Mesoscale analyses of West African summer climate: focus on wave disturbances

A mesoscale climate data set is created from simulations with a regional (limited area) model over West Africa on a 0.5 grid, covering six summers (June– September), 1998–2003. The Regional Model 3 (RM3) is the latest version run at the National Aeronautics and Space Administration Goddard Institute for Space Studies and the (Columbia University) Center for Climate Systems Research. RM3 simulations are driven by synchronous lateral boundary data four times daily from the National Center for Environmental Prediction reanalysis (NCPR) on a 2.5 grid and sea-surface temperatures from the reanalysis. Characteristics of African wave disturbances (AWD) are analyzed from the mesoscale data set and compared to prior published descriptions. Results show a remarkably high correlation in time and space between RM3 modeled precipitation and Tropical Rainfall Measurement Mission daily estimates, although the RM3 underestimates the frequency of the most extreme (high and low) precipitation rates. In addition, regional model precipitation was validated against gridded seasonal means from the East Anglia University data set and against a time series of daily rain gauge observations near Niamey, Niger. RM3 700 mb circulation shows evidence of considerable interannual variability in spectral properties that relate to AWD. Spectral amplitudes for 700 mb meridional wind time series peak most often in the range of 4 to 6-day periods over swaths traversed by AWD, often detected along 17 N and 4 N. AWD also create westward propagating bands of alternating southerlies and northerlies in NCPR, European Center for Mediumrange Weather Forecasting 40-year reanalysis (ERA-40) and RM3 700 mb circulations. RM3 700 mb meridional winds are highly correlated with corresponding NCPR and ERA-40 values, but are usually weaker, so RM3 AWD generally have lower amplitudes. Significant rain events are not always associated with AWD. The RM3 shows good potential for sensitivity experiments that will contribute to our understanding of the physical mechanisms underlying the variability of the West African monsoon. An RM3 West Africa mesoscale climatology could also prove useful for future climate research.