The influence of spatial wind inhomogeneity on flow patterns in a small lake

A two-dimensional vertically averaged flow model was applied to study the circulation patterns in Lake Belau (Northern Germany). The lake is north–south oriented, with a maximum extension of 2.2 km and is situated in a dead ice hole with steep slopes on the east and west coasts. A part of the coastal area is covered with forest, creating a strong shelter from prevailing south-westerly winds. The system of shallow water equations is discretized with the modified Utnes (1990) scheme which is characterized by a semi-decoupling algorithm. The continuity equation is rearranged to Helmholtz equation form. The upwinding Tabata (1977) method is used to approximate convective terms. Extensive wind (more than 60 observations) and current measurements (5 points) conducted all over the lake enabled us to verify the simulation results with observations. Under spatially homogeneous wind conditions the model predicts a two cell circulation system, that covers most part of the lake. Taking the spatial variation of wind speed due to shelter into account, the flow field changes drastically. The two cell system is replaced by one large cell, with a strong reverse jet along the western shore. The sheltering effect of the surrounding hills and vegetation have a pronounced effect on the circulation pattern. It appears that in general this fact cannot be neglected in numerical lake flow simulations.