A two-dimensional numerical model of the lake-land and sea-land breezes over Kenya

Abstract

A two-dimensional, hydrostatic, primitive equation model is used to describe the initiation and evolutfon of the lake and sea breezes over Kenya as a function of surface heat and momentum fluxes. The model has 12 vertical levels and 45 grid points in the horizontal. In the soil, 12 levels with a constant spacing of 2.5 cm are used. The horizontal minimum resolution is 20 km except near the lateral boundaries where a coarser resolution is employed. The vertical resolution is variable with a fine resolution near the ground along~ith-areasonably regular transition to 1 km spacing used higher up. The model is used to study the mesoscale circulations which develop over a mountain barrier and a flat terrain and to discuss the effects of these circulations on the thunderstorm and hailstorm formations over the Kenya highlands. At the start of the integration, the atmosphere is assumed to be at rest. Orography is given corres- . ponding to the Kenyan topographical map. The equations of motion are solved by a semiimplicit forward-upstream differencing technique. Vertical velocity is diagnosed by vertical integration. One of the main features of the model is the inclusion of the parameterisation of the surface heat budget, and shortwave and longwave radiative fluxes. Results show that the mesoscale flows in the form of the Lake Victoria breeze, the upslope winds, and the Indian Ocean breeze converge above the Kenya highlands where serious hailstorms are observed almost daily. The results further show that the combined lake/land and sea/land breezes and upslope and downslope winds produce a more intense circulation during both day and night than the lake/land and sea/land breezes over a flat terrain. The predicted convergence patterns qualitatively agree with ·ari the existing observational studies of cumulus convection organisation over the Kenya highlands during daytime and over Lake Victoria at night. The model simulations show that the lake-land and sealand breezes and upslope and downslope flows have an important influence on the formation and movement of hailstorms/thunderstorms over the tea-growing areas of the Kenya highlands during day and over the lake during the night respectively. The results of this study have an important bearing on the energy and water budget over Kenya and for the cloud modification program over the Kenya highlands.