Drag coefficient and turbulence intensity in conifer canopies

The relationship between wind speed and drag coefficient was experimentally investigated in two mixed conifer forests of the Italian Alps. Drag coefficient was calculated from the mean momentum equation while canopy architecture was described using optical measurements of gap fraction. Two different formulations of the momentum equation were tested: the common one, where drag forces are expressed in terms of the squared mean velocity (¯u2), and an alternative one in which drag forces depend on the averaged product of instantaneous wind intensity and instantaneous longitudinal wind component |U|u. Values of drag coefficients computed according to the first approach, decrease with increasing wind speed in both sites and show different average values between sites (0.15 ± 0.14 and 0.34 ± 0.45). Drag coefficients obtained with the second approach do not show a clear dependence on wind intensity, and the mean drag coefficients of the two sites become more similar (0.09 ± 0.06 and 0.12 ± 0.12). According to these results the scaling based on |U|u seems more appropriate to characterise the physics of the phenomenon.