Abstract
Northwest 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 this region is evaluated. Satellite-derived vegetation products are applied in the model. Based on comparison with observational data and Reanalysis II data, the model generally captures major features of the NWC summer energy balance and circulation. These 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 the precipitation located mainly in the high elevation areas surrounding NWC. The sensitivity of the summer energy balance and circulation over NWC and surrounding regions to land surface processes is 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. 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, weaker vertical descending motion, and thus an increase in the precipitation over this region.
Highlights
Title The observed and simulated major summer climate features in northwest China and their sensitivity to land surface processes
Long et al (2003) pointed out that: (1) many land surface parameters used in the models are derived from experiments carried out in humid regions, which produce improper energy and water transfer over arid and semi-arid soils; and (2) rather crude land surface conditions are prescribed in many models for climate studies in NWC, e.g., the surface bulk transfer coefficient, the ordinary variables on finer spatial and temporal scales, etc
The temperatures simulated by the NCEP general circulation models (GCMs)/SSiB are much closer to the National Meteorological Information Center of China (NMICC) data (Fig. 2b), which are higher than the Reanalysis II data by 8–10°C
Summary
The coupled NCEP GCM/SSiB (Simplified Simple Biosphere; Xue et al, 1991) model is used, which has 28 vertical levels and a T62 (approximately 2◦) horizontal resolution. A vegetation map based on satellite observations is used in the NCEP GCM to specify the land cover condition (Hansen et al, 2000; Xue et al, 2004) We refer to this as the SSiB classification map, with a resolution of 1 km. General features of NWC summer climate and their simulation experiments are averaged This set of experiments, which uses the SSiB classification map, is referred to as Control, and is evaluated by using the Second Global Soil Wetness Project (GSWP-2; Dirmeyer et al, 2006) data, NCEP/DOE (Department of Energy) Reanalysis II data (Kanamitsu et al, 2002), and weather station data provided by the National Meteorological Information Center of China (NMICC). The 2-m air temperature has the same pattern over NWC (figure omitted), consistent with the surface skin temperature field
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