Regional analysis of West African monsoonal convective systems

West Africa has been studied extensively in terms of African Easterly Wave (AEW) propagation and structure. The characteristics and effects of deep convection on the surrounding environment have also been investigated to better parameterize such interactions in large-scale models. This deep convective signal is often seen in the form of Mesoscale Convective Systems (MCSs), largely associated with AEW passages during the monsoon season. Due to the zonally-banded nature of average precipitation, studies have focused on large domains. Localized precipitation variability has largely been ignored. Consequently, small regional characteristics are not well understood, and recent findings suggest a more complex zonal and meridional variability in precipitation than has typically been ascribed to the West African region. As precipitation is closely related to the number of large convective systems during each monsoon season, it is important to understand the characteristics of convection in West Africa and relationship to larger scale forcing. Despite early studies of individual convective systems, the long-term characteristics of convective systems have not been closely examined in this region. Studies of convective systems on a short climatological scale (11 years; defined by available data) were conducted using TRMM satellite and ECMWF reanalysis data throughout West Africa. The University of Utah TRMM precipitation and cloud feature database was used to establish convective intensity and system characteristics (e.g. vertical reflectivity profiles, 30 dBZ echo heights, 85 GHz ice scattering signal, lightning flash density) in multiple, small regional domains (2° x 2° box) within the study area (30°E – 30°W, 0 – 30°N) during the monsoon months of July - September. In conjunction to the fine-scale analysis, data were parsed depending on evidence of AEW activity to yield a clear picture of convective variability as a function of AEW presence/forcing. This information is crucial for understanding interactions over the range of spatial scales associated with the West African monsoon. This study highlights differences in regional convective characteristics which may be important in determining feedbacks with AEWs.

Data from the African Monsoon Multidisciplinary Analyses (AMMA) and NASA-AMMA (NAMMA) 2006 field campaigns were used for a comparative analysis with TRMM satellite products. The fine-resolution ground-based radar data from AMMA were used to ascertain a clear picture of the 2006 monsoon season convection at each site. Examination of AMMA/NAMMA radar data from three sites were emphasized: Niamey, Niger; Kawsara, Senegal; and Praia, Cape Verde Islands. The ground-based radar domains corresponded to continental, coastal and maritime locations, respectively. The Niamey (MIT) and Praia (TOGA) are C-band Doppler radars, while the Kawsara (NPOL) is an S-band dual-polarization Doppler radar. The latitudinal location of all radars was near 15°N, a transitional region in terms of the northward propagation of the Gulf of Guinea moist air mass transported during the monsoon season. This transition region allows for observations of systems characterized by a variety of spatial extents and regime types. Reflectivity data from the ground-based radars were used to derive rainfall estimates for comparison with TRMM-based rainfall observations for the corresponding period. Profiles of vertical reflectivity for convective and stratiform components of convective systems were also compared to complement the coarser resolution TRMM precipitation radar with high resolution ground-radar data.