A 3-DIMENSIONAL NUMERICAL-MODEL OF DEEP VENTILATION IN TEMPERATE LAKES

Because of the variation in the temperature of maximum density with de pth, it is known that the upper (above 250 m) and lower water columns circulate separately in deep temperate lakes. While near-surface water overturns twice per year, the deep waters often mix only partially du ring a complete annual cycle. It is believed that deep convection is t riggered by storm surge, forcing of some of the relatively cold upper water column downward through its compensation depth, so that it becom es unstable and sinks. The scale and intensity of the resulting deep w ater forming plumes are studied numerically. A high-resolution model b ased on the nonhydrostatic Boussinesq equations is used. Deep water fo rmation is initiated by applying a statically unstable, initial temper ature profile over various regions of the domain at depths below 300 m . The grid spacing is small enough to resolve individual plumes which carry surface water to the lake bottom. The domain of study is also la rge enough that the geostrophic Eady (1949) wave breakup of newly form ed deep water can be observed. It is found that when the initial insta bility is applied in midbasin, remote from the solid sidewalls, relati vely little fresh deep water is formed and the fluid quickly reaches a geostrophic and hydrostatic balance. Vigorous vertical mixing ceases after 4 days, and the resulting baroclinic fluid has a well-defined an d predictable scale. In contrast, when the initial instability is appl ied immediately adjacent to a solid boundary, vigorous plume motion co ntinues for the duration of the numerical simulations (12 days), produ cing a much greater volume of fresh deep water. A parametric study inv estigates the scale and intensity of this boundary mixing. It is found that reduced surface water temperature, Coriolis acceleration, or hig her horizontal diffusion coefficients increase the rate of deep water renewal.