Investigating the role of different processes on the water vapor budget in the stratosphere by modelling HDO

Abstract

The water isotopologue HDO is included into the hydrological cycle of the ECHAM/MESSy Atmospheric Chemistry (EMAC) model to investigate the respective impacts of the processes controlling the water vapor budget in the stratosphere. During phase transitions and during chemical reactions in the stratosphere (i.e. CH4-oxidation) water faces equilibrium and kinetic isotope-fractionation processes. This leaves an isotopic fingerprint in the HDO/H2O-ratio (i.e. the _D-value), conserving information about the process-based history in the water compound. Hence this additional acquirement allows us to study the relative importance of a) direct injection of water and ice into the stratosphere by overshooting convection, b) dehydration of air in the Tropical Tropopause Layer during ascent by cirrus cloud formation and c) the in-mixing of older stratospheric air which has experienced H2O and HDO formation from CH4-oxidation. A comparison of the model results with satellite observations from the MIPAS instrument of the stratospheric ‘tape-recorder’-signal in both, the HDO and the _D-value, will be presented. Moreover we analyze the effect of different convection schemes on the results. This sensitivity study provides an estimate of the uncertainty range of the impact of overshooting convection on the water vapor budget in the lower stratosphere which is under debate of playing a crucial role.