Kinematic structures, diabatic profiles, and precipitation systems in West Africa during summer 2006

Abstract

West Africa is a region characterized by great spatial contrasts in temperature, precipitation, and topography, which combine to create many complex and interesting weather phenomena. In particular, the area is home to a seasonal monsoon, propagating easterly waves, and some of the most intense thunderstorm systems on Earth. These types of events have both local and global effects - precipitation variability has a major bearing on regional water resource issues, while West Africa is also the source of many of the disturbances that develop into tropical cyclones in the North Atlantic Ocean. Unfortunately, atmospheric data has historically been very sparse in West Africa, leading to an incomplete understanding of many of these meteorological features and a corresponding difficulty in modeling them accurately. An exceptional opportunity for improvement on these fronts exists thanks to the African Monsoon Multidisciplinary Analysis (AMMA) field campaign, which collected an unprecedented quantity of observations throughout the region, with the most concentrated effort during the summer of 2006. This work uses a gridded analysis of radiosonde measurements obtained during AMMA and places those observations in the context of AMMA radar data and satellite rainfall estimates to examine the patterns of kinematic and diabatic quantities in West Africa relative to the summer monsoon phase, easterly wave disturbances, precipitation systems, and the diurnal cycle. Many unique aspects of West African weather compared to conditions elsewhere in the tropics are revealed by this study. The meridional transitions related to the West African monsoon comprise the predominant control on the location and intensity of precipitation at seasonal time scales, with variations in convective activity related to the Madden-Julian Oscillation contributing at 25 to 60 day periods. On shorter time scales of two to six days, easterly wave disturbances look to be the principal factor governing the timing of rainfall events, though especially persistent cold pools and residual cloudiness generated by thunderstorm systems also act as constraints on convective evolution on the days following a precipitation episode. One of the most distinctive traits of the study region in West Africa compared to other tropical areas is the particular prevalence of convective downdrafts, chiefly those associated with mesoscale zones of stratiform precipitation in thunderstorm complexes. These features, along with the gravity waves forced by their characteristic heating pattern, have an especially large influence on the time-mean atmospheric structure relative to the majority of the tropics. A comparison of the diabatic profiles from the AMMA dataset with those from other field projects indicates that the signals of both convective downdrafts and diurnal variations of the planetary boundary layer are much stronger in West Africa than in the previously studied regions. Beyond the mentioned differences, though, the AMMA profiles show resemblance to those from both western Pacific and eastern Atlantic field campaigns. The vertical patterns of atmospheric variables tend to be complex and multi-layered in West Africa, suggesting that the area is home to an especially diverse cloud population, with contributions from numerous height regimes prominent enough to influence the mean state. Meridional differences within the domain of the analysis are evident, including indications of more intense convective updrafts toward the north, stronger effects of boundary layer mixing in the north, and a greater net influence of mesoscale convective system downdrafts toward the south. The diurnal cycle of precipitation appears most prominently shaped by convective initiation near areas of high topography and the subsequent development and long-distance propagation of extensive, well-organized thunderstorm systems, though there seem to be effects related to diurnal flow patterns near the Gulf of Guinea coast too. Inland, moisture transport achieved by the nocturnal low-level jet is a key influence on rainfall, with mixing by the daytime boundary layer playing an important function as well. Changes in the relative contribution and intensity of deep convective and stratiform heating and moistening patterns arise among different times of day and night, as the leading precipitation regime transitions from developing deep convection at midday to organizing thunderstorm systems by evening and propagating thunderstorm complexes with extensive stratiform rainfall overnight. The analyses in the present work demonstrate a few different issues and caveats that need to be considered when utilizing observational or remote sensing datasets. Namely, the timing of radiosonde launches and the spacing of the sounding site array combined to create a delay between when convective systems passed the Niamey, Niger measurement site and when their effects were detected in the gridded AMMA sounding data. Similarly, infrared satellite rainfall estimates from the Tropical Rainfall Measuring Mission (TRMM) are shown to have a time lag of about three hours between when precipitation actually occurs and when it appears in the estimate product, complicating the intended use of the data in evaluating the diurnal cycle of rainfall.