Cirrus and water vapor transport in the tropical tropopause layer: a modeling study

Abstract. We carry out simulations of tropical-tropopause-layer (TTL) cirrus which are formed and forced by a large-scale equatorial Kelvin wave. We find that, even under the influence of the large-scale wave, the radiatively induced mesoscale dynamics in TTL cirrus actively contributes to transport of water vapor in the vertical direction. In a typical TTL cirrus, the heating that results from absorption of radiation by ice crystals induces a mesoscale circulation. Advection of water vapor by the radiatively induced circulation leads to upward advection of the cloudy air. Upward advection of the cloudy air is equivalent to upward transport of water vapor when the air above the cloud is drier than the cloudy air. On the other hand, ice nucleation and depositional growth, followed by sedimentation and sublimation lead to downward transport of water vapor. The net direction of transport is determined by the relative magnitudes of the upward advection of water vapor and the downward transport associated with microphysical processes.