Abstract
Abstract. This paper presents an improvement of a one-dimensional lake hydrodynamic model (Simstrat) to characterize the vertical thermal structure of deep lakes. Using physically based arguments, we refine the transfer of wind energy to basin-scale internal waves (BSIWs). We consider the properties of the basin, the characteristics of the wind time series and the stability of the water column to filter and thereby optimize the magnitude of wind energy transferred to BSIWs. We show that this filtering procedure can significantly improve the accuracy of modelled temperatures, especially in the deep water of lakes such as Lake Geneva, for which the root mean square error between observed and simulated temperatures was reduced by up to 40 %. The modification, tested on four different lakes, increases model accuracy and contributes to a significantly better reproduction of seasonal deep convective mixing, a fundamental parameter for biogeochemical processes such as oxygen depletion. It also improves modelling over long time series for the purpose of climate change studies.
Highlights
1.1 Hydrodynamics of vertical mixing in lakesLakes are recognized as sentinels of changes in climate and catchment processes (Shimoda et al, 2011; Adrian et al, 2009)
This paper presents an improvement of a onedimensional lake hydrodynamic model (Simstrat) to characterize the vertical thermal structure of deep lakes
The reduction factors result in a seasonal filtering of the wind, with a clear tendency to curb the transfer of wind speed energy to basin-scale internal waves (BSIWs) during the wintertime, while the time series during the stratified period remains almost unchanged
Summary
1.1 Hydrodynamics of vertical mixing in lakesLakes are recognized as sentinels of changes in climate and catchment processes (Shimoda et al, 2011; Adrian et al, 2009). The complex hydrodynamic processes occurring in stratified lakes are mainly governed by the combination of surface heat flux and wind stress (Bouffard and Boegman, 2012) The former sets up a density stratification by warming the near-surface water, which floats on top of the cold deep water. This stratification pattern isolates the lower parts of the lake (hypolimnion) from the surface layer (epilimnion) and acts as a physical barrier reducing vertical fluxes. The latter, wind stress, brings momentum into the system and thereby contributes to mixing. Basin-scale internal waves (hereafter BSIWs) play a crucial role in the transport of mass and momentum in the lake, driving horizontal dispersion and vertical mixing, with important implications for biogeochemical processes (Bouffard et al, 2013; Umlauf and Lemmin, 2005)
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