Attributes of West Africa Convective Storms at the Land-ocean Transition

The objective of this dissertation was to investigate influences of surfaceatmosphere energy exchanges, atmospheric boundary layer (ABL) thermodynamics, and microphysical conditions on West African mesoscale convective systems (MCS). West African MCS are the primary rain producers in the Sahel and are linked to tropical cyclogenesis. In West Africa, continental and maritime thermodynamic processes and frequent dust events impact the genesis and life cycle of MCS. Research objectives were achieved using observations obtained in Senegal during August and September 2006 as part of NAMMA (NASA African Monsoon Multidisciplinary Analyses). Results indicate that MCS became weaker and disorganized as they transitioned from land to ocean. Initial MCS strength and organization were associated with favorable conditions such as high specific humidity (>18 g kg -1 ), high convective available potential energy (>2000 J kg -1 ), and strong wind shear. MCS weakening was linked to less favorable thermodynamic, dynamic, and microphysical conditions over the ocean. Such results indicated that warm and moist conditions over the coast were prerequisites for MCS to develop into tropical storms. Thermodynamic and dynamic attributes of the ABL remained variable in response to strong onshore flow, dust layers, and MCS occurrence. During onshore flow, mixed layer (ML) depths reached maximum values (~1 km) during day and night. Onshore flow produced deep, warm, and moist MLs by horizontally advecting substantial amounts of moisture and energy. Dust impacted ABL thermodynamics by decreasing incoming shortwave and increasing incoming longwave radiation. When both advection and dust were considered, a 1-dimension model realistically predicted the ML thermodynamic attributes. Influences of surfaceii atmosphere energy exchanges and aerosol attributes were investigated with a cloud resolving model. Model results indicated that gradients in surface energy exchanges and aerosol attributes (as expressed in a modified Köhler theory) can explain the life cycle and attributes of MCS as they propagate from land to ocean. The overall conclusion of this dissertation is that, to investigate cloud development, cyclogenesis, and rainfall over West Africa, numerical models need to explicitly represent continental-to-maritime gradients in thermodynamics, dust concentrations, and the chemical composition of aerosols that directly govern the microphysical processes in MCS.

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