Atmospheric Chemistry and Physics Cirrus and water vapor transport in the tropical tropopause layer –

Abstract. In a simulation of a tropical-tropopause-layer (TTL) cirrus forced by a large-scale equatorial Kelvin wave, the radiatively induced mesoscale dynamics of the cloud ac-tively contributes to the transport of water vapor in the verti-cal direction. In a typical TTL cirrus, the heating that results from ab-sorption of radiation by ice crystals induces a mesoscale cir-culation. Advection of water vapor by the radiatively induced circulation leads to upward advection of the cloudy air. Up-ward 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 de-positional growth, followed by sedimentation and sublima-tion lead to downward transport of water vapor. Under the conditions specific to our simulation, the up-ward transport of water vapor by the mesoscale circulation dominates the downward transport by microphysical pro-cesses. The net result is upward transport of water vapor, which is equivalent to hydration of the lower stratosphere. Sensitivity to model conditions and parameters will be dis-cussed in a follow-up paper.