In situ water vapour measurement, and a case study of the drying mechanism of the tropical lower stratosphere

During an experiment performed in tropical regions, water vapour content was measured in the lower stratosphere, using long duration balloons. A set of profiles show a vertical structure consistent with what has been named the tape‐recorder effect. One of the results, when the balloon descends to 100 hPa, shows a value of the water‐vapour mixing ratio as low as 2.8 parts per million by volume, in a layer approximately 1 km thick at about 80 hPa. This layer is delimited by two minima of air temperature, suggesting that the drying is a consequence of a vigorous convective process. This kind of process can be so strong that the convective cloud overshoots the tropopause and forms an anvil of ice crystals which grow and finally fall under gravity. Some of the characteristics of the METEOSAT cloud imagery and of the ECMWF analyses could be interpreted as the signature of this type of convective process. Nevertheless one cannot discard other mechanisms, such as the generation of stratospheric buoyancy waves by tropical convection. These waves can cross the tropopause and form thin ice clouds in the lower stratosphere. In this hypothesis, also, the ice falls and transports moisture downwards.