Lake-air interaction plays an important role in controlling local weather and climate. This study aims to reveal the dynamical effect of water-land roughness contrast on precipitation during a summer rainfall event, which was detected by rain gauge and remote sensing observations around Lake Selin Co (a large Tibetan lake). During this event, precipitation amount to the west (downwind) of the lake was much higher than that to the east (upwind). Numerical experiments based on Weather Research and Forecasting model (WRF) were conducted to investigate this phenomenon. High-resolution WRF simulations can reproduce the precipitation contrast between downwind and upwind of the lake, but fails with the lake aerodynamic surface roughness enhanced by either enlarging the length directly or replacing the lake with adjacent land cover. Further analyses indicate that the water-land surface friction contrast directly leads to acceleration of wind above the water surface and deceleration over the land surface, enhancing the air convergence and precipitation over land area downwind of the lake. By contrast, the heat flux and evaporation from the lake are small in summer and hardly show important effects on precipitation around the lake. Therefore, the dynamical effect of the lake surface plays a dominant role in the precipitation enhancement downwind of the lake during the summer.