Numerical Investigation of the Regional Climate Effect of the Lake Nasser

Lake Nasser is the largest artificial reservoir in the world, with a water area of about 5000–6000 km 2 . This study aims to investigate the impact of the Lake on the local or regional climate in Upper Egypt with its surroundings using a high-resolution regional climate model. The numerical experiment is conducted with the RIEMS model with a boundary condition constrained by reanalysis ERA5 for 2010–2012. Two experiments are performed with and without the lake at the surface. The results show that the lake plays a role similar to a smooth, wet, cooling island in the hot desert. The cooling comes from lake water evaporation, which decreases the daily temperature difference resulting from the drop in daily maximal temperature and increase in minimum temperature. The smoothness of the lake water has the surface moisture flow accelerated and has local potential precipitation decreased and increased farther with additional northward moisture flows near the surface induced by the lake. Finally, the mechanism of the lake's impact is explored with the difference in horizontal moisture flux represented by the streamlines, whose pattern approximately coincides with the corresponding precipitation difference during the period. The main finding is that the lake has a significant impact not only on the local climate but also on regional climate, especially regional precipitation. It leads to an additional decrease in precipitation over the lake basin and an increase farther from the basin. This is the first time that a regional climate effect of an artificial lake has been proved by numerical experiment.