Weather and Climate Systems

Mechanism of Low-Level Moisture Transport Associated with West African Summer Monsoon System

Issa Lélé
Cooperative Institute for Mesoscale Meteorological Studies & School of Meteorology

12 February 2014, 11:00 AM

National Weather Center, Room 5930
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The origins and relative contribution to the mean atmospheric moisture transport by both the time-mean circulation and the synoptic (period less than 10 days) and climate (period greater than 10 days) anomalies into the semi-arid region of the West African Sudan-Sahel are examined from the vertically-integrated atmospheric moisture budget based on NCEP-NCAR Reanalysis data. We will also focus on the interannual variations, in particular for the phases of the West African Monsoon (WAM) life cycle, from moisture build-up and onset to maturity and withdrawal. Results show that the magnitude of zonal fluxes into the Sudan-Sahel region is larger than that of the southerly fluxes, and that the prevailing strong westerly transport extends poleward to 20oN. Large scale anomalies of moisture flux drive much of the mean moisture transport from ocean to land, and the moisture flux convergence is suggested to be the primary source of water vapor needed to support enhanced convection over West Africa during the monsoon season.

The study also uses the Weather Research and Forecasting (WRF) model to investigate the role of changing regional circulations and their relations to water vapor transport and associated rainfall progression over West Africa. First, an attempt is made to understand the general impact of various physical schemes and their interactions on WAM rainfall forecasts. Overall, the model succeeds in reproducing the main features of the geographical distribution and seasonal cycle of moisture fluxes, and the associated rainfall. The greatest variability in simulations comes from changes in the choice of convective scheme, although notable impacts also result from changes in the microphysics.


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