Dehydration in the tropical tropopause layer of a cloud-resolving model

Dehydration in the tropical tropopause layer (TTL) is studied using a three-dimensional cloud-resolving model. A horizontally homogeneous slow ascent representative of the upward branch of the Brewer--Dobson circulation is imposed and the model is run to radiative-convective equilibrium. The simulated fluxes of mass and heat within the TTL are strongly dominated by the slow ascent. Furthermore, the slow ascent dominates in transporting moisture across the cold point into the stratosphere. The final stage of dehydration within the TTL, which sets the moisture content of air entering the stratosphere, takes place outside of cumulus clouds and is enhanced by small-scale, convectively generated buoyancy waves. Convective overshoots into the TTL rather hydrate than dehydrate the TTL in the simulation.

Additional dehydration occurs when wave-like perturbations typical of observed Kelvin waves are superposed to the slow ascent. This additional dehydration is achieved through a substantially modified cloud structure in the TTL due to those waves.