Wednesday 24 August 2005

G7
1515-1700 hours

303
Using GPS stations to retrieve precipitable water vapour in West Africa: first results and perspectives for the AMMA project
Bouin, Marie-Noelle¹, Bock, Olivier², Walpersdor, Andrea³, Doerflinger, Erik⁴, Masson, Frederic⁴
1 Institut Geographique National, Marne La Vallee, France
2 Service d'Aeronomie, France
3 LGIT
4 ISTEEM, France
Author email: bouin@ensg.ign.fr
The Global Positioning System (GPS) is now widely use for precipitable water vapour (PWV) applications in the mid latitudes. Within the frame of the AMMA (African Monsoon Multidisciplinary Analysis) project, we now implement a meso scale GPS network for atmospheric purposes in West Africa: 3 semi permanent stations during the Enhanced Observing Period (2005 to 2007), completed by 3 temporary stations during the 2006 Monsoon period. We first assess the quality of GPS derived PWV in that part of the world by comparing existing GPS data precision with other latitudes results. We analyse the orbit accuracy impact, the quality of the observations, the accuracy of positioning as well as the ionospheric activity influence. We then compare PWV retrievals from GPS on IGS or newly installed (Cotonou) stations to radiosondes (RS) and ERA40 reanalyses from ECMWF. Comparisons of GPS results versus RS show a quite large bias (3-4 mm) likely due to a dry bias in this type of radio sondes and standard deviation of 3-4mm. Reanalyses comparisons show similar effects, since RS data are assimilated in the model. The analysis of PWV variability over of a five years period shows distinct features on five IGS stations in the AMMA domain. Annual, semi-annual, and shorter time-scale signals are evidenced. Correlation with convective activity, precipitation, and Monsoon surges are under study. The deployment of this AMMA GPS network leads to near real time PWV monitoring as well as densification of the sites used to define the reference frame in West Africa.

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