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    Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

    Source: Weather and Forecasting:;2009:;volume( 024 ):;issue: 004::page 1064
    Author:
    Knippertz, Peter
    ,
    Fink, Andreas H.
    DOI: 10.1175/2009WAF2222221.1
    Publisher: American Meteorological Society
    Abstract: Precipitation during the boreal winter dry season in tropical West Africa is rare but occasionally results in significant impacts on the local population. The dynamics and predictability of this phenomenon have been studied very little. Here, a statistical evaluation of the climatology, dynamics, and predictions of dry-season wet events is presented for the region 7.5°?15°N, 10°W?10°E. The analysis is based upon Global Precipitation Climatology Project (GPCP) merged satellite?gauge pentad rainfall estimates and 5-day 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) precipitation forecasts, and covers the 23 dry seasons (November?February) during 1979/80?2001/02. Wet events are defined as pentads with an area-averaged precipitation anomaly of more than +200% with respect to the mean seasonal cycle. Composites of the 43 identified events indicate an association with a trough over northwestern Africa, a tropical plume on its eastern side, unusual precipitation at the northern and western fringes of the Sahara, and reduced surface pressure over the Sahara, which allows an inflow of moist southerlies from the Gulf of Guinea to feed the unusual dry-season rainfalls. The results give evidence for a preconditioning by another disturbance about 1 week prior to the precipitation event. The ERA-40 forecasts show a high temporal correlation with observations, a general wet bias, but a somewhat too low number of wet events. With 53% of all identified events correctly forecasted and only 32% of forecasted events not verified, the model shows moderate skill in contrast to the prediction of many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborates the previously proposed hypothesis that a strong extratropical influence enhances the quality of predictions in this region. The results should encourage weather services in West Africa to take advantage of available dry-season precipitation forecasts in terms of the dissemination of early warnings.
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      Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

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    http://yetl.yabesh.ir/yetl1/handle/yetl/4211423
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    contributor authorKnippertz, Peter
    contributor authorFink, Andreas H.
    date accessioned2017-06-09T16:32:40Z
    date available2017-06-09T16:32:40Z
    date copyright2009/08/01
    date issued2009
    identifier issn0882-8156
    identifier otherams-69722.pdf
    identifier urihttp://onlinelibrary.yabesh.ir/handle/yetl/4211423
    description abstractPrecipitation during the boreal winter dry season in tropical West Africa is rare but occasionally results in significant impacts on the local population. The dynamics and predictability of this phenomenon have been studied very little. Here, a statistical evaluation of the climatology, dynamics, and predictions of dry-season wet events is presented for the region 7.5°?15°N, 10°W?10°E. The analysis is based upon Global Precipitation Climatology Project (GPCP) merged satellite?gauge pentad rainfall estimates and 5-day 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) precipitation forecasts, and covers the 23 dry seasons (November?February) during 1979/80?2001/02. Wet events are defined as pentads with an area-averaged precipitation anomaly of more than +200% with respect to the mean seasonal cycle. Composites of the 43 identified events indicate an association with a trough over northwestern Africa, a tropical plume on its eastern side, unusual precipitation at the northern and western fringes of the Sahara, and reduced surface pressure over the Sahara, which allows an inflow of moist southerlies from the Gulf of Guinea to feed the unusual dry-season rainfalls. The results give evidence for a preconditioning by another disturbance about 1 week prior to the precipitation event. The ERA-40 forecasts show a high temporal correlation with observations, a general wet bias, but a somewhat too low number of wet events. With 53% of all identified events correctly forecasted and only 32% of forecasted events not verified, the model shows moderate skill in contrast to the prediction of many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborates the previously proposed hypothesis that a strong extratropical influence enhances the quality of predictions in this region. The results should encourage weather services in West Africa to take advantage of available dry-season precipitation forecasts in terms of the dissemination of early warnings.
    publisherAmerican Meteorological Society
    titlePrediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics
    typeJournal Paper
    journal volume24
    journal issue4
    journal titleWeather and Forecasting
    identifier doi10.1175/2009WAF2222221.1
    journal fristpage1064
    journal lastpage1084
    treeWeather and Forecasting:;2009:;volume( 024 ):;issue: 004
    contenttypeFulltext
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    DSpace software copyright © 2002-2015  DuraSpace
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