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The Never-Ending Struggle to Simulate the West African Monsoon

Reference
Kothe, S., Luthi, D. and Ahrens, B. 2014. Analysis of the West African Monsoon system in the regional climate model COSMO-CLM. International Journal of Climatology 34: 481-493.
According to Kothe et al. (2014), "the West African Monsoon (WAM) is a seasonal wind reversal that is mainly caused by a temperature gradient between the ocean and land surface," whereby "high temperatures over the Sahara and Sahel region induce development of a heat low, which initiates a northward displacement of the inter-tropical convergence zone (ITCZ) and transport of moist air over West Africa," which phenomenon brings West Africa most of its precipitation. Against this backdrop, Kothe et al. "investigated the WAM representation, and the impact of surface temperature uncertainties in three regional climate simulations with the model COnsortium for Small-scale MOdelling-Climate Limited-area Model (COSMO-CLM)," where "regional simulations were driven by present-day climate simulations with the global climate model ECHAM5, and by the re-analysis data ERA-Interim."

The three researchers report finding that (1) "in large parts of the African tropics, precipitation was underestimated by up to 50%," (2) "even larger overestimations occurred along the Western coastlines and the Ethiopian Highlands," (3) "an overestimation of precipitation in the Sahel region resulted from a northerly overextension of the monsoon system in the model simulations," (4) there was "a tendency of COSMO-CLM to produce too much convection," (5) "precipitation was underestimated in large parts of the African tropics," (6) there was "a negative bias (too low) in OLR [outgoing longwave radiation], arising from high convective clouds being far too optically thick (due to the excessive ice/liquid in the clouds)," (7) there was "a negative bias in precipitation (too much condensed water stays in clouds and does not rain out)," (8) "the convective activity of COSMO-CLM, measured by an OLR-based index, was especially over the Angola Basin much too high," (9) "precipitation was underestimated in large parts of the continental African tropics," and that (10) "the model strongly overestimated convection in particular over the Gulf of Guinea."

In concluding their study, Kothe et al. write "a lot of work has been done in the last years in the field of convection schemes (Braconnot et al., 2007; Zanis et al., 2009), but the convection schemes are still a major error source," with the result that "even with state-of-the-art climate models the simulation of complex atmospheric systems, such as the West African Monsoon, is still subject to errors (and will probably always be) [italics added for emphasis]," which is why the title of this review begins with the words "The Never-Ending Struggle."

Additional References
Braconnot, P., Hourdin, F., Bony, S., Dufresne, J.L., Grandpeix, J.Y. and Marti, O. 2007. Impact of different convective cloud schemes on the simulation of the tropical seasonal cycle in a coupled ocean-atmosphere model. Climate Dynamics 29: 501-520.

Zanis, P., Douvis, C., Kapsomenakis, I., Kioutsioukis, I., Melas, D. and Pal, J.S. 2009. A sensitivity study of the Regional Climate Model (RegCM3) to the convective scheme with emphasis in central eastern and southeastern Europe. Theoretical and Applied Climatology 97: 327-337.

Archived 6 May 2014