Wind sheltering of small lakes by complex terrain

Lakes with a surface area of less than 10 km² account for over 50% of the global cumulative lake surface water area, and make up more than 99% of the total number of global lakes, ponds, and wetlands. Within the boreal regions, a significant proportion of land cover is characterized as lakes or wetlands, which affect land-atmosphere fluxes. Most of these small water bodies are surrounded by complex terrain and forest. The atmospheric boundary layer over the surface of a small lake depends strongly on wind sheltering.  Wind blowing over a small lake or wetland is highly variable and affects sensible and latent heat fluxes and the lake heat budget. Wind mixing of the lake surface layer affects thermal stratification and air-water gas transfer, e.g. O₂, CO₂, and CH₄.  As the wind blows from the land to the lake, wake turbulence behind trees and other shoreline obstacles leads to a recirculation zone and enhanced turbulence. This wake flow delays the development of wind shear stress on the lake surface, and the fetch required for the shear stress to fully develop may be ~1 km. This wind sheltering must be considered when determining the effect of wind on lake stratification, surface gas transfer, lake water quality and fish habitat. 

We will present wind tunnel measurements of turbulent wake flows behind canopies of various sizes, field measurements of the wind over a sheltered lake and a new method to quantify wind sheltering of lakes that takes into account lake size, shape and the surrounding landscape. The model is validated against field data for 36 Minnesota lakes.