Relationship between Nighttime Wind Speeds and Thermal Conditions above a Sloping Forest

Abstract

Researchers have examined relationships between wind speeds and thermal conditions above a slope for pure drainage flow with nearly no disturbance by shear flow, though disturbances by shear flow are commonly present. This study examined the relationship between wind speeds and thermal conditions above a slope when wind speed is affected by shear as well as drainage flow. Data used in this study were obtained at a sloping forest site in Thailand, which had been also used in our previous studies (Komatsu et al. 2003, 2005). The canopy height (h) of the forest was approximately 30 m above ground level and wind speeds were measured at 34 m, 43 m, and 50 m (i.e., 1.13h, 1.43h, and 1.67h, respectively).
After classifying measured wind speed data based on vertical air temperature differences, the mean and minimum wind speeds of each temperature-difference class were determined. Here, mean and minimum wind speeds refer to means and minimums among wind speeds for a specific temperature difference class. Regarding the 34-m wind speeds, mean wind speeds generally increased with increasing temperature difference and minimum wind speeds were non-zero, indicating the effect of drainage flow was generally dominant over the effect of shear flow at this height. Regarding the 50-m wind speeds, mean wind speeds decreased with increasing temperature difference and minimum wind speeds were nearly zero regardless of the temperature difference, indicating the effect of shear flow was dominant over the effect of drainage flow at this height. The 43-m wind speed data showed results that were intermediate to those of the 34- and 50-m wind speed data.
Results of such analysis bring information on the significance of the drainage flow effect on wind speed data measured at different heights above a slope. Thus, such an analysis would be useful for examining vertical structures of wind flow above a slope when both drainage and shear flow effects are present.