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- Title
Observations of precipitable water vapour over complex topography of Ethiopia from ground-based GPS, FTIR, radiosonde and ERA-Interim reanalysis.
- Authors
Mengistu Tsidu, G.; Blumenstock, T.; Hase, F.
- Abstract
Water vapour is one of the most important green house 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 green house gas but also because of its role in amplifying other feedbacks in general circulation models. In recent decades, monitoring of water vapour on regular and continuous basis is becoming possible as a result of increase in the number of deployed Global Positioning Satellite (GPS) ground-based receivers at a faster pace. However, Horn of Africa region remains a data void region in this regard until recently when some GPS ground-receiver stations have been deployed to monitor tectonic movements in the Great Rift Valley. This study seizes this opportunity and the installation of Fourier Transform Infrared Spectrometer (FTIR) at Addis Ababa to assess the quality and comparability of Precipitable Water Vapour (PWV) from GPS, FTIR, radiosonde and ERA-Interim over Ethiopia. The PWVs from the three instruments and reanalysis show good correlation in the range from 0.83 to 0.92. The radiosonde PWV shows dry bias with respect to other observations and reanalysis. ERA-Interim PWV shows wet bias with respect to all while GPS PWV exhibits wet bias with respect to FTIR. The intercomparison between GPS and ERA-Interim is extended to seven other GPS stations in the country. Despite the sensitivity of GPS PWV to uncertainty in surface pressure in general, observed surface pressure is used only at four GPS stations. 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. In contrast to comparison at Addis Ababa, the comparison between GPS and ERA-Interim PWVs over seven other GPS stations shows difference in the magnitude and sign of bias of ERA-Interim with respect to GPS PWV from station to station. This variation is also visible across different seasons. The main cause of the variation is linked to variation in ECMWF model skill over different regions and seasons which might be related to poor observational constraint from this part of the globe and sensitivity of model convection scheme to orography. The latter is consistent with observed wet bias over some highland stations and dry bias over few lowland stations. However, there are also exceptions to this inference at few stations suggesting other factors such as proximity to water bodies and vegetations might have a role. The skill of ECMWF in reproducing realistic PWV varies with season showing large bias during warm and wet summer for most of the GPS sites.
- Subjects
ETHIOPIA; PRECIPITABLE water; TOPOGRAPHY; GLOBAL Positioning System; FOURIER transform infrared spectroscopy; RADIOSONDES; ELECTRON ring accelerators
- Publication
Atmospheric Measurement Techniques Discussions, 2014, Vol 7, Issue 9, p9869
- ISSN
1867-8610
- Publication type
Article
- DOI
10.5194/amtd-7-9869-2014