School of Meteorology (Defense)

Atmospheric Moisture Transport Associated with the West African Monsoon System: an Observational study and Evaluation of a WRF Dynamical Downscaling Simulation

M. Issa Lélé
OU School of Meteorology

21 April 2014, 3:00 PM

National Weather Center, Room 2107
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The West African Monsoon (WAM) is generally viewed as planetary-scale sea-breeze circulations, caused by contrasts in the thermal properties between oceans and land surfaces that lead to apparent moisture sources and sinks on which the West African boreal summer rainfall is inherently dependent. But, predicting the WAM moist convection remains a major challenge for weather and climate models. Here we use reanalysis data to estimate the relative contributions to mean atmospheric moisture transport by both the time-mean circulation and by the synoptic and climate anomalies. The study suggests that, while both the synoptic and the large-scale circulation move moisture meridionally from the tropical South Atlantic Ocean toward the Gulf of Guinea region during the spring season (April-May) coinciding with the WAM onset period, climate-scale circulation (low-frequency) drives much of the mean moisture zonally from the eastern Atlantic onto the Sahelian region during the core monsoon season (July-September). In general, during the Sahelian dry years, the contribution of the low-frequency variability to the mean moisture transport into the region is small, while that of the synoptic is displaced southward. We also show that intraseasonal wet (dry) monsoon events are generally the manifestations of the superposition of the 2-9, 10-25, and 30-90 day intraseasonal oscillation associated with enhanced (suppressed) moisture transport and convection. These results suggest that understanding potential changes in the West African hydrological cycle may require an understanding of corresponding changes in low-frequency time-scales atmospheric variability.

Using simulations with the Weather Research and Forecasting (WRF) model, we test the sensitivity of the cumulus and the microphysics schemes to represent the moist convection, which reveals the interactions between convection and moisture flux. We show that none of the schemes presently in the WRF was obviously best at all times in simulating WAM variability. The greatest variability in forecasts is found to come from changes in the choice of convective scheme, although notable impacts also occurred from changes in the microphysics schemes. Specifically, changes in convective schemes had the largest impact on the simulated rain rate, while simulations of total domain rainfall were influenced by choices of both the microphysics and convective schemes. However, WRF has been able to simulate reasonably well the gross pattern of many regional features, such as the magnitudes of the monthly accumulated precipitation, the low- and upper-level winds including the location of the AEJ, the monsoon depth, and the time and passage of westward propagating storms crossing the West African domain.

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