Eddy diffusion of water vapour and heat near the ground

Abstract

Abstract An experimental study has been made of the factors involved in the turbulent transport of water vapour and heat in the lowest layer of the atmosphere over well exposed level grassland. Measurements were made over periods of 1 hr. of the water loss from isolated but otherwise naturally exposed sections of the surface layers of the soil and quantitative arguments advanced for adopting them as a reasonable approximation to the true evaporation loss from the ground surface. The incoming and reflected components of solar radiation, the temperature distribution in the soil down to 16 in. and the vertical profiles of temperature, humidity and wind speed in the air up to a height of 2 m. were observed at the same time, and samples taken to provide necessary data on the physical properties of the soil. The net flux of long-wave radiation was computed from the temperature and humidity structure of the atmosphere as given by the present low-level measurements and routine upper-air soundings. The data prescribe the vertical turbulent flux and the vertical gradients of the water vapour and heat content of the air, from which maybe evaluated the vertical components of the eddy diffusivities for water vapour and heat (Kv and KH) as customarily defined. In the absence of thermal stratification of the surface air layers Kv is shown to be identical with the eddy diffusivity for momentum (Km) defined by the explicitly established logarithmic law relating the aerodynamic drag and vertical wind shear over a rough surface. The modification of Kv by unstable and stable thermal stratifications and the rapid decrease of stability influence as the ground surface is approached are both quantitatively demonstrated, and a unique relation between parameters involving Kv, the vertical wind shear and the vertical temperature gradient is indicated. No completely satisfactory a priori explanation can as yet be given for the latter relation, though in unstable conditions Kv is found to be identical with Km computed from a recent wind-profile law which does not involve the temperature gradient explicitly and has only been established in functional form. Diroct comparison of Kv and KH reveals a reasonable approach to equality in stable conditions but shows that the latter coefficient is systematically and substantially the greater in unstable conditions. The latter feature is qualitatively in keeping with recent trends in the theoretical concepts of turbulent transport. The bearing of the results on the problem of indirectly evaluating natural land evaporation is briefly discussed and attention drawn to the implied superiority of the present ‘ hydro-dynamical’ approach over the classical ‘heat-balance’ method.