Numerical simulation of the planetary boundary layer over Kenya using a meso-scale model

Abstract

A two dimensional dry meso-scale numerical model has been used to simulate the growth of the Planetary Boundary Layer (PBL) over Kenya. The model is primitive equation hydrostatic, uses a terrain following vertical co-ordinate system, smooth topography and parameterises the PBL prognostically. The main feature of the model suitability for this study is the solution of the surface heat budget, and the parameterisation of shortwave and longwave radiative fluxes at the surface and in the atmosphere. The domain of study is bounded by latitudes 40N and 405 and longitudes 340E and 410E. There were 38 grid points in the East-West direction. The horizontal resolution was 30km and the time step 90 seconds. The zero gradient boundary conditions at the lateral boundaries were imposed. In the vertical, there were 11 variable levels with the finest resolution within the first 2000m. For the solution of the surface energy balance, the soil layers were defined at a constant grid size of 5cm in a soil depth of 55cm. The schemes used for numerical integration of the equations were the stable leap-frog scheme in space and forward difference in time. Experiments characteristics were based on performed variable to simulate surface the PBL. vii All the experiments applied surface heating. In the first experiment, the domain was taken as all flat land. The second experiment focused on the effects of sea/lake breeze on the growth of the PBL by considering an hypothetical flat piece of land between water. The third experiment considered a purely land domain but with varying topography. The fourth experiment was run over the domain of variable terrain with Lake Victoria to the west, and Indian ocean to the east. The results showed a rapid PBL growth of about 300 metres per hour, the fastest growth rate ocurring during the first six hours of solar heating. The maximum PBL depth was about 5000m and was obtained at the high-ground region where the upslope flows and lake/sea breeze winds converge~ The sea breeze flow produced PBL depths of about 1600m, while upslope flows produced larger values of 5000m. Convergent winds obtained at the high-ground region agreed quite well with the observations of the convergence over the Kenya highlands. The computed profiles of the potential temperature and specific humidity showed the PBL developing during the day-time as a mixed layer.