An Evaluation of Eddy Flux Variation in the Atmospheric Boundary Layer

Abstract

An evaluation is made of the typical variation with height and time of mean and eddy flow of momentum in the atmospheric boundary layer using carefully determined wind profile data obtained over a separation of 200 m in a downwind direction. Twenty-three half-hour runs were used comprising almost the whole of three days of daylight hours under conditions of no cloud. The measurements were recorded up to a height of 16 m at a site specially selected for horizontal uniformity. The overall average wind differences observed were approximately constant with height, implying a linear dependence with height of the mean vertical velocity w?. The average value of w? at the 16-m level was ?0.54 cm sec?1, with individual values ranging from ?1.49 to 0.81 cm sec?1. The departure of the eddy flux of momentum from the corresponding surface value was also found to be approximately proportional to height. The overall average departure of the eddy flux at the 16-m level was 16.8%, with excursions extending from ?44 to +55%. The enforced neglect of certain terms in the relevant equations may lead to overestimates of the quoted w? values and eddy flux variations. The importance of these results to basic studies of flux-gradient relations is discussed. In fundamental experiments the need to obtain an extensive body of data at good quality sites is emphasized.