ON THE MAGNITUDE AND APPARENT RANGE OF VARIATION OF THE VON KARMAN CONSTANT IN THE ATMOSPHERIC SURFACE-LAYER

The half-century history of the experimental evaluation of the von Kar man ''constant'' in the atmospheric surface layer is reviewed, and evi dence indicating that this well-known scaling factor, k, is actually a weak variable that decreases with increasing Reynolds number is discu ssed. A combined average of k = 0.390 with a standard error of +/-0.01 0 is found from two field studies, a result which indicates that there is only one chance in 40 that the true value of the scaling factor in the population from which the experimental sample was drawn could hav e been as large as the laboratory value of 0.40. Based on experimental and theoretical results given by others, it is suggested that Ic vari es from a maximum of 0.41 in light winds over open water and smooth la nd surfaces, to a minimum near 0.37 in stronger winds over forests and cities. This range is shown to imply that a working value of Ic = 0.3 9 +/- 1% is appropriate for flows over surfaces characterized by 0.7 < z(0) < 8.7 cm, a roughness subrange which corresponds to a wide varie ty of terrain types from sparsely vegetated level plains to mixed crop lands with occasional hedges and trees.