Turbulent heat transfer from a sparsely vegetated surface: Two-component representation

Abstract

The conventional calculation of heat fluxes from a vegetated surface, involving the coefficient of turbulent heat transfer, which increases logarithmically with surface roughness (commonly taken as about 0.12 of the plant height), appears inappropriate for highly structured surfaces such as desertscrub or open forest. An approach is developed here for computing sensible heat flux from sparsely vegetated surfaces, where the absorption of insolation and the transfer of absorbed heat to the atmosphere are calculated separately for the plants and for the soil. This approach is applied to a desert-scrub surface for which the turbulent transfer coefficient of sensible heat flux from the plants is much larger than that from the soil below, as shown by an analysis of plant, soil and air temperatures measured in an animal exclosure in the northern Sinai. The plant density is expressed as the sum of products (plant-height) x (plant-diameter) of plants per unit horizontal surface area (the dimensionless silhouette parameter of Lettau). The solar heat absorbed by the plants is assumed to be transferred immediately to the airflow. The effective turbulent transfer coefficientkg-eff for sensible heat from the desert-scrub/soil surface computed under this assumption increases sharply with increasing solar zenith angle, as the plants absorb a greater fraction of the incoming irradiation. The surface absorptivity (the co-albedo) also increases sharply with increasing solar zenith angle, and thus the sensible heat flux from such complex surfaces (which include open forests) is a much broader function of time of day than when computed under constantkg-eff and constant albedo assumptions. The major role that desert-fringe plants play in the genesis of convection and advection cannot be evaluated properly in the conventional calculations.