Among the important physical characteristics of a lake are whether it stratifies seasonally, and if so, the depth to which wind-mixing is limited by the stratification. It is generally known that sufficiently shallow lakes tend to remain isothermal throughout the year and that the depth of the thermocline in stratified lakes correlates positively with the surface area of the lake. Observations from lakes in several different regions of the temperate zone of the northern hemisphere show that whether a lake stratifies depends on both the maximum depth and the surface area of the lake, whereas the depth of the thermocline depends primarily on the surface area. A modification of previously published scaling arguments provides a plausible theoretical basis for some of this behavior. These arguments account for additional shear-induced mixing associated with the fundamental internal seiche in small lakes and with near-inertial motion in big lakes. For lakes of crossbasin diameter less than 5,000 m (surface area less than 25 km1, an estimate of the depth of the thermocline, h, at the time of maximum heat content is given by: h :::: 2.0 (-'-) 112 L 112 7 g..:lp where t is the wind stress associated with late summer storms, .dp is the density contrast between epilimnion and hypolimnion typical for lakes in that region near the time of maximum heat content, g is the gravitational acceleration, and L is the square root of the surface area of the lake. A consistent set of units must be employed.
This is a discussion of some aspects of the physical behavior of the Great Lakes written for scientists with backgrounds in disciplines other than physics. The basic physical characteristics of Great Lakes basins are summarized. These characteristics are determined by the facts that (i) the basins are closed, (ii) the basins are large enough so that the Coriolis force is an important component of their dynamics, (iii) the principal source of mechanical energy is the wind, and (iv) the basins are vertically stratified in summer. Discussion of large-scale horizontal motions includes both currents and diffusion. The advection–diffusion equation is used as a framework for a discussion which includes a summary of the basic problem confronting hydrodynamic modellers, the parameterization of turbulence phenomena in terms of mean flow variables. Vertical transfer processes are considered, notably the measurement of vertical fluxes of heat and momentum and the computation of eddy diffusion coefficients, the prediction of thermal structure in terms of net surface wind stresses and heat fluxes, and the interactions of waves, currents, and turbulence in the thermocline. The dynamical structure of the coastal zone is outlined, and the review concludes with recommendations for future work on the understanding of vertical turbulent transports, the climatology of Great Lakes coastal zones, and an operational approach to verifying and improving numerical models of lake circulation.
Previous studies of the central basin of Lake Erie have indicated, on the basis of lakewide budgets of heat and dissolved oxygen, that the thickness of the hypolimnion and the interaction of the hypolimnion with the overlying fluicl are important factors governing the dissolved oxygen concentration in the near-bottom water. Data collcctcd during an intensive field program in 1979 contain an example of an event during which both the thickness and temperature of the hypolimnion incrcasc due to an erosion of the thermocline from below. This thickening of the hypolimnion requires two conditions. First, the thermocline region, or mctalimnion, must be thick enough so that the effects of the energetic surface mixing processes are confined to the upper portion of the thermocline.And second, the currents in the thin hypolimnion must be strong enough to entrain overlying metalimnion water down into the hypolimnion.An analysis of the current meter data suggests the source of turbulent energy driving the mixing is shearing stresses at the bottom. During the entrainment event, the contribution to bottom dissolved oxygen supply is about 10% of the daily demand.
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