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Numerical Simulation of the Impact of Urbanization on the Microclimate
over Nairobi Area
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
limited-area 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
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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.
The results show that topography and wind regimes are the main 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 landuse/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.
Up-slope/down-slope 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, viz., 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.
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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.