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Wind Sheltering of a Lake by a Tree Canopy or Bluff Topography


A model is developed to quantify the wind sheltering of a lake by a tree canopy or
a bluff. Specifically, we introduce a simple model to predict a wind-sheltering coefficient a priori for use in 1-D lake hydrodynamic/water quality models. The model is developed from velocity measurements in a boundary layer wind tunnel, by investigating mean velocity and surface shear stress development downwind of two canopies and a bluff. The model is validated with field measurements over an ice-covered lake. Both wind tunnel and field experiments show that reduced surface shear stress extends approximately 50 canopy heights downwind. The reduction in total shear force is parameterized by a wind-sheltering coefficient; that is, a reduction of wind-affected area. While all measurements are made on solid surfaces, the wind-sheltering coefficient is shown to be applicable to the lake surface. Although several canopy characteristics, such as height, roughness, and porosity affect the transition of wind velocity and surface shear stress on a lake, a relationship based on canopy height alone provides sufficient input for a realistic estimate of the wind-sheltering coefficient. The results compare well with wind-sheltering coefficients estimated by calibration of lake water temperature profile simulations for eight lakes.