| dc.description.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. | en |
