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Journal Article: BibTeX citation key:  Davisa
Davis, J., Knippertz, P. & Fink, A. H. The predictability of precipitation episodes during the West African dry season. IN Quarterly Journal of the Royal Meteorological Society, 139. 1047–1058.
Added by: Andreas Fink 2012-12-30 18:22:39    Last Edited by: Andreas Fink 2013-08-07 08:01:18
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Categories: Atmospheric processes
Keywords: Dynamics, Early warning system
Creators: Davis, Fink, Knippertz
Collection: Quarterly Journal of the Royal Meteorological Society

Peer reviewed
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Abstract
Precipitation episodes in tropical West Africa (7–15◦N, 10◦W–10◦E) during the
dry season from November to March are rare, but can have significant impacts on
human activities reaching from greening of pastures to spoiling harvests and health
implications. Previous work has shown a link between these unseasonal rainfalls and
extratropical disturbances via a decrease of surface pressure over the Sahara/Sahel
and a subsequent inflow of moist air from the Gulf of Guinea. This paper supports
the previously stated hypothesis that the extratropical influence leads to a high
rainfall predictability through a careful analysis of operational 5 day forecasts from
the European Centre for Medium-Range Weather Forecasts’ (ECMWF) ensemble
prediction system (EPS), which are evaluated using Global Precipitation Climatology
Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) precipitation
estimates for the 11 dry seasons 1998/99-2008/09. The long-term regional average
of ensemble-mean precipitation lies between the two observational datasets, with
GPCP being considerably wetter. Temporal correlations between the ensemble
mean and observations are 0.8. Standard probabilistic evaluation methods such as
reliability and relative operating characteristic (ROC) diagrams indicate remarkably
good reliability, sharpness and skill across a range of precipitation thresholds.
However, a categorical verification focusing on the most extreme ensemble mean
values indicates too many false alarms. Despite the considerable observational
uncertainty the results show that the ECMWF EPS is capable of predicting winter
rainfall events in tropical West Africa with good accuracy, at least on regional spatial
and synoptic time-scales, which should encourage West African weather services
to capitalize more on the valuable information provided by ensemble prediction
systems during the dry season.
Added by: Andreas Fink    Last Edited by: Andreas Fink