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| Titel |
Observations of precipitable water vapour over complex topography of Ethiopia from ground-based GPS, FTIR, radiosonde and ERA-Interim reanalysis |
| VerfasserIn |
G. Mengistu Tsidu, T. Blumenstock, F. Hase |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 8 ; Nr. 8, no. 8 (2015-08-13), S.3277-3295 |
| Datensatznummer |
250116530
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| Publikation (Nr.) |
 copernicus.org/amt-8-3277-2015.pdf |
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| Zusammenfassung |
| Water vapour is one of the most important greenhouse gases. Long-term changes
in the amount of water vapour in the atmosphere need to be monitored not only
for its direct role as a greenhouse gas but also because of its role in
amplifying other feedbacks such as clouds and albedo. In recent decades,
monitoring of water vapour on a regular and continuous basis has become
possible as a result of the steady increase in the number of deployed global
positioning satellite (GPS) ground-based receivers. However, the Horn of
Africa remained a data-void region in this regard until recently, when some
GPS ground-receiver stations were deployed to monitor tectonic movements in
the Great Rift Valley. This study seizes this opportunity and the
installation of a Fourier transform infrared spectrometer (FTIR) at Addis
Ababa to assess the quality and comparability of precipitable water vapour
(PWV) from GPS, FTIR, radiosonde and interim ECMWF Re-Analysis (ERA-Interim)
over Ethiopia. The PWV from the three instruments and the reanalysis show
good correlation, with correlation coefficients in the range from 0.83 to
0.92. On average, GPS shows the highest PWV followed by FTIR and radiosonde
observations. ERA-Interim is higher than all measurements with a bias of 4.6 mm compared
to GPS. The intercomparison between GPS and ERA-Interim was
extended to seven other GPS stations in the country. Only four out of eight
GPS stations included simultaneous surface pressure observations. Uncertainty
in the model surface pressure of 1 hPa can cause up to 0.35 mm error in GPS
PWV. The gain obtained from using observed surface pressure in terms of
reducing bias and strengthening correlation is significant but shows some
variations among the GPS sites. The comparison between GPS and ERA-Interim
PWV over the seven other GPS stations shows differences in the magnitude and
sign of bias of ERA-Interim with respect to GPS PWV from station to station.
This feature is also prevalent in diurnal and seasonal variabilities. The
spatial variation in the relationship between the two data sets is partly
linked to variation in the skill of the European Centre for Medium-Range
Weather Forecasts (ECMWF) model over different regions and seasons. This
weakness in the model is related to poor observational constraints from this
part of the globe and sensitivity of its convection scheme to orography and
land surface features. This is consistent with observed wet bias over some
highland stations and dry bias over few lowland stations. The skill of ECMWF
in reproducing realistic PWV varies with time of the day and season, showing
large positive bias during warm and wet summer at most of the GPS sites. |
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