Determination of Atmospheric Water Vapour Isotopic Composition using Multi-Platform Instruments and Models over Ethiopia: Implications for Water Cycle
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Date
2014-11
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Addis Ababa University
Abstract
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 cm1 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),
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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
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Keywords
Ethiopia Water Vapour Isotopic Composition