Analyses of the Observed and Simulated Major Summer Climate Features of Northwestern China and Their Sensitivity to Land Surface Processes

Northwestern China (NWC) is a typical arid and semi-arid region. In this study, the main summer climate features over NWC are presented and the performance of an atmospheric general circulation model (NCEP GCM/SSiB) over the region is evaluated. Satellite-derived vegetation products are applied in the model. Based on a comparison with observed and Reanalysis II data, the GCM generally simulates the major features of the NWC summer energy balance and circulation. These major features include: a high surface temperature center dominating the planetary boundary layer; widespread descending motion; an anticyclone (cyclone) located in the lower and middle (upper) troposphere, covering most parts of central NWC; and precipitation located mainly in the high elevation areas surrounding NWC.

The sensitivity of the summer energy balance and circulation over NWC and the surrounding regions to land surface processes is preliminarily assessed with specified land cover change. In the sensitivity experiment, the degradation over most parts of NWC, except the Taklimakan Desert, decreases the surface-absorbed radiation and leads to weaker surface thermal effects. In northern Xinjiang and surrounding regions, less latent heating causes stronger anomalous lower-level anticyclonic circulation and upper-level cyclonic circulation, leading to less summer precipitation and higher surface temperature there. Meanwhile, the dry conditions in the Hexi Corridor produce less change in the latent heat flux. The circulation change to the north of this area plays a dominant role in indirectly changing lower-level cyclonic conditions, producing more convergence and weaker vertical descending motion, and thus an increase in precipitation over this region.