The formation mechanism of the arid climate in northwest China is investigated using regional climate model. One may think that the arid climate in this region is caused by the rain-shadow of the surrounding mountains. Two kinds of long-term numerical experiments are carried out with real topography and with modified topography which does not contain the TianShan and the Altai Mountains. Simulated rainfall distribution with real topography shows maximum precipitation in upwind slope of the mountain and minimum precipitation in downwind side of it, which is consistent with the feature of rain-shadow in other regions of the world. However, numerical experiment without the TianShan Mountains also simulates the area of distinct minimum precipitation in the same location. The arid climate, therefore, is formed even though the rain-shadow effect is absent. Whether the TianShan Mountains exists or not, the subsidence tends to prevail over the arid region in summer, and suppresses the precipitation when synoptic scale disturbances are situated in the vicinity of the desert region.
The sensitivity of the subsidence over the arid region during summer is investigated by other series of regional climate model experiments. An experiment without diabatic heating shows very gentle vertical motion over the arid region. The subsidence is strongly enhanced in case that the diabatic heating processes are added in the model. Additionally, horizontal and vertical structures of the vertical motion are very similar even when diabatic heating by only radiation process is included in the model. The elevated surface of the mountainous region, especially the Tibetan Plateau, is heated in the model as a result of solar radiation. Since atmospheric circulation around the eastern Asia should be strongly influenced by the elevated heat source, the other experiments are conducted assuming an artificial heating over the Tibetan Plateau. Then, the simulated feature of the vertical motion is very similar with that by full diabatic heating model. These results suggest that the mechanical effect of the Tibetan plateau is not a principal reason for the subsidence. On the other hand, thermal effects of the plateau induce the prominent subsidence. The interaction between Rossby mode and subtropical westerly gives the subsidence over the arid region during boreal summer, since subtropical westerly jet locates north of the plateau. Consequently, precipitation is suppressed by the stationary descending motion around the arid region in northwest China, which is caused by the interaction between zonal wind and diabatic heating over the Tibetan Plateau.