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The dynamical interplay between stratification, waves, and wind can is best summed up in using the conceptual model proposed by J. Imberger in 1990 and supported by the more recent measurements by A. Wuest and others. A lake behaves like an engine that is powered by the wind and does work against the potential energy gradient embodied in the stratification. Approximately 2% of the wind energy flux

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Figure 18 Observations from Lake Biwa (Japan) in 1993 showing steepened nonlinear basin-scale internal wave front and associated nonlinear internal waves. (a) Wind speed collected at 10-min intervals showing the passage of a storm event; (b) nonlinear response of basin-scale internal wave field showing steepened wave front (2 °C isotherms); (c) magnified view of shaded region in panel b showing details of NLIWs (2 °C isotherms). (d) Magnified view of shaded region d in panel b showing shear instabilities resulting from enhanced shear at the base of the surface layer during the strong wind event (1 °C isotherms); (e) Magnified view of shaded region e in panel b showing a V2 expansion of the metalimnion resulting from a thermocline jet that forms after a period of metalimnion compression (1 °C isotherms). The bottom isotherm in panels b and c is 10 °C. Adapted from Boegman L, Imberger J, Ivey GN, and Antenucci JP (2003). High-frequency internal waves in large stratified lakes. Limnology and Oceanography 48: 895-919.

enters the lake; of this ^80% is dissipated in the surface layer and ^20% is transferred to the basin-scale internal wave field. The basin-scale seiches energized by the wind are frictionally damped as they swash along the lake bed and by degeneration into progressive high-frequency internal waves, shear instabilities, and eventually turbulence. Approximately 90% of the seiche energy is lost energizing turbulent dissipation and mixing in the TBBL; < 1 / 4 of which first passes through the nonlinear internal

Buoyancy periods (f/N) 10000 100 1

Buoyancy periods (f/N) 10000 100 1

Figure 19 Spectra of the vertically integrated potential energy signals from Lakes Pusiano, Kinneret and Biwa showing basin-scale seiches (0-10~4 Hz), freely propagating nonlinear wave groups with sinusoidal profiles (—10~4 Hz) and solitary wave profiles (—10~3 Hz), and shear instabilities (—10~2 Hz). Nmax denotes the maximum buoyancy frequency. Adapted from Boegman L, IveyGN, and Imberger J (2005). The degeneration of internal waves in lakes with sloping topography. Limnology and Oceanography 50: 1620-1637.

instabilities ^

Free gravity^ waves

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Figure 19 Spectra of the vertically integrated potential energy signals from Lakes Pusiano, Kinneret and Biwa showing basin-scale seiches (0-10~4 Hz), freely propagating nonlinear wave groups with sinusoidal profiles (—10~4 Hz) and solitary wave profiles (—10~3 Hz), and shear instabilities (—10~2 Hz). Nmax denotes the maximum buoyancy frequency. Adapted from Boegman L, IveyGN, and Imberger J (2005). The degeneration of internal waves in lakes with sloping topography. Limnology and Oceanography 50: 1620-1637.

wave field prior to wave breaking and > 3/4 of which is lost to frictional swashing in the TBBL. The remaining —10% of the seiche energy results in intermittent shear instability in the basin interior. The overall mixing efficiency of the turbulence is —15% leading to an upwards buoyancy flux that works to weaken the stratification and raise the centre of gravity of the lake. The lake engine is extremely inefficient with only —0.06% of the wind work acting to irreversibly mix the stratification; the bulk of the wind work is lost to frictional viscous dissipation, which due to the large heat capacity of water has no significant effect on the lake temperature. From this model, it is clear that while internal waves control biogeochemical mixing and transport within a stratified waterbody, they are not able to significantly influence the stratification upon which they propagate. Consequently, their existence depends entirely upon the seasonal stratification cycle set up by the surface thermodynamics.

See also: The Benthic Boundary Layer (in Rivers, Lakes, and Reservoirs); Currents in Stratified Water Bodies 1 : Density-Driven Flows; Currents in Stratified Water Bodies 3: Effects of Rotation; Currents in the Upper Mixed Layer and in Unstratified Water Bodies; Density Stratification and Stability; Mixing Dynamics in Lakes Across Climatic Zones; Small-Scale Turbulence and Mixing: Energy Fluxes in Stratified Lakes.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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