Effect of climatic changes on stratification and deep‐water renewal in Lake Constance assessed by sensitivity studies with a 3D hydrodynamic model

Abstract

The implications of climatic change on Lake Constance are studied by long‐term hydrodynamic model simulations. In contrast to earlier studies that mostly have applied one‐dimensional models, this investigation utilizes a three‐dimensional hydrodynamic model and thus profits from the advantages of a spatial representation of lake bathymetry and hydrophysical processes. Model adaptation and validation are based on half a century of vertically resolved temperature recordings (1961 to 2011). Three different horizontal grid layouts are used to test the sensitivity of the thermal stratification and effective vertical turbulent diffusivities ( D V eff ) determined with the heat‐budget method to grid resolution. D V eff calculated from observations and from simulations with different grid resolutions agree rather well. However, in the deep water simulated D V eff are overestimated if the basin is resolved only by few grid cells. The investigation of the effect of climatic changes on Lake Constance is focused on the effects of altered air temperatures and wind velocities on deep‐water renewal. Numerical tracers are used as indicators of the winter vertical transport and mixing. Effects on stratification and mixing strongly depend on the seasonal course of the climatic changes. Warmer winter temperatures result in reduced deep‐water exchange and more frequent years with incomplete mixing. Increased air temperatures in summer have almost no effect on deep‐water renewal in winter but increase water‐column stability during summer and autumn. Increased wind speeds influence vertical mixing and surface heat fluxes and cause higher deep‐water temperatures if wind speeds are higher during the summer.