| dc.description.abstract | 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
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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
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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)
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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
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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. | en |
