Relationships between rainfall characteristics and environmental predictors in the West African Region
Open Access
- Author:
- Hamilton, Holly Laurielle
- Graduate Program:
- Meteorology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 20, 2015
- Committee Members:
- Jenni Evans, Dissertation Advisor/Co-Advisor
- Keywords:
- Precipitation
West African Monsoon
topography
African Easterly Waves
African Easterly Jet
rainfall
mesoscale convective systems
regional modeling
Weather and Research Forecasting model
WRF - Abstract:
- Mesoscale Convective Systems (MCS) are a substantial source of the water required for agriculture and human consumption in West Africa. Understanding the lifecycle behaviour of West African mesoscale storms poses unique challenges as these systems traverse strong thermodynamic gradients in their westward porpagation from land to ocean. MCS in this region are known to be associated with African Easterly Waves (AEWs). The topography of eastern Africa, namely the Ethiopian Highlands and Darfur Mountains have been shown to play an intricate role in the genesis of these waves through MCS initiation in that region. We investigate the topographic influence on the formation of these systems using the Weather Research and Forecasting model by recreating the atmospheric state over an eight day period for three cases with varying topography: realistic, half-height, and no topography. As a case study, we use a MCS observed on August 30-31 during the NASA African Monsoon Multidisciplinary Analysis field experiment of 2006. This sensitivity study reveals that the topography influences the development and structure of the reference MCS event. The model persisted in simulating the MCS in conjunction with an AEW in each run, which highlights the robustness of the well-known connection between the two systems in West Africa. The development of the simulated MCSs differ in each run due to their interaction with the AEW and the onshore flow that are in turn impacted by the topography. We examine further the topographic influences on the evolution and energetics of AEW and the formation of convective systems within these AEW. The Weather Research and Forecasting (WRF) model is employed to simulate the evolution over a 36-day period for three cases with varying topography: realistic, half-height, and no topography. An energetics analysis for AEW reveals that baroclinic processes that develop low-level waves north of the jet strengthens due to a stronger monsoonal flow in the flattened topography environment. A noticeably weaker contribution from barotropic and baroclinic instabilities at the jet level in West Africa in the flattened topography simulation explains the weakening of those waves at that level. The results show that topography in Africa plays more of a role in the wave development as they propagate westward rather than in their initiation over East Africa. Satellite rainfall estimates reveal a consistent rainfall maximum offshore West Africa during the monsoon rainy season. A 16-year rainfall climatology is conducted to examine the cause of such copious amounts of rainfall. Composites of daily rainfall and mid-level meridional winds centered on the days with maximum rainfall in August show that the day with heaviest rainfall follows the strongest mid- level northerlies. Reflectivity and rain type composites show that stratiform rain dominates the region. The composites suggest that the dominant contribution to the offshore rainfall maximum derives from the trailing stratiform portion of mesoscale convective systems in the northerly phase of African Easterly Waves propagating off the continent. Sensitivity simulations done with the WRF model recreates the rainfall maximum and indicates the weakening of the maximum as topography on the African continent is flattened.