Eddy diffusion of momentum, water vapour, and heat near the ground

Abstract

An account is given of an experimental study of the factors which control the vertical turbulent transport of momentum, water vapour, and heat in the first 2 m of air above a short grass surface. The results of fifty-one observations, each of which extended over 30 min, are presented. Measurements were made of the vertical profiles of wind speed, temperature and humidity, and of the rate of evaporation and the aerodynamic drag of the surface. Measurement of the heat-balance components was also attempted. On any one occasion it was not generally possible to observe all the desired quantities. It is found that the wind speed and temperature profiles always have the same form, and that this form is shared on the majority of occasions by the humidity profile. The profiles depart from the logarithmic form found in adiabatic conditions in the manner suggested by Deacon. In near adiabatic conditions as defined by small numerical values of the Richardson number the established laboratory law relating the surface drag to the fluid velocity over a rough surface in turbulent flow is found to hold over the site and a value is deduced for von Kármán's constant. In conditions other than adiabatic it is shown that the explicit wind profile law suggested by Deacon holds in unstable but not in stable conditions. In the latter conditions equations proposed by Rossby & Montgomery and by Holtzman are found to represent the observations with reasonable precision. The friction coefficient of the surface is computed and found to be independent of wind speed but to increase in value in unstable conditions. Values of the eddy diffusivity for momentum, water vapour and heat are obtained on a direct observational basis from the expressions from which the diffusivities are normally defined, and it is found that the diffusivities for momentum and vapour are identical over the range of stability experienced. The magnitude of the diffusivity for heat often appears to be approximately the same as that for momentum or vapour, but exceptions occur when it is much larger than the other two. The exceptions do not appear to be related to stability.