The Effect of Atmospheric Stability on the Surface-Layer Characteristics in a Low-Wind Area of Tropical West Africa

• Published in: Boundary-layer meteorology 1997-11, Vol.85 (2), p.309-323
• Main Authors: Jegede, O.O., Fasheun, T.A., Adeyefa, Z.D., Balogun, A.A.
• Format: Article
• Language: English
• Subjects: Meteorology; Wind speed
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1023/A:1000447600185

This study details the observed effects ofatmospheric stability on characteristics of thesurface layer in a low wind speed (U = 1.5 m s-1)regime of tropical West Africa. Theaerodynamic roughness length, z0, anddisplacement height, d, obtained from profilewind-speed data at our bush land site (height [asymptotically =]2 m)have values of 0.24 ± 0.10 m and 1.54 ± 0.04 mrespectively. In the unstable range (-2.5 < Ri < -0.1; Riis gradient Richardson number), thestandard deviation in wind speed fluctuations, σu, increased from 0.57 ± 0.19 m s-1 toa maximum of 0.7 ± 0.2 m s-1 in near-neutralconditions, and in the stable range, the parameterdecreased rapidly to 0.41 ± 0.15 m s-1 at Ri [asymptotically =] 0.2.In the same stability range, the horizontal winddispersion, σ[theta], decreased withincreasing stability from 19 ± 8 deg. to 13 ± 5 deg.The surface-layer integral quantity, σu/u*, when plottedas a function of stability, is in agreement with theempirical results. The ratio ofsensible heat flux (estimated) to the net radiationranged between 0.1 and 0.2 at nighttime,increasing to about 0.5 during the daytime, and showeda strong dependency on season.[PUBLICATION ABSTRACT]