Wind sheltering of a lake by a tree canopy or bluff topography

Abstract

[1] A model is developed to quantify the wind sheltering of a lake by a tree canopy or a bluff. The experiment-based model predicts the wind-sheltering coefficient a priori, without calibration, and is useful for one-dimensional (1-D) lake hydrodynamic and water quality modeling. The model is derived from velocity measurements in a boundary layer wind tunnel, by investigating mean velocity profiles and surface shear stress development downwind of two canopies and a bluff. The wind tunnel experiments are 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 from the transition. The reduction in total shear force on the water surface is parameterized by a wind-sheltering coefficient that is related to the reduction of wind-affected lake 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 its height, aerodynamic roughness, and its porosity affect the transition of velocity profiles and surface shear stress onto a lake, a relationship based on canopy height alone provides a sufficiently 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.

Automated calculation of surface energy fluxes with high-frequency lake buoy data

“…The program does not include quality control procedures and assumes that instrument calibration has been performed. The algorithms also assume that the input data is measured on the lake, particularly wind speed, which will vary from that measured on land (Markfort et al 2010) and will have a substantial influence on the turbulent surface fluxes. A wind-sheltering coefficient is often applied to land-based wind measurements in numerical models to account for the difference in roughness lengths…”

Physical responses of small temperate lakes to variation in dissolved organic carbon concentrations

“…For all years from 1989 to 2010, we used NTL-LTER observations ice-on and ice-off dates, DOC concentrations, and meteorological driver data to simulate openwater temperatures for Trout Bog. After simulating and adjusting the model for size-specific parameters, such as the wind sheltering coefficient (Markfort et al 2010), we randomized yearly DOC concentrations (holding the derived relationship between DOC and K d constant), and randomly iterated through simulation years. This process was repeated for 1000 simulations, and we calculated the volumetric average water temperature for each simulation.…”

Transparency, Geomorphology and Mixing Regime Explain Variability in Trends in Lake Temperature and Stratification across Northeastern North America (1975–2014)

“…The strength of thermal stratification is increasing more rapidly in lakes of intermediate depth (Figure 6f) with larger heat budgets and more resistance to increases in total thermal content throughout the water column than smaller lakes [19]. While the smallest lakes are often polymictic, smaller dimictic lakes can be sheltered from wind and turbulent mixing, with minimal transference of the climate signal below the thermocline, resulting in increased stratification strength [18,59,63]. Similarly, reduced mean annual wind speed in recent decades in this region [64] may lead to enhanced thermal stratification.…”