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The relationship between African easterly waves and daily rainfall over West Africa: observations and regional climate simulations

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Abstract

The relationship between summer African easterly waves (AEWs) and daily rainfall is assessed in West Africa for 1998–2008 using various reanalyses, satellite-derived rainfall products, and a regional climate model (RCM) run at 90- and 30-km resolutions. 3–5 and 6–9 day AEWs are extracted by filtering daily 700 hPa meridional wind time series at 1°W and 11.5°N, and 1°W and 17.5°N, respectively. Both observed and simulated rainfall anomalies are of larger magnitude over West Africa during 3–5-d than 6–9-d AEWs. The RCM simulates larger rainfall rates in phase with the 3–5-d wave trough instead of ahead, unlike the observations, and overestimates the intensity and spatial coverage of rainfall associated with 6–9-d AEWs. The observed and simulated co-variability between 3–5-d (6–9-d) AEW activity and daily rainfall is strong (weak) and mostly located south (north) of 15°N. However, the RCM overestimates the spatial coverage of the AEW–rainfall relationship in the longitudinal (latitudinal) direction in the case of 3–5-d (6–9-d) AEWs. Observed and simulated daily intense rainfall events, extracted using a percentile threshold approach, are mostly located south of 15°N during summer. The observed relationship between their frequency of occurrence and active 3–5-d AEWs is maximal west of 8°E, while extends up to southern Chad in both RCM simulations. Their magnitude is also largely overestimated by the RCM, indicating an exaggerated coupling between the wave activity and the convection. Finally, observed and simulated 3–5-d AEWs establish the most favorable synoptic conditions for the development of intense rainfall events over West Africa.

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Acknowledgments

Support from the U.S. Department of Energy Office of Science (award DE-FG02-10ER65092) is gratefully acknowledged. WRF was provided by the University Corporation for Atmospheric Research (http://www.mmm.ucar.edu/wrf/users/download/get_source.html). The authors acknowledge the Texas Advanced Computing Center (TACC) at the University of Texas at Austin for providing the high performance computing and database resources that have contributed to the research results reported within this paper. http://www.tacc.utexas.edu. TRMM 3B42V7 data used in this study was acquired using the GES-DISC Interactive Online Visualization ANd aNalysis Infrastructure (Giovanni) as part of the NASA’s Goddard Earth Sciences (GES) Data and Information Services Center (DISC). We also thank George Kiladis and the other anonymous reviewer for their constructive comments.

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  1. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX, 78712, USA

    Julien Crétat, Edward K. Vizy & Kerry H. Cook

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Crétat, J., Vizy, E.K. & Cook, K.H. The relationship between African easterly waves and daily rainfall over West Africa: observations and regional climate simulations. Clim Dyn 44, 385–404 (2015). https://doi.org/10.1007/s00382-014-2120-x

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