Contrasts between organized convection during the peak and tail ends of the West African monsoon

During the peak of the wet season the convective environment of Niamey has less convective inhibition and is at a latitude where the troughs of African easterly waves (AEWs) sometimes pass. At this time the African easterly jet (AEJ) is over Niamey. Convection typically develops ahead of the AEW trough associated with the adiabatic forcing for ascent there. At the beginning and end of the wet season the environment near Niamey is marginal with respect to the development of organized moist convection. Perturbations of the inter-tropical discontinuity (ITD) which separates the dry Saharan air layer from the moist monsoon air are critical. These perturbations are associated with vortices which propagate along the ITD and develop in association with extratropical potential vorticity streamers and African easterly waves (AEWs). Intra-seasonal variability also impacts the latitude of the ITD.

The Massachusetts Institute of Technology (MIT) C-band radar was operated at the Niamey, Niger International Airport from July-September 2006 and 2007 performing volume scans roughly every 10 min. The synoptic scale environment and mesoscale structure of organized convective events impacting Niamey during these two years is summarized. We focus on the difference between convective events during favorable and marginal convective environments associated with seasonal and intra-seasonal north-south movements of the ITD. To characterize the synoptic environment in these two regimes we utilize radiosondes and European Center for Medium-Range Weather Forecasting Interim reanalyses. The characteristics of the convection are examined using the MIT radar observations. The relative amounts of convective and stratiform rainfall and the vertical structure of divergence and reflectivity for cases in these two regimes are examined in the context of changes in the synoptic-scale environment. Two WRF simulation case studies are presented to examine these results in more detail. These simulations were chosen to be representative of the northern and southern ITD position regimes.