The influence of stability on the turbulent transfer of heat, moisture and momentum fluxes in the surface layer over an agricultural farmland at a tropical location

 

 

Abstract

Data from the Nigerian Micrometeorological Experiment (NIMEX-1) recently conducted between 15^th February and 10^th March 2004, at a tropical agricultural field site in Ile-Ife (7 55^oE, 4 56^oE), south-western Nigeria have been used to investigate the effects of stratification (characterized here as the gradient Richardson number, Ri) on the scale of turbulent transfers of heat, moisture and momentum in the surface layer.

The vertical profile (at several levels) of temperature, moisture and wind up to 15m-height measured using sensitive cup anemometers, Frankenberger-type psychrometers, and a windvane were sampled at every 1 sec. and stored subsequently as 1 min. averages. The turbulent fluxes of 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), while sensible and latent heat fluxes were also determined using the Bowen ratio energy balance method. The friction velocity, u_* and eddy diffusivities for heat (K_h), moisture (K_w) and momentum (K_m) were also obtained from the data.

The results obtained showed that the heat fluxes from the eddy covariance and Bowen ration energy balance techniques were in very good agreement and vary by about 10%. Results also show that there is a significant dependence of u_{* ,} K_h , K_w and K_m on the surface layer stability, with a sharp decrease of these turbulence parameters with increasing stability. In the free convection regime (of low wind speed, U < 1.5 ms^-1, and intense surface heating, net radiation greater than 750 Wm^-2), the behaviour of the ratio of eddy diffusivities Kh/Km with the stability parameter, Ri is unlike that predicted from earlier studies. An empirical relationship that appears to fit the observed behaviour has been suggested.