rate, and the much smaller mass of ice water relative to water vapor. Results strongly support the notion that the UT is predominantly moistened by water vapor advected to drier surroundings from the diverging cloud top areas, in association with vertical convergent transport of water vapor which is also responsible for the formation and maintenance of cirrus clouds.