Warm water vapour pixels over high clouds as observed by METEOSAT

Deep convective clouds are known to play an important role in the interaction and exchange between troposphere and stratosphere. Simultaneous observations of these clouds in the infrared window region and the water vapour absorption band, reveal that the equivalent brightness temperature in the latter is larger than in the former. The larger brightness temperatures in the absorption band are caused by the presence of stratospheric water vapour, which absorbs radiation from the cold cloud top and emits radiation at stratospheric temperatures, which are normally higher than at the top of the troposphere. In addition to stratospheric temperature the brightness temperature difference depends on the amount of stratospheric water vapour and on the actual location of the cloud top with respect to the tropopause. With a constant stratospheric water vapour content, the differences increase when the cloud top approaches the tropopause and decrease again if the cloud penetrates into the stratosphere. Hence, in deep-convective areas it is possible to estimate the tropopause temperature from the simultaneous observations in the infrared window and the water vapour band in successive images.