The observations in the pre-monsoon seasons of 1996 and 1997 indicated that two moraine-dammed glacial lakes, Tsho Rolpa and Imja, in the Nepal Himalayas have the different thermal structure in spite of the similar solar radiation, air temperature and wind system at the weather stations. The difference of thermal conditions in the lakes possibly results from the depletion in wind velocity over Imja Lake, which is caused by the screening of the upwind end-moraine to dead-ice zone 20-25 m higher than the lake surface. In order to clarify the topographic screening effect, three-dimensional numerical simulations of airflow around Tsho Rolpa and Imja Lakes were performed by building up a topographic model of actual size in the calculation domain. Simulated results indicated that the wind velocity at 2 m above the Imja Lake surface is 33-42 % smaller than that above the Tsho Rolpa Lake surface for the constant wind velocites of 1-5 m/s at 2 m above the points corresponding to the weather stations. This evidences the significant screening effect on the thermal structure of Imja Lake, since the wind force per unit lake area is proportional to the square of wind velocity at a certain height above the lake surface. The airflow simulations indicate that, as a barrier, the glacier terminus (ice cliff) leeward of Imja Lake decreases the wind velocity near the lake surface as much as does the upwind end-moraine to dead-ice zone.