The Roughness Length for Heat and Other Vegetation Parameters for a Surface of Short Grass

Abstract

Observations are presented that were made in the lower 2 m of the atmosphere and in the soil near the Cabauw mast in the Netherlands. The surroundings of the mast are horizontally homogeneous and the soil is covered with short grass. In the air, the wind, temperature, specific humidity, and different radiation fluxes were measured, whereas in the soil, observations of the temperature and soil heal flux were made. From the observations, the ratio of the roughness length for heat and momentum has been calculated and is shown to depend strongly upon the friction velocity u*. From a theoretical analysis of the heat transfer from the vegetation towards the air, it is shown that the ratio of the roughness length for heat and momentum is a function of the friction velocity, the leaf-area index (LAI), and the Prandtl number. It is shown that the observational results compare reasonably well with the theoretical prediction. In addition to the ratio of the roughness length for heat and momentum, the roughness length for moisture, albedo, and canopy resistance has also been calculated from the observations.