Impact of wind direction on diurnal and seasonal changes in wind profiles

Abstract

We explored the relationship between directional variation (changes in direction from a reference point) in vegetation and wind profiles, and propose an empirical wind profile model that may reproduce the wind profile within the canopy (such as secondary wind maximum) and reduce calculation loads. Based on the results of our observations in secondary broad-leaved forest, we clarified the variation in secondary wind maximum and aerodynamic parameters in wind direction, and assessed the influence of forest structural heterogeneity on the wind profile. Wind blew from specific directions depending on the time of day and season, and secondary wind maxima were observed in particular wind directions. Outlier estimations of aerodynamic parameters were determined for wind directions of 45°, 120-210°, and 300°, and these did not show a logarithmic distribution. Our proposed model reproduced the wind profile within and above the canopy, closely resembling K-theory and closure models, and reduced the required number of calculations and observations. The influence of forest structural heterogeneity was greatest in the upper part of the canopy (9.14 m), and reproducibility of the wind profile was improved by 0.05 m/s (3.4% on average) by changing the forest structural parameters of the model according to the wind direction. It was clear that forest structural heterogeneity did not cause critical errors in estimation of the wind profile, even at our study site, which had complex forest structure.

Influence of evaporation from the forest floor on evapotranspiration from the dry canopy

“…The values of d and z 0 were determined using the least squares method from the observed wind profile at heights of 18Ð8 m, 14Ð4 m, and 11Ð0 m above the canopy. Further details regarding the d and z 0 values were given by Daikoku et al (2007). The value of Ts can be calculated by assigning the calculation result of H from the eddy covariance technique, with calculated ga and observed Ta in Equation (3).ˇcan then be obtained by applying the Ts value in Equation (5).…”