FLake is a bulk model. It is based on a
two-layer parametric representation of the evolving temperature profile and
on the integral budgets of heat and kinetic energy for the layers in question.
The structure of the stratified layer between the upper mixed layer and the
basin bottom, the lake thermocline, is described using the concept of
(assumed shape) of the temperature-depth curve. The same concept is used to
describe the temperature structure of the
thermally active upper layer of bottom sediments and of the ice and snow cover.
The result is a computationally efficient bulk model that incorporates much of
the essential physics.
FLake incorporates (i) a flexible parameterisation of the temperature profile in the thermocline, (ii) an advanced formulation to compute the mixed-layer depth, including the equation of convective entrainment and a relaxation-type equation for the depth of a wind-mixed layer, both mixing regimes are treated with due regard for the volumetric character of solar radiation heating, (iii) a module to describe the vertical temperature structure of the thermally active layer of bottom sediments and the interaction of the water column with bottom sediments, and (iv) a snow-ice module. Empirical constants and parameters of FLake are estimated, using independent empirical and numerical data. They should not be re-evaluated when the model is applied to a particular lake. In this way, FLake does not require re-tuning, a procedure that may improve an agreement of model results with a limited amount of data but should generally be avoided as it greatly reduces the predictive capacity of a physical model.
In order to compute fluxes of momentum and of
sensible and latent heat at the lake surface, a parameterisation scheme is
developed that accounts for specific features of the surface air layer over
lakes. The scheme incorporates (i) a fetch-dependent formulation for the
aerodynamic roughness of the water surface, (ii) advanced formulations for the
roughness lengths for potential temperature and specific humidity in terms of
the roughness Reynolds number, and (iii) free-convection heat and mass transfer
laws to compute fluxes of scalars in conditions of vanishing mean wind.