Some effects of corn and oak forest canopies on cold air drainage

Abstract

Nocturnal cold air drainage was examined in corn and oak canopies, and over a bare field on a hillside in Storrs, Connecticut. Smoke tracers, and vertical profiles of wind and temperature showed drainage flow development and persistence to be markedly affected by surface cover. In corn, the drainage flow maximums were in the order of 1 m s −1 above the canopy and 0.4 m s −1 inside the canopy. In the leafed hardwood forest, drainage flow only developed above the canopy. Only ground drainage flows developed in the forest during the leafless phase. Cold air drainage flows within the corn and leafless oak canopies were often maintained during periods of high local windspeeds (> 5 m s −1) when drainage flow above the canopies and over bare ground was no longer apparent. Measured flow rates were compared to predicted rates from models developed by Fleagle, and Petkovsek and Hocevar. We found the form assumed for the frictional force in these models inadequate for use with katabatic air flows in tall roughness elements. In some cases the Cionco canopy flow model, modified for stability, simulated the within canopy flows.