A numerical study of the effects of the roughness length on the meso-scale flow patterns over Kenya

Abstract

A three-dimensional numerical model was used in this study to examine the effects of changes in the surface roughness length on the meso-scale flow patterns over Kenya. Three experiments were performed. The first experiment was performed to test the ability of the model in reproducing the meso-scale circulations over Kenya. In the second experiment, the roughness length was uniform over the whole domain (homogeneous roughness terrain) . In the third experiment, the roughness length at each grid point was specified according to the terrain characteristics (heterogeneous roughness terrain). The initial fields were observations for May 6 1986. Results of the first experiment showed that the model was able to give realistic simulations of the meso-scale flow over Kenya. The model simulated the afternoon sea/lake breezes and upslope flows on either side of the Kenya highlands. In the morning, the land breeze and downslope flows from the Kenya highlands were also simulated. Maximum afternoon low-level rising motion, meso-scale convergence and precipitation were located over the western highlands. Results from the second experiment showed that changes in the surface roughness had significant impacts on the development of meso-scale flows over Kenya.The 'simulated boundary layer wind speed decreased as the surface roughness was increased. The results further showed enhanced turbulent mixing of the boundary layer heat and moisture, and increased convective rainfall over (vii) the western highlands, when the surface roughness was increased. Non-convective rainfall contributed relatively less to the total simulated meso-scale rainfall over Kenya. In the third experiment, it was found that the simulated boundary layer wind field compared well with the mean flow patterns for May. The simulated temperature field indicated high values over the coastal areas, the lake region and the arid and semi-arid northern parts of Kenya. Low temperatures were concentrated over the central highlands. These patterns compared well with the observed mean patterns for the month. High values of relative humidity were simulated over the coastal areas and western highlands in agreement with the observations. Rising motion accompanied by high meso-scale precipitation was simulated over western Kenya, while the eastern highlands including the arid/semi arid areas of northern Kenya were dry. These results compared -- better with the mean fields and the observed weather patterns for May 6 1986. From the results obtained in this study one may infer that modification of the land surface characteristics through human activities like deforestation and overgrazing, would have considerable impacts on the meso-scale weather systems in Kenya. Such changes would have far r~aching socio-economic impacts. Proper planning of land use activities is therefore recommended in order to minimize human induced modification of the surface characteristics