J. Meteor. Res.  2014, Vol. 28 Issue (5): 836-848   PDF    
http://dx.doi.org/10.1007/s13351-014-4010-x
The Chinese Meteorological Society
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Article Information

LI Qian, XUE Yongkang. 2014.
The Observed and Simulated Major Summer Climate Features in Northwest China and Their Sensitivity to Land Surface Processes
J. Meteor. Res., 28(5): 836-848
http://dx.doi.org/10.1007/s13351-014-4010-x

Article History

Received January 24, 2014;
in final form May 26, 2014
The Observed and Simulated Major Summer Climate Features in Northwest China and Their Sensitivity to Land Surface Processes
LI Qian1 , XUE Yongkang2,3    
1. Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China;
2. Department of Geography, University of California, Los Angeles, CA 90095-1524, USA;
3. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1524, USA
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 tem- perature 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 domi- nant 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.
KeywordsNorthwest China (NWC)     arid and semi-arid regions     atmospheric general circulation model     land cover change     land surface processes    
1. Introduction

Approximately 50% of the l and surface in Chinais arid or semi-arid, over 80% of which is in NorthwestChina(NWC)(Dong et al., 2006). NWC is locatednorth of 35°N and west of 107°E, and includes western Inner Mongolia, Gansu Province, Ningxia Hui Autonomous Region, and Xinjiang Uygur AutonomousRegion. Most of the l and surface in NWC is typicallyGobi and desert l and with sparse vegetation. In thenorthern and western mountain areas, the dominantvegetation types are needle-leaf evergreen forest and grassl and. The annual precipitation over NWC is lessthan 50 mm, but annual potential ev aporation reachesgreater than 3000 mm. As the situation in manyarid and semi-arid regions, there is large sensible heatflux in NWC, which is transferred to the atmosphere, afecting the general circulation and the East Asianmonsoon(Bueh et al., 2002; Huang et al., 2006; Zhou and Huang, 2006). Such a region is often called the\thermal pad" in the Eurasian continent(Huang et al., 2006). Despite the above known facts, there have beenfew studies on the climate variability and sensitivityof NWC to l and surface processes. Underst and ing theclimate and environmental change of NWC, as well asthe associated physical processes, is a major objectivein climate change research in this part of the world.

Despite their usefulness, general circulation models(GCMs)or regional climate models(RCMs)havebeen only moderately used to study the role of l and surface process in the climate change of arid areas inNWC(e.g., Long et al., 2003). One study, which employed the l and surface model(LSM)of the NationalCenter for Atmospheric Research(NCAR)(Wang et al., 2002), showed that sensible heat flux is an important part of the energy balance in the arid areasof NWC, but latent heat flux should not be ignoredwhen precipitation occurs. Using a two-dimensionalmesoscale soil-plant-atmosphere model, Zhang and Zhao(1998)studied the diurnal variation of soil water and physical variables of leaf surfaces, as well asthe spatial surface energy budget distribution overtwo contrasting surfaces(oasis and desert)in NWC.However, in the above-mentioned model-based studies, l and surface processes are not well represented inthe models. Long et al.(2003)pointed out that:(1)many l and surface parameters used in the models arederived from experiments carried out in humid regions, which produce improper energy and water transferover arid and semi-arid soils; and (2)rather crudel and surface conditions are prescribed in many modelsfor climate studies in NWC(because quite limited observ ational data are available for descriptions of l and surface vegetation characteristics and l and surface processes), e.g., the surface bulk transfer coeffcient, theordinary variables on fner spatial and temporal scales, etc. Although a few feld experiments have been conducted and some important results are obtained, e.g., the Heihe River basin Field Experiment(HEIFE; Hu and Gao, 1994), they are insuffcient for large-scale climate and environmental change studies in NWC.

In this study, we introduce satellite-derived l and surface conditions for NWC. Previous studies have indicated the importance of vegetation characteristics, such as leaf area index(LAI), on regional and globalclimate, and that unrealistic vegetation presentationin GCMs can increase the model bias in the simulation of water and energy transfer between the nearsurface atmosphere and l and (e.g., Buermann et al., 2001; Friend and Kiang, 2005). Nowadays, more dataon these crucial characteristics are available for climate research with the development of remote sensing and retriev al methodologies(Kang et al., 2007).We apply satellite-derived LAI(referred to as RSLAI)in this study on NWC regional climate. Buermannet al.(2001)pointed out that the introduction ofRSLAI in the NCAR GCM can improve simulationsof near-surface temperature and rainfall over some regions. Meanwhile, experiments by Kang et al.(2007)showed that simulations of near-surface climate in theEast Asian summer monsoon regions can be greatlyimproved by using RSLAI and other satellite-derivedl and surface data in the NCEP GCM. The presentstudy assesses for the frst time the capability of theNCEP GCM by using RSLAI and a satellite-derivedvegetation map(Xue et al., 1996)in simulating theclimate of NWC.

Another aim of the current work is to ev aluatethe possible efects of l and surface processes on theclimatic features of NWC in summer. As the socialeconomy and population density continue to increasein the region, there is evidence that l and use and the quality of the environment in NWC are changing considerably, and the surface ecosystem is becoming increasingly fragile(e.g., Xu et al., 2002). Thesurface vegetation cover has seriously deteriorated invast parts of NWC since 1995, as identifed by multiple sources, e.g., the NOAA's Advanced V ery HighResolution Radiometer(A VHRR) and the T elevisionInfrared Observation Satellite's Operational V erticalSounder(TIROS-TOVS), and as documented in Xu etal.(2007). When l and cover changes, l and variablessuch as albedo, surface roughness, and bulk transfercoeffcient also change, which leads to variations insurface heat fluxes and ultimately results in surfacetemperature anomalies. Accordingly, a sensitivity ex-periment in which the vegetated parts of NWC arealtered to bare ground is conducted in the presentstudy. Using dramatic changes to l and conditions toev aluate the impacts of l and surface processes has beenwidely used in regional studies. This may also serve asa preliminary platform upon which more comprehensive studies can be built. F or example, this methodhas been applied in l and cover change studies over theAmazon(e.g., Shukla et al., 1990), the Sahel(e.g., Xue and Shukla, 1993), and East Asia(e.g., Y atagai and Yasunari, 1995; Xue, 1996; Fu, 2003).

Many statistical analyses and numerical experiments over diferent regions of China have shown thatl and cover change exerted great impacts on local precipitation and temperature(e.g., Xue, 1996; Zheng et al., 2002a, b; Ding et al., 2005). F or NWC, acorrelation analysis with observ ational data, including reanalysis data from the ECMWF, and NOAAA VHRR and TIROS-TOVS satellite remote sensingdata, showed that the changes in l and cover couldhave resulted in a decrease in total cloud cover and an increase in air temperature, as well as impacts onother regions' climate(Xu et al., 2007). Using theFifth-Generation Penn State/NCAR Mesoscale Model(MM5), and based on observed data, Chen et al.(2009)examined how the variation in regional climatebetween the 1970s and 1990s in NWC relates to l and cover changes, with a particular focus on the impact onsurface temperature and circulation. Although therehas been progress in studying the impact of l and coverchange on the precipitation over NWC, compared withl and cover change studies in other regions such as theSahel, the interacting processes and associated physical processes including the surface energy balance and water balance, are not fully understood for NWC.Therefore, in this study, the efects of l and surface processes on the major summer climate features in NWCare preliminarily identifed and ev aluated by using al and degradation scenario.

The remainder of the paper is structured as follows. The model and experimental design are described in Section 2. In Section 3, we present and discuss the simulated results, in which the capabilityof the model in using satellite-derived vegetation products to simulate the summer surface energy balance and circulations is highlighted. In Section 4, we discuss how the summer climate features over the NWCare afected by l and cover change. Finally, in Section5, we provide some concluding remarks. 2. Model description and experimental design

In this study, the coupled NCEP GCM/SSiB(Simplifed Simple Biosphere; Xue et al., 1996)modelis used, which has 28 vertical levels and a T62(approximately 2°)horizontal resolution. As a forecasting model, the NCEP GCM is also used for climatestudies. The fluxes of momentum and surface energy, skin temperature, as well as albedo, are provided tothe GCM by the biophysical model, SSiB.

Satellite-derived vegetation products provideseveral vegetation properties used in the NCEPGCM/SSiB. The satellite product is the F ourierAdjusted, Sensor and solar zenith angle corrected, Interpolated, Reconstructed Normalized Diference V egetation Index(F ASIR-NDVI), which covers 17 yr from1982 to 1998(Los et al., 2000). This dataset includesLAI, vegetation cover infraction(VCI), leaf fraction, and surface roughness length, and is described in detail by Los et al.(2000). Meanwhile, a vegetation mapbased on satellite observ ations is used in the NCEPGCM to specify the l and cover condition(Hansen et al., 2000; Xue et al., 1996). We refer to this as theSSiB classifcation map, with a resolution of 1 km2.When aggregating the vegetation map to the GCMgrid points, the cover types are grouped into 12 SSiBvegetation types(Xue et al., 1996), and then the maintype is selected in each T62 cell(Fig. 1a).

Fig. 1 NCEP GCM/SSiB l and cover classifcation mapin Northwest China.(a)SSiB classifcation map and (b)Kuchler classifcation map. Type 1: tropical rain forest;type 2: broadleaf deciduous trees; type 3: broadleaf and needle leaf trees; type 4: needle leaf evergreen trees; type5: needle leaf deciduous trees; type 6: broad leaf trees withground cover; type 7: grassl and ; type 8: broadleaf shrubswith ground cover; type 9: broad leaf shrubs with bare soil;type 10: dwarf trees with ground cover; type 11: desert;type 12: crops; type 13: permanent ice.

Using the NCEP GCM/SSiB, we conduct experiments with three diferent initial conditions, integratedfor fve months from May to September. The three different initial atmospheric conditions are obtained fromthe NCEP/NCAR global reanalysis dataset. The ensemble simulated results are then averaged to obtainthe fnal results. The F ASIR RSLAI data interpolatedfrom a resolution of 1°to the model grids are used. Inthis study, the simulated results in the summer season(June, July, and August; JJA)of the ensembleexperiments are averaged. This set of experiments, which uses the SSiB classifcation map, is referred to asControl, and is ev aluated by using the Second GlobalSoil Wetness Project(GSWP-2; Dirmeyer et al., 2006)data, NCEP/DOE(Department of Energy)Reanalysis II data(Kanamitsu et al., 2002), and also weatherstation data provided by the National MeteorologicalInformation Center of China(NMICC).

In order to examine the responses of NWC summer climate to l and degradation, we conduct an idealized experiment in which the l and degradation conditions in the NCEP GCM/SSiB are provided by another vegetation map. We refer to the experiment using the SSiB classifcation map as case S1(as in Control, mentioned above), and the experiment based onthe l and classifcation of Kuchler(1983) and Matthews(1984, 1985)as case S2(referred to as the Kuchler classifcation map; Fig. 1b).

Over NWC and the surrounding areas, signifcantdiferences exist between the two classifcation maps(SSiB and Kuchler). In the SSiB classifcation, thel and cover type is assigned as grassl and and shrubl and as well as desert in central-southern Xinjiang, whichis close to reality(Chen, 1994). However, the whole ofNWC is classifed as desert in the Kuchler classifcation. In the l and degradation experiment, vegetationparameters such as LAI over the l and degradation areas are changed and assigned from a table based onvegetation types(Dormen and Sellers, 1989; Xue et al., 1996). We use the two diferent classifcation mapsto obtain the maximum response of l and cover change.The results from this preliminary experiment shouldprovide useful information regarding the impact and mechanisms involved, constituting a helpful step formore realistic assessments in the future. In addition, since the Kuchler classifcation has been used in previous GCM studies(e.g., Xue et al., 1996), the diference between these two cases also indicates simulationuncertainty due to improper specifcation of l and conditions. The only diference between the S1 and S2experiments is the diferent vegetation maps used. 3. General features of NWC summer climate and their simulation

The major summer circulation and other climatefeatures of NWC have been reported by a number ofdiagnostic studies(e.g., Wu and Qian, 1996). Sensibleheat release and strong downward °ow from the northern slope of the Tibetan Plateau(TP)make NWC asignifcant summer \thermal pad, " when the planetaryboundary layer is dominated by a high surface temperature center and widespread descending motion in thetroposphere over the region. In this section, the majorcharacteristics of the surface energy balance, surfacetemperature, vertical motion, vorticity felds, precipitation over NWC, and their simulation, are presented and discussed. 3.1 Evaluation of the NCEP GCM/SSiB

The l and skin temperature in NWC, especiallythe T aklimakan desert, Badain Jaran desert, T enggerdesert, and the surrounding Gobi area, is extremelyhigh during daytime in spring and summer due to little cloud cover, abnormally low humidity(< 20%), and high solar radiation(Zhou and Huang, 2008).According to the GSWP-2, NMICC, and ReanalysisII data, a high skin temperature center is located inNWC in summer. Meanwhile, the temperature alongthe southern boundary of NWC has a sharp gradientdue to the cold temperature of the TP(Figs. 2a-c).The NCEP GCM/SSiB produced this skin temperature feature, including the warmest centers located inthe T aklimakan and Badain Jaran deserts(Fig. 2d).The temperatures simulated by the NCEP GCM/SSiBare much closer to the NMICC data(Fig. 2b), whichare higher than the Reanalysis II data by 8-10°. The2-m air temperature has the same pattern over NWC(fgure omitted), consistent with the surface skin temperature feld.

Fig. 2 JJA mean surface temperature(℃).(a)GSWP-2, (b)Reanalysis II, (c)NMICC observational data, and (d)NCEP GCM/SSiB.
3.2 Surface energy balance

The high albedo of deserts and Gobi relects alarge amount of solar radiation over NWC. F urthermore, the high skin temperature also leads to largeamounts of surface upward longwave radiation(fgureomitted), which produces a low surface net radiationcenter in summer over NWC(Figs. 3a and 3b). Sincethe surface energy balance variables in Reanalysis IIdata are not so reliable, here we only use the GSWP-2data for ev aluation. The NCEP GCM/SSiB simulatesonly one low center over the whole region(Fig. 3a).Meanwhile, large sensible heat flux is released to theatmosphere, which accounts for approximately 75% ofnet radiation(Figs. 3c and 3d). Although intensitydiferences in the net radiation and sensible heat fluxhave been reported in some studies based on reanalysis and observ ational data(e.g., Zhou, 2009), the strongerefect of the "thermal pad" over NWC is still illustrated by the NCEP GCM/SSiB results.

Fig. 3 JJA mean(a, b)surface net radiation(Wm-2) and (c, d)sensible heat flux(W m-2).(a, c)NCEP GCM/SSiB and (b, d)GSWP-2
3.3 Vertic al motion and vorticity felds

Over the southern part of NWC, ascending vertical motion occurs in the entire troposphere, whichis related to vertical airflow over the TP. Meanwhile, accompanying subsiding currents occur to the north ofthe plateau(Fig. 4a), covering central and northernXinjiang Uygur Autonomous Region, northwesternGansu Province, and Inner Mongolian AutonomousRegion, as well as some parts of other central Asiancountries.

Fig. 4JJA mean vertical velocity(10-2Pa s-1)at 500hPa. (a)Reanalysis II, (b)NCEP GCM/SSiB, and (c)diference between NCEP GCM/SSiB and Reanalysis II.

The NCEP GCM/SSiB simulates the widespreaddescending and ascending motion at 500 hPa over theabove-mentioned areas. However, there are two descending centers over NWC at 500 hPa in the Reanalysis II data(Fig. 4a): one over the north of XinjiangUygur Autonomous Region, and the other over InnerMongolian Autonomous Region. The model results, however, only show one descending motion center(Fig. 4b), and with much stronger intensity. The verticalmotion at 700 hPa has the same pattern as at 500 hPa(fgure omitted).

There are strong anticyclonic circulations associated with the descending motion in the lower layer, aswell as a strong and stable cyclonic convergence center at upper levels. The Reanalysis II divergence feldat 200 hPa shows a wide cyclonic b and with its center located in the northern part of NWC(Fig. 5a).The NCEP GCM/SSiB simulates the strong cyclone;however, it moves to the northwest(Fig. 5b). At 500hPa, the Reanalysis II data show a wide anticycloneb and from Xinjiang to Inner Mongolia, with its centerlocated over the T aklimakan desert(Fig. 5d). TheNCEP GCM/SSiB produces this pattern, but withstronger intensity(Fig. 5e). The NCEP GCM/SSiBmodel also captures the key circulation feature on thenorthern side of the TP; in particular, the Asian jetstructure in summer, which can clearly be seen inFig. 5.

Fig. 5 JJA mean vorticity(10-6s-1) and wind(m s-1)at(a, b, c)200 and (d, e, f)500 hPa.(a, d)Reanalysis II, (b, e)NCEP GCM/SSiB, and (c, f)diference between NCEP GCM/SSiB and Reanalysis II.
3.4 Precipitation

Figure 6a shows the JJA observed rainfall databased on 6-yr daily weather station data provided bythe NMICC. The precipitation over NWC is sparse, but there are clear characteristics in terms of its distribution, i.e., little precipitation over the desert and Gobi areas. Two precipitation centers are located overnorthwestern Xinjiang, along the high elev ation areas and the Hexi Corridor. These two precipitation areas receive the main summer precipitation over NWC(Guo and Li, 2006). Guo and Li2006)describedthe role played by the Tianshan terrain, local rivers, and lakes in the high precipitation of northwesternXinjiang. The precipitation located along the HexiCorridor is mainly controlled by the subtropical synoptic system. The NCEP GCM/SSiB simulates themain patterns and also shows good performance interms of the precipitation rate(Fig. 6b), which is lessthan approximately 0.4 mm day-1in the Gobi region, around 0.8-1.2 mm day-1in northwestern Xinjiang, and roughly 0.8-4.0 mm day-1over the Hexi Corridor.

Fig. 6 JJA mean precipitation(mm day-1).(a)observation and (b)NCEP GCM/SSiB.

By and large, using the satellite-derived vegetation characteristics, the NCEP GCM/SSiB producesreasonable regional climate features over NWC.4. Impacts of l and surface processes on summer circulation and surface energy overNWC

In this section, case S2 using a map with markedl and degradation in NWC and case S1 using the SSiBclassifcation map(Fig. 1)are compared to examinethe sensitivity of summer climate features in NWC tol and surface processes.

In Fig. 1a, NWC is classifed as grassl and innorthern Xinjiang, northwestern Inner Mongolia, theHexi Corridor, and some surrounding central Asiancountries and Mongolia. In case S2, most of those areas are defned as bare ground(Fig. 1b). The l and cover changes in case S2 are generally consistent withthose presented in Chen et al.(2009). The surfacevegetation diference causes a substantial change insurface albedo. Figure 7 shows that, compared to caseS1, case S2 leads to higher albedo around centralsouthern Xinjiang by more than 0.03, and by morethan 0.05 in the Hexi Corridor and Mongolia. Thenet surface shortwave radiation in case S2 would decrease due to the higher surface albedo, and also thesurface net heat flux decreases by 20-40 W m-2innorthwestern Xinjiang, the Hexi Corridor, and Mongolia(Fig. 8a). Because of no transpiration in case S2over the degraded l and area, the latent heat flux in theHexi Corridor and in northern Xinjiang decreases by10-20 and 10-60 W m-2, respectively(Fig. 8c). However, due to the reduction of net radiation, the sensibleheat flux in case S2 increases by approximately 20-30W m-2in northern Xinjiang and by 10-30 W m-2inthe Hexi Corridor to balance the energy budget. Thediferences in surface energy balance change in thesetwo areas are related to the soil moisture state. Incase S2, soil moisture decreases greatly in northwestern Xinjiang, but very little in the Hexi Corridor(fgure omitted), due to dry soil conditions in this semiarid area, which are consistent with a large reductionin latent heat flux in northwestern Xinjiang and lesschanges in the Hexi Corridor. Also, the surface temperature increases by 5-6‰℃ in northern Xinjiang and by 1-3‰℃ in the Hexi Corridor, due to the surface netradiation change in case S2(Fig. 12a).

Fig. 7 JJA(a)diference of albedo between cases S2 and S1, and (b)surface albedo in case S1 in NWC.

Fig. 8 JJA diference of(a)surface net radiation(W m-2), (b)sensible heat flux(W m-2), and (c)latent heatflux(W m-2)between cases S2 and S1 in NWC. The dotted areas in(a-c)indicate the 90% confdence level

Changes in surface energy and water balance affect summer circulation over NWC. There are largechanges in the vorticity feld at 700 hPa(Fig. 9)inthe Hexi Corridor and northwestern Xinjiang, whichcorrespond to the area in NWC of large surface netradiation changes(Fig. 8a)in case S2.

Figure 9 shows that there are anomalous lowerlayer anticyclonic and upper-level cyclonic circulationsover northwestern Xinjiang, consistent with the largereduction in surface latent heating. This circulationchange is quite dominant over NWC and leads tolower-layer anomalous cyclonic circulation and anticyclonic circulations in the upper troposphere over theHexi Corridor in case S2, where the change in surfacelatent heat fluxes is weaker. In a previous l and coverchange study(e.g., Xue, 1996), it was identifed thatthe latent heat flux change makes a major contributionto the latent heating change in the lower and middletroposphere, and plays a major role in regional circulation change.

Fig. 9 JJA diferences between cases S2 and S1 in NWCfor wind(m s-1) and vorticity(10-6s-1)at(a)200 hPa and (b)700 hPa.

In addition, the moisture flux is convergent overthe mountain areas of northwestern Xinjiang and weakly divergent over the Hexi Corridor in case S1(Fig. 10a). In case S2, the moisture flux divergence inthe Hexi Corridor changes to convergence(Fig. 10b) and the vertical descending motion becomes weaker(Fig. 11). Meanwhile, the moisture flux convergencein northern Xinjiang is much weaker and the descending motion is stronger. Therefore, in case S2, suchsummer circulation changes cause a decrease in precipitation over northern Xinjiang and an increase inrainfall over the Hexi Corridor in summer(Fig. 12b).It can also be seen that there is an anomalous negative precipitation center over Mongolia(around 48°N, 105°E), where the change in the summer energy balance and circulations is consistent with that in northwestern Xinjiang. We do not discuss these changes inany further detail because they are beyond the scopeof the present paper.

Fig. 10 JJA mean moisture flux(mm day-1)(a)in caseS1 and (b)the diference of moisture flux(mm day-1)between cases S2 and S1 in NWC. The dotted areas in(b)indicate the 90% confdence level.

Fig. 11 JJA diference of vertical motion(Pa s-1)at 500hPa between cases S2 and S1 in NWC.

Fig. 12 JJA diference of(a)surface temperature(℃) and (b)precipitation(mm day-1)between cases S2 and S1 inNWC. The dotted areas indicate the 90% confdence level.
5. Conclusions and discussion

The major summer climate features of NWC and the performance of the NCEP/GCM SSiB in simulating these features over the region are presented inthis study, in which satellite derived vegetation products are used. The possible efects of l and surface processes over NWC on the summer energy balance and circulation are also ev aluated with specifed l and coverchanges.

Based on the comparison with Reanalysis II, GSWP-2, and NMICC observ ational data, the NCEPGCM/SSiB reproduces the main energy balance and summer circulation features over NWC. The major climate features(high surface temperature and descending motion centers located in both the lower and middle troposphere; the cyclone in the upper troposphere and the anticyclone in the lower atmosphere)are allreproduced. The precipitation of NWC is mostly located in its high elev ation areas. Both the modelresults and the GSWP-2 data illustrate that NWCacts as a "thermal pad" in summer. F urther researchwill be conducted to explore the relationship of thedefciencies between the model simulation and observations/Reanalysis II data, and the surface thermalforcing due to inadequate l and conditions and l and surface parameterization by using recently availableNWC feld observation data and other satellite data.

The sensitivity of the summer energy balance and circulation over NWC to l and surface processes is assessed for a l and degradation scenario by comparingcase S1 with case S2. When grassl and in NWC and surrounding regions is changed to desert, the surfacealbedo increases by 3%-5%, leading to weaker surfacethermal efects. In northern Xinjiang and surroundingregions, less latent heating causes stronger lower-levelanomalous anticyclonic circulation and moisture fluxdivergence, and upper-level anomalous cyclonic circulation, leading to less summer precipitation there. Onthe other h and, the dry conditions in the Hexi Corridor and nearby areas produce less variation in latentheat flux. The circulation change to the north of thisarea is dominant and leads to lower-level cyclonic conditions, more convergence, weaker vertical descendingmotion, and thus a slight increase in precipitation overthis region.

Our study is a preliminary investigation into theability of the NCEP GCM/SSiB in simulating the basic summer energy balance and circulation features ofNWC. We also assessed the efects of l and surface processes in NWC and surrounding regions on the regionalsummer climate. There are a few studies that have focused on the impact of l and cover change in NWC onthe regional climate. Some, however, e.g., the studyby Chen et al.(2009)based on the l and cover changebetween the 1990s and 1980s, have shown degradedareas in northern Xinjiang to become cooler with aslight increase in precipitation after l and degradation, which is diferent from the fndings of the presentstudy. More studies with diferent models and experimental designs as well as better l and cover changeinformation, are necessary to further explore the impacts of l and cover change on the climate of NWC.

When more observed data are available, we willapply them in further NWC climate simulations, alongwith satellite data. In order to improve our underst and ing of climate change in NWC and East Asia, we should also look to conduct further studies on theresponse of climate features, the water balance, and energy balance to l and degradation based on more realistic l and cover change data. This study shows thatl and cover change is one factor that may have a largeimpact on the regional climate of NWC. A comprehensive underst and ing of this and other factors, suchas the greenhouse efect and the efects of aerosols, should contribute to a better underst and ing of pastprecipitation change in the region(e.g., Jin et al., 2005; Chen and Dai, 2009), as well as the productionof more credible future projections.

Acknowledgments: The authors wish to thankDr. Zhou Degang and Dr. Liu Y ong from the Instituteof Atmospheric Physics,Chinese Academy of Sciences,for providing the NMICC observation surface temperature data and precipitation data.

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