Determination of Atmospheric Water Vapour Isotopic Composition using Multi-Platform Instruments and Models over Ethiopia: Implications for Water Cycle

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Addis Ababa University


Coupled stable isotopes of hydrogen and oxygen ( D and 18O) in water vapour, as well as in precipitation, represent valuable tools for quantifying the atmospheric processes in water cycle. Various physical processes associated with the state of phase changes of water potentially impart unique signal on the integrated isotopic fractionation of water vapour in the atmosphere. For instance, atmospheric processes such as condensation, evapo-transpiration and transport can be characterized quantitatively and qualitatively using Rayleigh distillation curves of D versus humidity. In this study, abundances of H2O and D determined from various satellites, models, and Fourier transform infrared spectrometer (FTIR) at Addis Ababa, Ethiopia are used to evaluate isotopic fractionation over Ethiopia and assess water vapour budget and cycle in the region. Volume mixing ratio (VMR) pro les of H2O and D are simultaneously retrieved from FTIR solar absorption spectra during June, 2009 to March, 2013. Fifteen spectral microwindows in the region between 2600 to 3200 cm􀀀1 are used to determine their pro les. For the peculiar features of the remote sensing product of water vapour isotopologues, it is essential applying aposteriori corrections to ensure that both H2O and D products are representative of the same air mass and also minimize the cross-dependence between them. Detailed error analysis of the retrieved species are also performed. H2O VMR pro les and integrated column amounts from FTIR are compared with the coincident satellite observations of Tropospheric Emission Spectrometer (TES), xiii xiv Atmospheric Infrared Sounding (AIRS) instruments, and Modular Earth Submodel System (MESSy) model. The mean relative di erences in H2O pro les of FTIR with TES and MESSy are generally lower than 27% within the altitude range of 3.6 and 8.9 km, whereas di erence from AIRS is lower than 45%. The mean relative di erences of integrated column amounts are within +3.5 to +15.4% for FTIR versus TES, whereas -9.4 to -28.6% for FTIR versus MESSy and AIRS. The corresponding standard deviations are within 21.7 to 33.6% among them. Thus, the retrieved H2O VMR and column amounts from a tropical site, Addis Ababa, is found to exhibit a general agreement with these instruments and model. Spatio-temporal variability of isotopic composition of water vapour using TES observations are examined over Ethiopia. The seasonal variation of D in vapour composition of the region is mainly controlled by di erences in sources of moisture owing to the seasonal movement of the ITCZ and local factors such as amount and temperature e ects. The role of atmospheric processes that contribute to the seasonal variability of isotope composition of water vapour at 682 and 510 hPa pressure levels over our site are characterized by tting the Rayleigh distillation curves of D versus H2O. Enrichment characteristics are identi ed at 682 hPa level in all seasons, which is likely caused by surface in uence. In addition, D vapour that falls below the Rayleigh curve is generally associated with moisture recycling in convective clouds, and this could partly describe some observations in summer season. On the other hand, D vapour that falls above the Rayleigh curve is typically associated with advective mixing which explains a large number of observations during Spring and Autumn seasons. However, winter observations are strongly in uenced by moisture mixing process. Further analysis of isotopic composition of water vapour from IsoGSM model follows the known seasonal cycles observed in precipitation from earlier studies suggesting that the dominant mode of water vapour variability is governed mainly by large scale climate system



Ethiopia Water Vapour Isotopic Composition