The Climatology of African Wave Disturbances in a Regional Model

Our understanding of the structure and behavior of African wave disturbances (AWD) has been derived from brief empirical studies or analyses/simulations using general circulation models. In each case the spatial resolution of the observational network or the computational grid has been an issue in the interpretation of results. The current effort studies the structure and behavior of AWD based on the results of a regional climate model (RCM) run on a grid with 0.5 deg spacing over a limited domain covering West Africa. The model is driven by four times daily synchronous NCEP reanalysis data at the lateral boundaries from June 1-September 30. Thus far, seven summers have been completed. Diagnostics from this system provide distinct advantages over those from coarser resolution global models and from previous results based on sparse networks of observations. The presentation will show composites of mid-tropospheric AWD circulation, associated precipitation patterns and near-surface divergence. Spectra and wavelets of the meridional wind at selected locations will highlight favored periodicities of AWD and intraseasonal variability. Trajectories of AWD during different seasons will be mapped by spatial distributions of the vorticity variance at 700 mb. Interannual variability of AWD characteristics will be related to the interannual variability of summer monsoon precipitation over West Africa. Sensitivity of AWD characteristics to land surface influences will be examined. RCM characteristics of AWD will also be compared to previous descriptions in the literature.