The boundary-layer flow-field regime over Nairobi

Abstract

Logarithmic wind profile equations account for the aerodynamics within the lower atmosphere. This paper employs logarithmic equations in various forms while incorporating changes in atmospheric stability to simulate the spatial and temporal structure of both the planetary boundary layer depth and the field of flow within it. Turbulent fluxes of sensible heat and momentum as a result of the impact of the synoptic flow on the frictionally active surface are assumed to be the only means of modifying the mixing heights. Cumulus convection, moisture flux, and the associated effects of latent heat change are ignored. As a case study, the procedure is designed to investigate diurnal/nocturnal and seasonal change by modelling using monthly time averages over a year s period, 1986, in Nairobi. To this end, a threet dimensional simple Eulerian model is attained in the analyses. Results show a reasonable resemblance between the nocturnal-diurnal and seasonal cycles of the computed mixing , " heights and the observed surface weather parameters, particularly wind speed and insolation. Seasonal variations were noticed for the estimated mixing heights during daytime only. The inversion strengths of 12~7 were comparable with those at the sinking arm of the Hadley cell of the general circulation. The inversion strengths however vary seasonally with the highest values in the period from June to August, and the lowest in the rainy months of April and October. The elevated inversions, though, are not responsible in determining ·the model depths of the planetary -xiil-· boundary layer owing to the lofty heights at which they occur. The inversions are substantially influenced by the upper level trade wind regime in addition to surface processes. Low ground heat storage, being just 10% to 0.5% of the global radiation, is due to the nature of the soil within the region. Horizontal and vertical variations in the wind field seem to be a consequence of the influence of the large forests, altered surface fabric, urbanisation and terrain undulations as they alter the synoptic flow.