The Modified Microclimate in Small, Square, Sheltered Plots
Windbreaks are ubiquitous modifiers of our microclimate. Their primary effects are wind "reduction" (though they may actually increase the gustiness of the wind), and shading. Secondary effects, on the temperature, humidity and evaporation, are considered their principal benefit, but are poorly understood. In fact, we are only beginning to be able to quantify the microclimatic impact of a windbreak.
Abstract (Argete & Wilson, 1989; Agric. & Forest Meteorol., Vol. 48).
The difference in microclimate between the centre of a small square sheltered field and the undisturbed flow was examined experimentally as a function of atmospheric stability for two plot sizes D/H=8,16 (where D is the plot side length and H is the windbreak height) and two values of the height to roughness length ratio H/z0=25,200. We isolated the short-term aerodynamically induced microclimate change by measuring short-term changes in the mean equivalent temperature Teq (a measure of the total heat content of the air: the hypothetical temperature that would be attained if the vapour present were condensed to release the latent heat).
During the day, Teq in the smaller plot (D/H=8) exceeded the undisturbed value by as much as about 5 T* (where T* is the equivalent temperature scaling parameter, determined essentially by the net radiation and the friction velocity; typical values of T* over a moist surface in summer are of order 1 C). A converse effect of similar magnitude occurred at night.
In contrast, the effect in the centre of the larger plot (D/H=16) was opposite in sign (by day or night), and smaller in magnitude. It is concluded that one may obtain a microclimatic benefit (increased temperature over most of the plot area) only by using a small plot size (D/H < apprx. 10). This is due to the turbulent wake of the shelter, which increases exchange with the overlying flow, except in a restricted zone lying in the immediate lee of the shelter, and bounded by a line from about shelter-top to the ground (at about) 10H downstream.
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Last Modified: 23 Dec 1998
