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The Interaction Between Deep Convection and Easterly Waves over Tropical North Africa: A Weather State Perspective

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  • 1 NOAA/Cooperative Remote Sensing of Science and Technology Center, The City College of New York, New York, New York
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Abstract

The interaction between deep convection and easterly waves over tropical North Africa is studied using a weather state (WS) dataset from the International Cloud Climatology Project (ISCCP) and reanalysis products from the European Centre for Medium-Range Weather Forecast, as well as radiative fluxes from ISCCP and a precipitation dataset from the Global Precipitation Climatology Project. Composite analysis based on 21 yr of data shows that stronger latent and radiative heating of the atmosphere are associated with stronger, more organized, convective activity than with weaker, less organized, convective activity, implying that any transition from less to more organized and stronger convection increases atmospheric heating. Regression composites based on a meridional wind predictor reveal coherent westward propagation of WS and large-scale wind anomalies from the Arabian Sea into East Africa and through West Africa. The analysis shows that enhanced, but unorganized, convective activity, which develops over the Arabian Sea and western Indian Ocean, switches to organized convective activity prior to the appearance of the African easterly wave (AEW) signature. The results also suggest that low-level moisture flux convergence and the upper-tropospheric wind divergence facilitate this change. Thus, the upper-level easterly waves, propagating into East Africa from the Indian Ocean, enhance one form of convection, which interacts with the Ethiopian highlands to trigger another, more organized, form of convection that, in turn, initiates the low-level AEWs.

Corresponding author address: Ademe Mekonnen, NOAA/CREST Center, T107 Steinman Hall, The City College of New York, CUNY, 140 St. and Convent Ave., New York, NY 10031. E-mail: amekonnen@ccny.cuny.edu

Abstract

The interaction between deep convection and easterly waves over tropical North Africa is studied using a weather state (WS) dataset from the International Cloud Climatology Project (ISCCP) and reanalysis products from the European Centre for Medium-Range Weather Forecast, as well as radiative fluxes from ISCCP and a precipitation dataset from the Global Precipitation Climatology Project. Composite analysis based on 21 yr of data shows that stronger latent and radiative heating of the atmosphere are associated with stronger, more organized, convective activity than with weaker, less organized, convective activity, implying that any transition from less to more organized and stronger convection increases atmospheric heating. Regression composites based on a meridional wind predictor reveal coherent westward propagation of WS and large-scale wind anomalies from the Arabian Sea into East Africa and through West Africa. The analysis shows that enhanced, but unorganized, convective activity, which develops over the Arabian Sea and western Indian Ocean, switches to organized convective activity prior to the appearance of the African easterly wave (AEW) signature. The results also suggest that low-level moisture flux convergence and the upper-tropospheric wind divergence facilitate this change. Thus, the upper-level easterly waves, propagating into East Africa from the Indian Ocean, enhance one form of convection, which interacts with the Ethiopian highlands to trigger another, more organized, form of convection that, in turn, initiates the low-level AEWs.

Corresponding author address: Ademe Mekonnen, NOAA/CREST Center, T107 Steinman Hall, The City College of New York, CUNY, 140 St. and Convent Ave., New York, NY 10031. E-mail: amekonnen@ccny.cuny.edu
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