Dehydration of the stratosphere

Abstract. Domain filling, forward trajectory calculations are used to examine the global dehydration processes that control stratospheric water vapor. As with most Lagrangian models of this type, water vapor is instantaneously removed from the parcel to keep the relative humidity with respect to ice from exceeding saturation or a specified super-saturation value. We also test a simple parameterization of stratospheric convective moistening through ice lofting and the effect of gravity waves as a mechanism that can augment dehydration. Comparing diabatic and kinematic trajectories, we find, in agreement with previous authors, that the additional transport due to the vertical velocity "noise" in the kinematic calculation creates too dry a stratosphere and a too diffuse a water-vapor tape recorder signal compared observations. The diabatic simulations, on the other hand, produce stratospheric water vapor mixing ratios very close to that observed by Aura's Microwave Limb Sounder. Convective moistening, which will increases stratospheric HDO, also increases stratospheric water vapor while gravity waves do the opposite. We find that while the Tropical West Pacific is the dominant dehydration location, dehydration over Tropical South America is also important. Antarctica also makes a contribution to the overall stratospheric water vapor budget by releasing very dry air into the Southern Hemisphere stratosphere following the break up of the winter vortex.