The microclimate in the centre of small square sheltered plots

Abstract

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 z 0=25, 200 . Following McNaughton (1988), we isolated the short-term aerodynamically induced microclimate change by measuring the short-term changes in the mean equivalent temperature T ̄ eq (a measure of the total heat content of the air). In the small plot, T ̄ eq exceeded the undisturbed value by as much as ∼5 T eq ∗ (where T eq ∗ is the equivalent temperature scaling parameter, determined essentially by the net radiation and friction velocity) during the day, with a converse effect of comparable magnitude occurring at night (i.e., reduction of the equivalent temperature in the small plot at night by an amount of order 5 T eq ∗ ). In contrast, the effect in the centre of the large plot was opposite in sign (by day and by night) to the changes observed in the small plot and smaller in magnitude. Hence, 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 ⪅10 ).