Numerical model simulations and airborne measurements have been used to study the mesoscale flow modification of the boundary layer due to the presence of a lake (34 km2) located in the southern boreal zone. The mesoscale model used is a three-dimensional hydrostatic model with a level 2.5 turbulence closure. The results from this study show that in general, differences in surface roughness are more important than differences in surface temperature for the flow modification. The dynamical effect due to the roughness difference between the lake and the surrounding land creates divergence over most of the lake because of the accelerating flow, while a convergence zone appears at the downwind shore because of the deceleration. This pattern is found during nighttime for all simulated cases with wind speeds from 4 to 17 m s-1. This pattern persists during daytime for moderate to high wind speeds but is reduced in strength owing to stable stratification over the lake. For wind speeds ≤6 m s-1 a lake breeze circulation develops during the afternoon. Complicated flow patterns occur when the forces due to friction and thermal pressure gradient are comparable in size. Available measurements qualitatively support the model results. ¿ 2001 American Geophysical Union