characterization of stability conditions in the atmospheric surface layer of Obafemi Awolowo University, Ile-Ife, Nigeria

Abstract

Micrometeorological from the Nigerian Micrometeorological Experiment (NIMEX) conducted between 24th February and 10th March 2004, at a tropical agricultural field site in Ile-Ife (7.550N, 4560E), south-western Nigeria has been used to characterize the stability conditions for the surface layer. The vertical profile (at several levels) of temperature and speed up 10 15m-height measured using sensitive cup anemometers, Frankenberger-type psychrometers, and a wind vane sampled at every I see, and stored subsequently as I min. averages. The turbulent fluxes off heat, moisture and momentum were measured directly with an eddy covariance system, consisting of an ultrasonic anemometer and a krypton hygrometer (sampled at 16 Hz and 8 Hz respectively), The result reveals that night conditions were always stable giving positive Richardson number hut just after sunrise they became unstable with a negative Richardson number. The unstable condition lasted during the day, and then returned to stable condition just before sunset. This unstable condition was intense When the surface temperature and solar radiation received was greater indicating strong turbulence in the layer. This behaviour is found to be the same with the diurnal variation of the Obukhov length. relationship between the gradient Richardson number (Ri) and the stability parameter (z//,) shows remarkably small sealter is nearly linear in unstable condition, during stable condition, Ri approaches a o/ about 0.35 as tends to infini1Y. The Friction velocity was to he dependent wind velocity. The friction velocity, eddy diffusivities for heat (Kh) and momentum (Kw). Turbulent Kinetic Energy (TKE) obtained from the data showed significant dependence on the Surface layer stability, with a sharp decrease of these parameters with increasing stability.