Impacts of Re-greening the Desertified Lands in Northwestern China: Implications from a Regional Climate Model Experiment

Abstract

This paper describes a study that investigates the local and regional effects of vegetation restoration in northern China via regional climate model simulations, and reports implications for the sustainability of vegetation under the altered rainfall regime. Ensemble simulations with the current vegetation cover and an idealized re-greening scenario for a test area in northwestern China (90 � –110 � E and 36 � –42 � N) were performed using large scale boundary forcing derived from 1998. The results indicate that such re-greening has both significant local and regional effects on the atmospheric circulation and rainfall distribution. Replacing desert and semi-desert areas with grass in the test area increases net radiation at the surface, and hence total heat flux from the surface to the atmosphere. This results in enhanced ascending motions and moisture supply to the atmosphere over the test area. Consequently, rainfall increases in the whole test area. However, the increase in rainfall largely occurs due to an increase in intensity rather than an increase in frequency. Lack of frequent rainfall, especially in the lowlands of the test area, makes it very difficult to maintain a vegetated surface. This implies that the current vegetation restoration activities will be largely limited to areas where water resources are relatively abundant, or they will depend heavily on irrigation. The increased runoff at higher elevations could be important for providing water for irrigation in the lowlands. The increase in rainfall in the highlands and far eastern parts of the test area, which already receive more frequent rainfall, may help support a restored vegetation cover in these regions. The enhanced ascending motions over the test area are compensated by increased subsidence to the east, centered over Yellow River Delta and Shandong Peninsula, resulting in a higher-pressure and anticyclonic circulation anomaly there. Consequently, rainfall decreases at these areas. This anticyclonic anomaly provides significant northeasterly low-level anomalous winds that enhance cyclonic shear vorticity in the Yangtze River Basin and South China when they meet southwesterly monsoonal flow. This causes strong ascending anomalies over southern China and the Sichuan Basin, and increases rainfall in these regions.