Impact of Internal Boundary Layer Development on Lake Hydrodynamics

The wind field over lakes can be spatially inhomogeneous because of meso- and microscale processes. Mesoscale processes (e.g., orographic winds) may have a more considerable impact in the case of large lake surface area, leading to the spatial variability of wind speed and direction. Micrometeorological processes also occur by the wind’s transition from land over to water surface. An internal boundary layer (IBL) may develop due to surface roughness change, as the water surface provides significantly less resistance to wind flow than the aerodynamically rough land surface. As a result, the wind speed increases along the fetch in the IBL over the lake surface. Therefore, the wind shear stress, the main driving force of waves and currents in lakes, also varies along the fetch. Measurements were carried out for four weeks in 2018 within an observational campaign in Lake Balaton to explore the IBL characteristics and establish a simple but reliable IBL model that can reproduce wind shear stress variability over the lake. We measured wind stress with the eddy-covariance (EC) technique at three locations along the fetch and by a sodar on the windward shoreline. We found a considerable spatial variability of wind stress caused by the IBL development and the wave state. We fit an analytic IBL model to the measured data that can quantitatively reproduce wind speed and wind stress development over the lake surface based on land observation of wind speed.

In Lake Balaton (area of 596 km², mean depth of 3.2 m), currents are almost only driven by the wind in the lack of throughflow; thus, we can simulate the lake’s hydrodynamics by providing an accurate wind forcing in both time and space. Despite the above-presented micro- and mesoscale processes, simple spatially homogeneous wind forcings are commonly used in lake models. We apply FVCOM for simulating 3D hydrodynamics in Lake Balaton using different wind forcings, including spatial interpolation from several wind stations and incorporating IBL effects. We assess different wind forcing models against flow profile measurements at an offshore location and water levels around the shoreline. Our analysis shows that measured water level fluctuations and flow direction profiles cannot be reproduced by applying a homogeneous vector-averaged wind stress field using ten anemometer data around the lake. In contrast, when multiple stations and the IBL development were considered in the wind forcing field interpolation method, the modeled current direction profiles matched better with the flow observations. We compare the modeled large-scale circulation patterns in the lake to show the critical role played by IBL development. The simulations confirmed that lake currents are very sensitive to wind forcing. Even though the wind directions had low deviations between different stations around the lake, their effect on the lake currents was significant. This work aims to support the calibration of lake models.