Simulations of regional climatic effects of vegetation change in China

In the context of a regional climate model (RegCM2), simulation is undertaken of climatic effects of changed vegetation over China with their possible mechanisms studied, reaching results which show that the increase in flood events over the Yangtze-Huai River valleys and the aggravation of north-China droughts are likely to be under thejoint action of desertification of the northern grassland, and southern degradation of evergreen broadleaf trees, the latter seeming to be the primary factor. Severe vegetation degeneration leads to a positive feedback between rainfall reduction and degradation with the consequence that the disaster expands steadily outward and the area affected is difficult to turn back, into farmland. In contrast, less heavy degeneration causes a negative feedback between deterioration and rainfall reduction, and the stricken sector is likely to be reclaimed when human damages are decreased or removed. However, degeneration strengthens surface runoff, allowing flood occurrence. Vegetation deterioration makes the climate even more inhospitable. On the other hand, experiments show that the vegetation enlargement of northern grassland will lead to a mild climate, responsible for precipitation reduction in the outer belts of afforestation, possibly causing newly-planted trees to die stepwise inwardly, a result that mean, the current climate in the grassy land does not seem to support a plan of large-scale afforestation there. Study of climatic response to changed surface roughness and albedo shows that the former more strongly affects rainfall, atmospheric circulation patterns and humidity field, and the latter strongly affects temperature. Moreover, surface-released effective thermal fluxes (sensible and latent heat) follow the change in vegetation, and so does their Bowen ratio, thereby resulting in a change in moist static energy distribution in the atmosphere, followed by a corresponding alteration of atmospheric stratification and vertical motion, which, in turn, affects vapour transportation that, in connection with changed vertical motion, leads to rainfall variation.