Vertical Profile of Wind Diurnal Cycle in the Surface Boundary Layer over the Coast of Cotonou, Benin, under a Convective Atmosphere

Abstract

The characteristics of the wind vertical profile over the coast of Cotonou during wind convective diurnal cycle were explored in this study. Wind data at 10 m above the ground and the radiosonde data in the lower 60 m of the surface boundary layer were used over the period from January 2013 to December 2016. Based on Monin–Obukhov theory, the logarithmic and power laws have allowed characterizing the wind profile. The error estimators of the Root Mean Square Error (RMSE) and the Mean Absolute Error (MAE) were, respectively, evaluated at 0.025; 0.016 (RMSE; MAE) and 0.018; 0.015. At the site of Cotonou, the atmosphere is generally unstable from 09:00 to 18:00 MST and stable for the remainder of the time. The annual mean value of the wind shear coefficient is estimated at 0.20 and that of the ground surface roughness length and friction velocity are, respectively, of 0.007 m, 0.38 m·s−1. A comparative study between the wind extrapolation models and the data was carried out in order to test their reliability on our study site. The result of this is that whatever the time of the year is, only the models proposed (best fitting equation) are always in good agreement with the data unlike the other models evaluated. Finally, from the models suitable for our site, the profile of wind convective diurnal cycle was obtained by extrapolation of the wind data measured at 10 m from the ground. The average wind speed during this cycle is therefore evaluated to 8.07 m·s−1 for August which is the windiest month and to 4.98 m·s−1 for the least windy month (November) at 60 m of the ground. Considering these results, we can so consider that the site of Cotonou coastal could be suitable for the installation of wind turbines.

Vertical Profile of Wind Speed in the Atmospheric Boundary Layer and Assessment of Wind Resource on the Bobo Dioulasso Site in Burkina Faso

“…Despite this limit, these low values enable us to validate these various fitting equations based on the power law model and the logarithmic law as extrapolation models of the wind speed at the Bobo-Dioulasso site. Both laws can therefore be used to model the profile of the vertical wind speed at our study site as reported by the studies in [23] [45]. Near the ground, there is a significant wind speed variation that would be due to the impact of roughness and obstacles encountered on the ground, which decreases with altitude.…”

“…σ is the variance of the air temperature and ( ) 2 σ ω , the variance of the vertical wind component. According to [23], the standard deviation of the vertical wind component ( ) σ ω can be estimated from the parameter ( ) v σ which is the standard deviation of the horizontal wind component:…”

“…Table 2 details the various atmospheric stability classes according to Obukhov length. The roughness length and the friction speed can be calculated according to the various stability classes of the atmosphere by changing variables and using the Equation (1) [23]:…”

“…This method is easier to use in general for engineering studies [24] and thus enables to address difficulties encountered in using the linear-log law in terms of input Table 2. Atmospheric stability class according to Obukhov length [23].…”

“…Statistical tests of the root mean squared error (RMSE) and mean absolute value (MAE) were used to assess the errors made by the prediction. These indicators are the most used and the model is better when they are close to zero [23]. Using Equations (15) and (16), errors between speeds measured and those estimated are assessed […”