Abstract
Stable Water Isotopologues (SWIs) are important diagnostic tracers for understanding processes in the atmosphere and the global hydrological cycle. Using eight years (2002–2009) of retrievals from Odin/SMR (Sub-Millimetre Radiometer), the global climatological features of three SWIs, H216O, HDO and H218O, the isotopic composition δD and δ18O in the stratosphere are analysed for the first time. Spatially, SWIs are found to increase with altitude due to stratospheric methane oxidation. In the tropics, highly depleted SWIs in the lower stratosphere indicate the effect of dehydration when the air comes through the cold tropopause, while, at higher latitudes, more enriched SWIs in the upper stratosphere during summer are produced and transported to the other hemisphere via the Brewer–Dobson circulation. Furthermore, we found that more H216O is produced over summer Northern Hemisphere and more HDO is produced over summer Southern Hemisphere. Temporally, a tape recorder in H216O is observed in the lower tropical stratosphere, in addition to a pronounced downward propagating seasonal signal in SWIs from the upper to the lower stratosphere over the polar regions. These observed features in SWIs are further compared to SWI-enabled model outputs. This helped to identify possible causes of model deficiencies in reproducing main stratospheric features. For instance, choosing a better advection scheme and including methane oxidation process in a specific model immediately capture the main features of stratospheric water vapor. The representation of other features, such as the observed inter-hemispheric difference of isotopic component, is also discussed.
Highlights
Stratospheric water vapor plays a critical role in the climate system
The dry bias seems to be a systematic bias in all Global Climate Models (GCMs). Another difference between Exp1 and the observed values is the large values in the lower stratosphere in winter hemisphere over high latitude regions, which are apparent in Odin/Sub-Millimetre Radiometer (SMR) (Figure 4a,b) but do not appear in the LMDZ4strato simulation (Figure 9a,b), where the large values are replaced by low values in the JJA mean (Figure 9b)
We have explored eight years of measurements obtained from Odin/SMR, by investigating the climatological features of stratospheric SWIs
Summary
Stratospheric water vapor plays a critical role in the climate system. Changes in stratospheric water vapor affect the fluxes of longwave (infrared) and shortwave (solar) radiation, and thereby influence the temperature in the stratosphere and troposphere [1,2]. The measurements of normal and deuterated water from the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) were used to map global climatological behavior of HDO along with the HDO/H2O ratio in the upper troposphere- lower stratosphere, but for this dataset measurements were made in the tropics only during February, April, August and October [19]. Odin/SMR SWIs data have much longer time span than, e.g., MIPAS, and higher than, e.g., ACE-FTS, which retrieves HDO from 6.5 to 37.5 km in altitude [21] These invaluable data have not been fully explored to study the stratospheric isotopic structures, atmospheric hydrological cycle and related dynamical processes. GCM simulations show that the distribution of water isotopes is sensitive to the parameterization of cloud physics and the representation of upper tropospheric processes, and are useful in constraining modeled cloud physics and investigating the mechanisms of stratosphere–troposphere exchange of water vapor [22].
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