Abstract
This study surveyed the levels of boundary layer surface turbulence across a West African climate region. Five years (2011-2015) temperature and wind speed data at synoptic hours 0000 Hr, 0600 Hr, 1200 Hr and 1800 Hr within 0.125° grid resolution was sourced from Era-Interim Reanalysis platform at 1000 mbar pressure level. Using the Richardson (Ri) number technique, results showed that mechanical turbulence of Ri range 0.04 - 0.57 dominates across the surface layer for study locations of Port Harcourt, Enugu, Jos, Kano and Maiduguri than thermal turbulence. However, the least turbulent area was the coastal zone of surveyed region. Results also indicated that the vertical height (L) at which thermal turbulence replaces mechanical turbulence across study locations ranged from 20 - 250 m with lowest replacement levels (20 - 50 m) occurring mainly in the coastal area of Port Harcourt during periods of dawn. The most turbulent periods in the southern coastal location of study region were during key rainy periods from June-August while that for the rest far northern inland areas occur during the dry season/early rainy periods i.e. November-May. The implication of the lower surface turbulence/replacement level within coastal domains most especially during periods of dawn is that emission releases near surface layer will be dispersed after initial rise due to buoyancy at horizontal levels thereby increasing ground level pollutants concentration across sensitive receptors that are close to emission source. At heights of thermal turbulence replacement, emission releases will be transported vertically and then dispersed further away from emission sources, thus impacting sensitive receptors at farther distances. This is the atmospheric boundary layer dynamics that makes ground level pollution worse in the coastal city of Port Harcourt in recent times during periods of dawn. Efforts must be made by concerned Stakeholders towards ensuring that emissions are reduced during the periods of dawn within and around coastal environments.
Highlights
The surface layer is the layer in which humans, animals, and vegetation live and where most anthropogenic activities take place
This study examines surface layer turbulence across tropical climate zones in Nigeria using Richardson number (Ri) technique
The lower height of thermal turbulence (TT) replacing mechanical turbulence (MT) in Port Harcourt across the diurnal hours is an indication of the smaller diurnal range of ambient air temperature over the area arising from its close proximity to large water bodies which enriches the moisture content of the lower atmosphere than the rest study areas
Summary
The surface layer is the layer in which humans, animals, and vegetation live and where most anthropogenic activities take place. It is the bottom part of the boundary layer that connects the atmosphere to ground surface and is very vital regardless of its lesser share within the overall atmosphere due to its significant turbulent exchanges [1]. Boundary layer fraction is primarily responsible for low-level convergence and divergence of flow in the regions of lows and high surface pressures, respectively. The frictional convergence in a moist boundary layer is responsible for low-level convergence of moisture in low-pressure regions. Almost one-half of this loss on an annual basis occurs within the ABL, even though the ABL comprises only a tiny fraction (less than 2%) of total kinetic energy of the atmosphere
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