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Frequency of Sahelian storm initiation enhanced over mesoscale soil-moisture patterns

Abstract

Evapotranspiration of soil moisture can affect temperature and humidity in the lower atmosphere, and thereby the development of convective rain storms. Climate models have illustrated the importance of soil-moisture–precipitation feedbacks for weekly rainfall totals in semi-arid regions, such as the Sahel1. However, large variations exist between model feedbacks, and the mechanisms governing the strength and sign of the feedback are uncertain. Here, we use satellite observations of land surface temperatures and convective cloud cover over West Africa—collected during the wet seasons between 2006 and 2010—to determine the impact of soil moisture on rainfall in the Sahel. We show that variations in soil moisture on length scales of approximately 10–40 km exert a strong control on storm initiation—as evidenced by the appearance of convective cloud. The probability of convective initiation is doubled over strong soil-moisture gradients compared with that over uniform soil-moisture conditions. We find that 37% of all storm initiations analysed occurred over the steepest 25% of soil-moisture gradients. We conclude that heterogeneities in soil moisture on scales of tens of kilometres have a pronounced impact on rainfall in the Sahel, and suggest that similar processes may be important throughout the semi-arid tropics.

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Figure 1: Sensitivity of MCS initiation to land surface properties.
Figure 2: Mesoscale surface variability around initiation point.
Figure 3: Distribution of LSTA gradients associated with MCS initiations.
Figure 4: Schematic depicting the impact of soil-moisture heterogeneity on convective initiation.

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Acknowledgements

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, the UK, the US and Africa. The authors were funded by the European Community’s Sixth Framework Research Programme and the UK NERC project NE/B505597/1. We would like to thank D. Parker and J. Polcher for many valuable discussions on this topic. We also thank LandSAF for the provision of land surface temperature data, EUMETSAT for cloud data, R. de Jeu for soil-moisture retrievals and M. Tomasini for assistance with MCS tracking.

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C.M.T. and F.G. conceived the study and wrote the paper, A.G. analysed the LST data, C.M.T. developed the cloud tracking, F.G. determined the atmospheric sensitivities, P.P.H. devised statistical tests, R.J.E. and F.C. evaluated the tracking and M.D.K. performed the wavelet analysis. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Christopher M. Taylor.

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The authors declare no competing financial interests.

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Taylor, C., Gounou, A., Guichard, F. et al. Frequency of Sahelian storm initiation enhanced over mesoscale soil-moisture patterns. Nature Geosci 4, 430–433 (2011). https://doi.org/10.1038/ngeo1173

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