| dc.description.abstract | The present study investigated the effect of land-use/land-cover changes, including
urbanization of the metro area (urban built-up area) and landscape changes (topography), on
the microclimate over Cosmopolitan Nairobi area (Province). The microclimate was
represented by the heat budget (thermal field), airflow (motion field) and moisture budget
(water substance) over the Province. The study was divided into three parts, namely,
observational analysis, numerical modelling and sensitivity experiments.
In the first part, analyses of various observed weather elements were done to
determine their spatial and temporal patterns over the area of study. This was accomplished
by examining the weather parameters in an endeavour to establish the impact of mesoscale
forcing generated by land surface inhomogeneity on the weather as well as to isolate man's
input in the space-time distribution of the weather elements. Two chief anthropogenic
processes that influence weather were examined, namely, changes in land-use patterns and
the artificial heating potential.
The second part of the study-numerical modelling-used a high-resolution limitedarea
model (LAM) with the acronym GESIMA (developed at Geesthacht, Germany) to
simulate the local microclimate represented by the thermodynamic, dynamical and moisture
structures over Cosmopolitan Nairobi area in a Control Experiment initialized with data for
17 November, 1990. This date was chosen because it displayed the weather patterns that
were typical of the average climatology over the study area. Model simulations were done for
24 hours to bring out the mean diurnal cycle of the meteorological elements over the study
domain. The control runs were achieved by performing numerical simulations with full
physics in which the modules for the physical processes that control weather (including the
dynamics, hydrodynamics and thermodynamics of the atmosphere) were fully activated. The
analysed model outputs were compared with results from observational analyses in order to
verify the skill of the model in replicating the observations.
The third part of the study consisted of sensitivity experiments and considered the
possible implication of man's activities through land-use/land-cover changes and topography
on the weather and microclimate over the Nairobi area. This was accomplished by
performing theoretical experiments for modified land-use/land cover and also for a flat
terrain.
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The results show that topography and wind regimes are the mam controllers of
surface temperature patterns. City expansion and deforestation are projected to modify the
surface temperature in the location of these features, although the magnitude of the changes
would not be substantial during the warm seasons. Despite the fact that the anthropogenic
heat released in Nairobi is currently small, its magnitude in future will be large enough to
alter the heat budget, and hence the weather activities.
The central business district (CBD) of Nairobi City and forests reduce horizontal flow
but enhance vertical air motion. Surface aspect is more important than land-use/land-cover
change patterns in determining the horizontal and vertical motion of air. The influence of the
surface on the simulated wind field was most significant within the planetary boundary layer
(PBL).
The vertical fluxes of heat, momentum and moisture in the PBL are controlled by
dissimilar mechanisms. Both surface processes and large-scale conditions influence these
fluxes. Vertical mixing and horizontal transfer suggest good ventilation during the day. Upslope/
down-~lope motion patterns indicate a possibility of re-circulation of air pollutants in
the City.
Decreased wind speeds have reduced the evaporation rate at the CBD and forests.
Consequently the CBD and forests have drier air. Moisture is advected to low-ground areas
at night and to upper grounds during the day. The concrete/asphalt material has resulted in
decreased soil moisture at the city centre due to reduced field capacity. Deforestation would
increase soil moisture loss thereby reducing soil wetness. The urban built-up area destabilizes
the air causing precipitation to form downwind. Rainfall distribution was mainly a result of
topographical variations, synoptic flow patterns and the ITCZ.
The model under-predicted most meteorological parameters, V1Z., the dynamic,
thermal and moisture fields, which represented the observed local climatology over the study
domain. There is, therefore, need for fine-tuning or customizing the model to improve its
treatment of the topographical inhomogeneity over the area of study.
Evidently, mesoscale influences emanating from land-use/land-cover changes on the
observed and simulated weather/microclimate characteristics in Nairobi are appreciable. The
forcing that showed the most influence on the weather patterns in the Cosmopolitan Nairobi
Province is topography. The effects of the metro-area and forests on the motion field
(airflow), as well as on the heat and moisture budgets are substantial and are experienced
throughout the Province in spite of the relatively small areas occupied by these features.
Further growth and development of the city is bound to have appreciable ramifications on the
weather distribution over the study area and the surrounding regions | en |
