Determination Of Atmospheric Water Vapour Isotopic Composition Using Multi-platform Instruments And Models Over Ethiopia Implications For Water Cycle

Coupled stable isotopes of hydrogen and oxygen ( D and 18O) in water vapour, asrnwell as in precipitation, represent valuable tools for quantifying the atmospheric processesrnin water cycle. Various physical processes associated with the state of phasernchanges of water potentially impart unique signal on the integrated isotopic fractionationrnof water vapour in the atmosphere. For instance, atmospheric processes such asrncondensation, evapo-transpiration and transport can be characterized quantitativelyrnand qualitatively using Rayleigh distillation curves of D versus humidity. In thisrnstudy, abundances of H2O and D determined from various satellites, models, andrnFourier transform infrared spectrometer (FTIR) at Addis Ababa, Ethiopia are usedrnto evaluate isotopic fractionation over Ethiopia and assess water vapour budget andrncycle in the region. Volume mixing ratio (VMR) pro les of H2O and D are simultaneouslyrnretrieved from FTIR solar absorption spectra during June, 2009 to March,rn2013. Fifteen spectral microwindows in the region between 2600 to 3200 cmÀ€€1 arernused to determine their pro les. For the peculiar features of the remote sensing productrnof water vapour isotopologues, it is essential applying aposteriori corrections tornensure that both H2O and D products are representative of the same air mass andrnalso minimize the cross-dependence between them. Detailed error analysis of thernretrieved species are also performed.rnH2O VMR pro les and integrated column amounts from FTIR are compared withrnthe coincident satellite observations of Tropospheric Emission Spectrometer (TES),rnxiiirnxivrnAtmospheric Infrared Sounding (AIRS) instruments, and Modular Earth SubmodelrnSystem (MESSy) model. The mean relative di erences in H2O pro les of FTIRrnwith TES and MESSy are generally lower than 27% within the altitude range of 3.6rnand 8.9 km, whereas di erence from AIRS is lower than 45%. The mean relativerndi erences of integrated column amounts are within +3.5 to +15.4% for FTIR versusrnTES, whereas -9.4 to -28.6% for FTIR versus MESSy and AIRS. The correspondingrnstandard deviations are within 21.7 to 33.6% among them. Thus, the retrieved H2OrnVMR and column amounts from a tropical site, Addis Ababa, is found to exhibit arngeneral agreement with these instruments and model.rnSpatio-temporal variability of isotopic composition of water vapour using TES observationsrnare examined over Ethiopia. The seasonal variation of D in vapour compositionrnof the region is mainly controlled by di erences in sources of moisture owingrnto the seasonal movement of the ITCZ and local factors such as amount and temperaturerne ects. The role of atmospheric processes that contribute to the seasonalrnvariability of isotope composition of water vapour at 682 and 510 hPa pressure levelsrnover our site are characterized by tting the Rayleigh distillation curves of Drnversus H2O. Enrichment characteristics are identi ed at 682 hPa level in all seasons,rnwhich is likely caused by surface inrnuence. In addition, D vapour that falls belowrnthe Rayleigh curve is generally associated with moisture recycling in convectivernclouds, and this could partly describe some observations in summer season. On thernother hand, D vapour that falls above the Rayleigh curve is typically associatedrnwith advective mixing which explains a large number of observations during Springrnand Autumn seasons. However, winter observations are strongly inrnuenced by moisturernmixing process. Further analysis of isotopic composition of water vapour fromrnIsoGSM model follows the known seasonal cycles observed in precipitation from earlierrnstudies suggesting that the dominant mode of water vapour variability is governedrnmainly by large scale climate system

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