FLUCTUATIONS OF THE HORIZONTAL WIND UNDER UNSTABLE CONDITIONS

The similarity relations for sigma(u)/u proposed by Panofsky et al. ( 1977) and Hojstrup (1982) have been verified using eddy-correlation da ta collected during the EFEDA-experiment, conducted over the extensive plain of La Mancha (Spain), where vine plants form a primary crop. Al so, the standard Monin-Obukhov relation is considered. It is found tha t the expressions by Panofsky et al. and Hojstrup both yield almost id entical results, and are better than the Monin-Obukhov expression. Als o, sigma(u) measured with a cup anemometer obeys the similarity expres sions well, provided that the effect of the variation of wind directio n on propeller wind speed is accounted for. The relationship of Panofs ky et al. works rather well even when the boundary-layer height scale h is replaced by a fixed height, h(c). Best results were obtained for h(c) = 1800 m. This height scale is possibly associated with the horiz ontal variability in the surface sensible heat flux pattern.