The Role of Monsoon Convection in the Dehydration of the Lower Tropical Stratosphere

Observations by radiosondes, satellites, and aircraft have shown that a minimum in water vapor mixing ratio in the lower tropical stratosphere (typically around 19-km altitude) is a climatological feature of the global water vapor distribution. The processes responsible for the formation and maintenance of this minimum are examined with the aid of a mesoscale dynamical model of tropical convection that includes bulk ice microphysics, radiative transfer, and surface processes. Model results suggest that convectively generated buoyancy waves may play an important role in dehydrating the tropical lower stratosphere. Vertical parcel displacements produced by buoyancy waves promote the formation of thin ice clouds in the lower stratosphere. Such clouds formed upwind of the convective region in all simulations where ice was allowed to form. When ice was allowed to precipitate, there was a decrease of ≈0.3 ppmm in total water mixing ratio at 19 km after a 30-h simulation. It is concluded that thin cirrus clouds produced by buoyancy waves may contribute significantly to the formation and maintenance of the observed water vapor minimum in the lower tropical stratosphere.