Land surface influences on the West African summer monsoon: Implications for synoptic disturbances
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The study examines regional atmospheric model (RM) simulations of the mean June–September (JJAS) climate and the implications of the mean state for the model representation of African wave disturbances (AWD). Two simulations are made with a version of the RM that computes soil moisture with multivariate functions that statistically relate it to rainfall, surface temperature, albedo, vegetation and terrain slope. These simulations differ according to the assignment of ground temperatures (Tg). While the control experiment is based on the fully interactive computation of soil moisture, the second experiment tests the response to fixed Tg whose seasonal means are more realistic than in the control within a swath along the Gulf of Guinea coast. A third simulation is made with the RM coupled to a sophisticated land surface process model (RM2). Results show a rather acute sensitivity of the mean circulation to land surface processes. The more realistic meridional temperature gradient created by fixing Tg in turn increased the vertical wind shear over West Africa and eliminated unrealistic westerly circulation at 700 mb. AWD composites were transformed from intense closed cyclonic circulations with copious rainfall to more realistic open waves that organized more moderate precipitation maxima. Lower vorticity variances in the specified change experiment imply that the open waves were characterized by more moderate vorticity extremes. Corresponding spectral amplitudes for 3–6 day periodicities of the 700 mb meridional wind were 40–80% of control values within the swath of maximum AWD activity. From among the three simulations, RM2 achieved seasonal mean precipitation, temperature, energy flux and circulation distributions that, despite some unrealistic features, were closest to observational evidence. RM2 AWD were much less intense and favored slightly longer periods. Results demonstrate that discrepancies in modeled ground temperatures caused by underestimating the cloudiness that intercepts short wave solar flux along a narrow swath of the West African coast have far-reaching consequences for the simulation of both the mean summer climate and individual synoptic disturbances. The study implies that this cooling along the Gulf of Guinea coast prevents AWD from developing into more intense storms with heavier precipitation.
- Land surface influences on the West African summer monsoon: Implications for synoptic disturbances
Meteorology and Atmospheric Physics
Volume 86, Issue 3-4 , pp 261-273
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- A1. Center for Climate Systems Research, Earth Institute at Columbia University, NASA/Goddard Institute for Space Studies, New York
- A2. Stinger Ghaffarian Technology, Inc., NASA/Goddard Institute for Space Studies, New York