ABSTRACT: Stratified lakes present challenges to numerical modeling not encountered in estuarine flows. The vertical mixing processes are small, both instantaneously and cumulatively, as evidenced by the small seasonal change in hypolimnion temperature found in monomictic temperate lakes. The small cumulative vertical flux which does exist is important for ecological processes, but the mixing is energized by a variety of sources and occurs predominantly within a benthic boundary layer at small scales relative to practical model grid resolution. The fundamental control for the cascade of energy from the wind into mixing and benthic boundary layer transport appears to be the basin-scale internal wave field, which can be modeled on a relatively coarse mesh. However, attention must be given to the processes in the surface layer that lead to the setup of a baroclinic tilt. Below the wind-mixed surface layer the advective motion is driven principally by the basin-scale internal waves, whose net transport is a function of nonlinear effects from topography and wave amplitude. A further modeling complication is the convoluted bathymetry often associated with reservoirs formed in drowned river valleys and many lakes formed naturally in geological rifts. Cold inflows associated with storm events may flow rapidly through the narrow bottom of reservoirs where model grid resolution is generally poor. In this paper we discuss some of the challenges in numerical modeling of stratified lakes and some of the techniques which we are using and developing to address these challenges.

EXTRACT: Figure 1.

Figure 1: Transformation of wind and thermodynamic energy into stratification. The energy from the wind and thermodynamics directly influences the TKE and stratification in the upper layer (epilimnion) while indirectly influencing the mixing across the thermocline (metalimnion) through entrainment and generation of internal waves. Evidence of Antenucci and Imberger (2000) points to the existence of direct forcing of non-hydrostatic internal waves by a mechanism that is as yet unknown.

Acknowledgments: Supported by the Centre Environmental Fluid Dynamics and the Centre for Water Research at the University of Western Australia, Kinneret Limnological Laboratories, Western Australia Waters and Rivers Commission, Western Australia Estuarine Research Found-ation, Sydney Catchment Authority, and the Australian Research Council under grant A89802317.