Global characterization of atmospheric hydrology with HDO measurements from Aura

The history of condensation and evaporation processes is captured by the isotopic composition of atmospheric water. While the analysis of the isotopic measurement from a global standpoint provides new insight to the role of the hydrologic cycle in climate, the richness of the measurements lies in the fact that the isotopes are indicative of processes rather than atmospheric state. Isotopic methods have been used extensively in previous studies to understand continental hydrology and specifically estimate recycling of water between the atmosphere and landscapes. Similarly the use of water isotopes in determining the sources of atmospheric water has been endorsed widely based on global model calculations. The isotopic composition of precipitation has been monitored since the 1950 globally, yet the existing vapor isotope datasets are inadequate to allow source estimation at anything but local scales and largely inadequate for examining the hydrology of the troposphere at large. New measurements from the Tropospheric Emission Spectrometer (TES) aboard the NASA Aura spacecraft address this need by providing estimates of vapor H2O and HDO in the lower and mid troposphere. Here, we use over 5000 observations scattered globally from 9 days of nadir observations taken in November 2004 and January 2005. The retrieved HDO and H2O profiles are averaged between 850 hPa and 400 hPa to attain a typical precision is approximately 1%, which translates to an error in HDO delta values of around 15 permil. The data show greater depletion toward the polar regions and is characteristic of preferential removal of heavy nuclides during condensation as water vapor moves pole-ward. These observations are consistent with precipitation measurements that also show a latitudinal gradient. In the extra-tropics we contrast results from cloud free profiles with low humidity (where evaporation from the ocean surface is more dominant) with those having clouds and high humidity (where condensations process are active). Because the isotopic signature associated with condensation differs from that of evaporation the new measurements clearly track the global hydrologic cycle as a function of the overturning atmospheric circulation From simple isotopic arguments, the tendency of the data cluster around some mean specific humidity and isotopic composition provides a quantification of the residence time of atmospheric water - one metric of the intensity of the hydrologic cycle. The distribution of points allows estimation of evaporation source location and an assessment of the range of cloud processes active globally. In the tropics, observation show more depletion than can be explained by simple distillation, and is evidence of water recycling by clouds. Specifically, the isotopic composition observed near tropical convection can only be explain if the cloud system is sustained by evaporation from the ocean surface as well as evaporation of falling precipitation. From mass balance, the recycled fraction is between 30-60% The need for evaporation from the ocean surface to explain the isotopic composition gives insight into the interaction between the energetics of tropical storm systems and ocean surface heat content, and thereby the role of such cloud systems in climate.