J. Otterman et al., TURBULENT HEAT-TRANSFER FROM A SPARSELY VEGETATED SURFACE - 2-COMPONENT REPRESENTATION, Boundary - layer meteorology, 64(4), 1993, pp. 409-420
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 o
f the plant height), appears inappropriate for highly structured surfa
ces such as desert-scrub or open forest. An approach is developed here
for computing sensible heat flux from sparsely vegetated surfaces, wh
ere the absorption of insolation and the transfer of absorbed heat to
the atmosphere are calculated separately for the plants and for the so
il. This approach is applied to a desert-scrub surface for which the t
urbulent 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 t
he northern Sinai. The plant density is expressed as the sum of produc
ts (plant-height) x (plant-diameter) of plants per unit horizontal sur
face area (the dimensionless silhouette parameter of Lettau). The sola
r heat absorbed by the plants is assumed to be transferred immediately
to the airflow. The effective turbulent transfer coefficient k(g-eff)
for sensible heat from the desert-scrub/soil surface computed under t
his assumption increases sharply with increasing solar zenith angle. a
s the plants absorb a greater fraction of the incoming irradiation. Th
e surface absorptivity (the co-albedo) also increases sharply with inc
reasing solar zenith angle, and thus the sensible heat flux from such
complex surfaces (which include open forests) is a much broader functi
on of time of day than when computed under constant k(g-eff) and const
ant albedo assumptions. The major role that desert-fringe plants play
in the genesis of convection and advection cannot be evaluated properl
y in the conventional calculations.