ATMOS stratospheric deuterated water and implications for troposphere-stratosphere transport

Abstract. Measurements of the isotopic composition of stratospheric water by the ATMOS instrument are used to infer the convective history of stratospheric air. The average water vapor entering the stratosphere is found to be highly depleted of deuterium, with 6Dw of-670 + 80 (67 % deuterium loss). Model calculations pre-dict, however, that under conditions of thermodynamic equilibrium, dehydration to stratospheric mixing ratios should produce stronger depletion to 6Dw of-800 to-900 (80-90 % deuterium loss). Deuterium enrichment of water vapor in ascending parcels can occur only in conditions of rapid convection; enrichments persisting into the stratosphere require that those conditions con-tinue to near-tropopause altitudes. We conclude that either the predominant source of water vapor to the up-permost troposphere is enriched convective water, most likely evaporated cloud ice, or troposphere-stratosphere transport occurs closely associated with tropical deep convection