Towards The Development Of A Forecasting Numerical Model For Kenya
Okeyo, Andrew E
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Two- and three-dimensional mesoscale numerical models are used to study the causes of frequent severe weather over Kenya. The two-dimensional version of the model has 56 grid- points in the east-west direction and 12 vertical sigma levels. This version uses extended grid system with a finer resolution at the centre of interest. The three-dimensiona+ version has 37 gridpoints in the east-west direction, 25 gridpoints in the north-south direction and 13 sigma levels in the vertical. This version uses a n~ed grid system in a two-way interactive manner with the resolution of the fine mesh being 40 km and that of the coarse mesh as 120 km. The two versions of the model are used to simulate the weather patterns for January and April of 1976 in Kenya. Part i- cular attention is given to the ~e$oscale systems and how they . influence convective-scale systems and the larger-scale environ- ment. The mesoscale systems of interest are the lake/sea breezes and mountain flows. The convective-scale systems of great concern are the thunderstorms/hailstorms and lightning hazards, while the larger-scale phenomenon is the Intertropical Convergence Zone (ITCZ). Four experiments were done using the two-dimensional model. These experiments were: 1. explicit moisture scheme 2. cumulus parameterisation scheme 3. passive moisture scheme 4. exclusion of water masses viii Results from these experiments show that the lake breeze circulation from Lake Victoria plays a significant role on the development of the convective-scale systems over the Kenya high- lands. The sea breeze from the Indian Ocean however plays little role on these activities. The results further show that the inclusion of moisture plays a fundamental part on the development of the hazardous weather over the Kenya highlands. In comparing an explicit moisture scheme, in which cloud and rain water are predicted explicitly, and a cumulus para- meterisation scheme, it is noticed that the cumulus parameterisa- tion scheme gives stronger winds, warmer troposphere and more precipitation than the explicit moisture scheme. The results of the cumulus parameterisation scheme are also found to be closer to the observations than those of the explicit moisture scheme. Five experiments were conducted using the three-dimensional model. These experiments were: 1. the cumulus parameterisation scheme (control experiment) 2. exclusion of water masses 3. flat terrain 4. passive moisture 5. increased moisture (wet season) , It is noticed from the results of the three- dimensional model experiments that Lake Victoria circulation is a significant source of heat and moisture necessary for the development of ix the frequent thunderstorms/hailstorms and lightning hazards over the Kenya highlands. Results from these experiments also indicate that high ground plays a significant role in the development of the hazardous weather activities by gathering the moisture necessary for condensation and subsequent precipitation. Other results from the three-dimensional model further show that the release of latent heat by condensational warming plays a significant role in the weather activities over the region. It is suggested that this phenomenon be accurately parameterised into the numerical simulations to help understand the physics and dynamics of the hazardous weather over Kenya especially above the highlands. In an experiment with increased moisture input (wet season) it was found that the model simulated more rainfall in most areas in Kenya than during the dry season. The results of this study will have a significant impact on the energy and water budgets. They will also be very useful for the improvements of weather forecasting and cloud modification programmes over Kenya.