West African convective storms during the land-ocean transition

The purpose of this presentation is to document the attributes of West African storms at the land-ocean transition during August and September 2006. As these storms move from continental to oceanic environments, they propagate through strong thermodynamic and microphysical gradients both at the surface and in the atmospheric boundary layer. These gradients are partly due to the presence and strength of the Saharan Air Layer (SAL), a layer of dry and dusty air that is unique to this region. Current efforts to simulate and forecast storm evolution in West Africa require an improved understanding of the interactions between cloud-development and rainfall processes, surface conditions, and microphysical composition (e.g., the concentrations of dust, sea salt, or other cloud condensation nuclei) in the troposphere. Thus, a goal of this research is to use storms sampled during NAMMA (NASA African Monsoon Multidisciplinary Analyses) to investigate storm changes at the land-to-ocean transition, particularly in light of expected lifecycle progressions and pre-existing thermodynamic and microphysical gradients. During the field experiment, storms were observed by several instruments, including the NASA S-Band polarimetric Doppler weather radar (NPOL), a meteorological flux tower, upper air soundings, and rain gauges. The presentation will include analyses of the diurnal surface fluxes near the coast on stormy days, cumulative frequencies of rainfall rates associated with storms, and cumulative frequencies of radar reflectivity values (in convective versus stratiform and continental versus oceanic regions). We also employ the National Center for Atmospheric Research (NCAR) polarimetric particle identification algorithm to identify vertical and horizontal distribution of hydrometeors over both land and ocean. Radar analyses reveal that convective storms during NAMMA had well-defined convective and stratiform regions over both land and ocean, but that storms became weaker (i.e., lower reflectivity values in the troposphere) and more disorganized over the Atlantic Ocean. The evolving horizontal and vertical hydrometeor composition (e.g., location and thickness of the cloud ice layer) will be presented to discuss how changes in these storms result in response to key features of the land-ocean transition, such as a cooler troposphere and favorability for warm-rain processes over the ocean.