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Scale decomposition of atmospheric water budget over West Africa during the monsoon 2006 from NCEP/GFS analyses

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Abstract

NCEP/GFS analysis is used to investigate the scale dependence and the interplay between the terms of the atmospheric water budget over West Africa using a dedicated decomposition methodology. The focus is on a 2-month period within the active monsoon period of 2006. Results show that the dominant scales of seasonal mean precipitation and moisture flux divergence over West Africa during the monsoon period are large scales (greater than 1,400 km) except over topography, where mean values of small scales (smaller than 900 km) are strong. Correlations between moisture flux divergences in monsoon and African Easterly Jet layers and precipitation indicate that precipitation is strongly correlated to moisture flux divergence via both large-scale and small-scale processes, but the correlation signal is quite different depending on the region and vertical layer considered. The analysis of the scales associated with the rainfall and the local evaporation over 3 different regions shows that positive correlation exists over the ocean between precipitation and evaporation especially at large scale. Over the continent south of the Sahel, the correlation is negative and driven by large scale. Over the northern part of Sahel, positive correlation is found, only at small scales during the active monsoon period. Lag correlation reveals that the maximum evaporation over the Sahel occurs 1–3 days after the maximum precipitation with maximum contribution from small-scale processes during the first day. This study shows that NCEP/GFS reproduces well the known atmospheric water budget features. It also reveals a new scale dependence of the relative role of each term of the atmospheric water budget. This indicates that such scale decomposition approach is helpful to clarify the functioning of the water cycle embedded in the monsoon system.

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Acknowledgments

This research was supported by the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) and the Fondation de l’Ecole Polytechnique. The first author is grateful to the Laboratoire de Météorologie Dynamique for the welcome as a visiting scientist. The authors thank Jean-Yves Grandpeix, Frédéric Hourdin and Jean-Philippe Lafore for very insightful and constructive discussions. Invaluable help with the GFS data from A. Deme is acknowledged. “Based on a French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, especially from France, UK, US and Africa. It has been the beneficiary of a major financial contribution from the European Community’s Sixth Framework Research Program. Detailed information on scientific coordination and funding is available on the AMMA International web site http://www.ammainternational.org”. The authors are also grateful to the two reviewers for their thoughtful and constructive reviews, which helped improving the manuscript.

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Correspondence to Soline Bielli.

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This paper is a contribution to the special issue on West African Climate, consisting of papers from the African Multidisciplinary Monsoon Analysis (AMMA) and West African Monsoon Modeling and Evaluation (WAMME) projects, and coordinated by Y. Xue and P. M. Ruti.

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Bielli, S., Roca, R. Scale decomposition of atmospheric water budget over West Africa during the monsoon 2006 from NCEP/GFS analyses. Clim Dyn 35, 143–157 (2010). https://doi.org/10.1007/s00382-009-0597-5

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