Numerical simulation of the influence of mesoscale forcing on the large-scale flow and weather over Ethiopia

Abstract

The influence of large-scale monsoonal winds and thermally-induced mesoscale circulations on the diurnal weather pattern over Ethiopia was investigated in this study using a three-dimensional limited area model known as the Regional Atmospheric Modeling System (RAMS), which was developed at the Colorado State University (CSU). Two numerical experiments were performed in this study. The simulations were done for 16 August 2000, a day within the long rains season over the country. Experiment 1 was devoted to examining the influence of large-scale monsoonal wind systems on the weather pattern over Ethiopia through unnested simulation. The second simulation (experiment 2) examined the influence of mesoscale circulations on the weather pattern over the country. The data to create the initial state of the atmosphere at OOOOUTC was accessed from the National Center for Atmospheric Research (NCAR). Starting at OOOOUTC the model was integrated forward in time for 24 hours. The model outputs were generated at a frequency of one hour. The results from the first experiment revealed that the atmospheric structure over the country was dominated by a shallow layer of the southeasterly monsoon winds at low level, northeasterly winds at mid-levels, and easterly wind aloft. These features were confirmed by the observations over the country during the season in which the simulation done. The results from the second experiment signified the importance of the mesoscale circulations like upslope and dowenslope flows on the space-time characteristics of weather over. Ethiopia. The results clearly identified the contribution of the local circulations on the large-scale flow comprising the upslope and dowenslope flows. It was therefore concluded from the results of this study that the interaction between mesoscale circulation and the large-scale synoptic winds plays a crucial role in the determination of the space-time characteristics of the weather observed over Ethiopia.