The Impacts Of Vegetation Change And Orography On Regional Weather And Climate In An Arid/semi-arid Region

Under the combined effect of global wanning and human activities, arid and semi-arid areas are faced with severe vegetation degradation, land degradation and desertification. The ecololy and environment in the arid and semi-arid regions of northern China, in particular, are under potential threaten to the vegetatioan degradation and landuses. A better understanding of the mechanism and the characteristic of weather and climate response to vegetation change is important for preventing semi-arid areas from further degradation and desertification.Using a meso-scale model (WRF) and long term observations, this paper first analysed and simulated the precipitation differences between an isolated mountain forest (Xinglong,103.84E,35.86N) and its surrounding area located at the arid/semi-arid region of northwest of China,in an attempt to understand the mechanisms responsible for the persistence of this isolated forest. The results indicate that the precipitation differences between Xinglong Mountains and its surrounding area are the largest in summer and autumn. The Xinglong mountain area is dominated by the southeastern wet flow in summer and autumn which provides a stable water vapor source for the region. In addition, the meso-scale high terrain blocks the water vapor from the southeastern flow and enhances precipitation. The contribution of local evapotranspiration to the precipitation is less than that of advection effects of water vapor. On the other hand, the rock tectonics of Xinglong Mountain covered by soil and forest also help to hold and store water for the growth of plants. Therefore, being different from the oasis, the persistence of the forest island (Xinglong mountains) in semi-arid regions is the consequence of the large-scale circulation, which transports more water vapor, the orography and the special geology which tend to help and store more water vapor for this area.Using a regional climate model, the climate responses to different degrees of vegetation degradation are further investigated in China-Mogolia arid and semi-arid areas (CMASA). The mechanisms of climate responses and the differences in climate responses upon different land-cover types are studied. Overall, vegetation decreases can cause an increase in the surface air temperature and a reduction in annual total precipitation. The total precipitation decreases nearly linearly with decreasing vegetation coverage. It is found that the reduction in total precipitation caused by vegetation changes is mainly due to the decrease in convective precipitation. Our analysis reveals the most vulnerable land-cover type in CMASA to the vegetation degradation is the dry grassland where vegetation decreases can cause significant reduction in convective precipitation. The local circulation responses to reduced vegetation cover are characterized by an anomalous anticyclone covering transient zone between the well vegetated surface and semi-desert area and this anomalous anticyclone is found to be related to the orography flow and variations in secondary circulation induced by the surface temperature gradient changes. This anomalous circulation tends to enhance the climate responses to vegetation changes over the transient zone. It is also found that the vegetation changes can weaken the decreasing trend of local precipitation and enhance the increasing trend of surface temperature. The effect of vegetation change on the local precipitation and surface temperature trends are relatively small compared to their corresponding background trends in CMASA. But vegetation change can largely affect the background trends in some areas.Finally, the comparison is made between the climate response of revegetation and vegetation degradation. The seasonal patterns of climate response to the vegetation change are also investigated. The results indicate that the climate responses to revegetation are overall opposite to those responses to vegetatation degradation, but both of revegetation and vegetation degradation affect the precipitation through changing convective precipitation. Our analysis reveals that the increasing amounts of evapotranspiration and precipitation induced by revegetation are smaller than the corresponding decreasing amounts induced by vegetation degradation owing to their different influences on Bowen ratio. And this makes regional climate of CMASA more sensitive to vegetation degradation than to revegetation. Our analysis indicates that the climate response to revegetation is most sensitive over deciduous forest while the responses over the transition zones between forest and grassland are not significant. A small increase and decrease in vegetation coverage can both lead to an increase in the moisture import. The climate responses induced by vegetation change are strongest in June and summer and are relatively weak in other seasons. It is also found that local circulation responses to revegetation are characterized by an anomalous cyclone tilting to the north which can enhance precipitation.