Research on Water-Vapor Distribution in the Air over Qilian Mountains

NO.1ZHANG Qiang, ZHANG Jie, SUN Guowu and DI Xiaohong107Research on Water- Vapor Distribution in the Airover Qilian Mountains *ZHANG Qiang'+(张强)， ZHANG Jie'(张杰)， SUN Guowu' (孙国武), and DI Xiaohong2(狄潇泓)1 Key Laboratory of Arid Climatic Changing and Reducing Disaster of Gansu Province;Key Opening Laboratory of Arid Climatic Change and Disaster Reduction of CMA;Institute of Arid Meteorology, China Meteorological Administration, Lanzhou 7300202 Lanzhou Regional Meteorological Centre, Lanzhou 730020(Received November 27, 2007)ABSTRACT .stations, distributions of atmospheric water- vapor and cloud motion wind over the Qilian Mountains areanalyzed. Moreover, on the basis of water-vapor and cloud motion wind analyses, relations of atmosphericwater-vapor distribution with precipitation, atmospheric circulation, and terrain are investigated. Theresults show that distributions of atmospheric water- vapor and precipitation in the Qilian Mountains areaffected by the westerly belt, the southerly monsoon (the South Asian monsoon and plateau monsoon), andthe East Asian monsoon. In the northwest Qilian Mountains, water-vapor and precipitation are entirelyaffected by the westerly belt, and there is no other direction water- vapor transport except westerly water-vapor fAux, hence, the northwest region is regarded as the westerly belt region. In the south and middleof the mountains, water-vapor is mainly controlled by the southerly monsoon, 37.7% of the total water-vapor is from the south, especially in summer, the southerly water-vapor flux accounts for 55.9% of thetotal, and furthermore the water-vapor content in the southerly fow is more than that in the westerlyflow. The southerly monsoon water-vapor is influenced by the South Asian monsoon from the Indian Oceanand the plateau monsoon in the Qinghai- Tibetan Plateau, thus, the south and middle region is calledsoutherly monsoon region. But in the northeast Qilian Mountains, the East Asian monsoon is the mainclimate system affecting the water-vapor. Besides west and northwest water-vapor fuxes, there are a lotof easterly water-vapor fAuxes in summer. The frequency of easterly cloud motion winds in summer halfyear accounts for 27.1% of the total, though the frequency is not high, it is the main water-vapor source ofsummer precipitation in this region, therefore, the northwest region is a marginal region of the East Asianmonsoon. On the other hand, atmospheric water-vapor, precipitation, and conversion rate of water-vaporinto precipitation are closely related with altitudes and circulation system. Generally, there is a peak valueof water-vapor content at the altitude from 3500 to 4500 m on the windward slope, but on the leeward slope,water-vapor monotonically decreases with altitude descending except for that in the East Asian monsoonregion. Water- vapor on the leeward is much less than that on the windward slope, and the maximal differencein water-vapor content between the two sides may reach about 4 49 kg m-2. Either the values of water-vaporcontent. precipitation or the conversion rate of water- vapor into precipitation all reach their maxima in theEast Asian monsoon regions, and correspondingly the peak value of water-vapor on the windward is alsolarge and occurs at a lower altitude in comparison with other two regions.Key words: satellite remote sensing data, Qilian Mountains, atmospheric water-vapor, cloud-motion wind,atmospheric circulation1. Introductionder on the Qinghai-Tibetan Plateau on the south andHexi Corridor on the north. Because of the increas-Qilian Mountains lie in the Central Eurasia, ex- ing water domino effect (Ding, 2003), the water-vaportending from the east of Wushaoling region to the west and precipitation are rich in the mountains (Yi et al,of Dangjin Mountain, in 37°- -40°N, 92°- 104°E. Itsal- 2003). All these factors keep permanent glacier andtitude is between 1700 and 5808 m, and the relativeperpetual snow which feed three inland rivers includ-diference of altitude is very large. The mountains bor- ing Heihe， Shule, and Shiyang Rivers. Therefore,* Supported jointly by the Ministry of Science and Technology of China under No. 2004BA901A16 and the Natural ScienceFoundation of Gansu Province under No. 3ZS051-A25-011.TCorresponding author: zhangq@gsma.gov.cn.中国煤化工MHCNM HG108 .ACTA METEOROLOGICA SINICAVOL.22 .Qilian Mountains play a role of natural reservoir of lie in the confuent region of the three circulation sys-Hexi Coridor for agriculture, ecology, and human tems (Zhang and Hu, 2002), and are probably infu-life. However, with economic development, popula- enced by three of them. However, in the mountains,tion increasing, and the change of ecological environ- there are only few weather stations such as Qilian,ment, water resources are seriously deficient. The ten- Tuole, and Menyuan Stations, and their altitude dif-dency to some extent has destroyed the organic struc- ferences are large, vertical climate characters are ob-ture of“valley climate-water resources-ecological sys- vious, and precipitation is extraordinarily infuencedtem”, and brought about some negative effects on eco- by circulation system and local environment.Becauseenvironment such as valley of inland river shrunk, un- of those factors, understanding the water-vapor dis-derground water level decreased, lake dried, and 0a- tribution around the mountains is constrained in thesis of downriver disappeared (Zhang and Hu, 2002). previous research.At present, those eco-environmental questions are fo-In this research, water-vapor content and cloudcused on the precipitation tendency of upriver of in- motion wind are retrieved from remote sensing data,land rivers, and more concerns are given to develop and the effect mechanisms of terrain and circulationand utilize the cloud water of Qilian Mountains.system on precipitation and water-vapor are also stud-To explore cloud water resources of Qilian Moun-iThe aim is to offer scientific guidance for ex-tains, it is primarily to objectively understand spa- ploiting cloud water over the Qilian Mountains andtial and temporal distribution characters of the water- to improve the efficiency of artificial enhancing rainvapor and the circulation system of atmosphere, be-and snow.cause of the notable effects of terrain and circulationsystems on the Qilian Mountains (Zhanget al, 2006). 2. Data and methodThe study results have shown that climate in North-2.1 The region under studywest China is infuenced by three circulation systems(Song and Zhang, 2003), i.e., the westerly belt, theQilian Mountains are located between GansuSouth Asian monsoon together with plateau monsoon，and Qinghai Provinces in Northwest China. Fig-and the East Asian monsoon. Qilian Mountains just ure 1 shows the mountains, with height contours40°NjShule River'/ Gansus 2000之30000000River5宁400038Shiyang River玛4000~?400Qinghai36929810102* EFig.1. Geographical distribution of the Qilian Mountains. Solid lines are height contours at intervals of1000 m.中国煤化工MHCNM HGNO.1ZHANG Qiang, ZHANG Jie, SUN Guowu and DI Xiaohong109at intervals of 1000 m, and the valley distribution of optical thickness and efficient radius of cloud particles.inland rivers. In Fig.1, Qilian Mountains are wide in2.3 Calculating methodrange, and terrain trend is from northwest to south-2.3.1The method of retrieving water- vapor contenteast with many landscapes over the mountains suchby remote sensingas grassland, forest, lake, snow cover, and glacier. Be-Water-vapor content is the water-vapor mass ofcause of more than 10 inland rivers such as Heihe,unit volume. In meteorology, water-vapor content isShule, Shiyang Rivers,and so on, Qilian Mountainsdefined as the amount of wet air mass of some depthform a typical natural environment chain linked byper unit area. At present, there are three methodswater, and combine precipitation in high mountainsto retrieve water-vapor content of the atmosphere,with glacier, snow cover, river, oasis, and desert (Hu,i.e., near-infrared method (Kaufman and Gao, 1992;2003). It becomes an eco-environmental union and aBennartz and Fscher, 2001), microwave method (AIlandscape of arid region in Northwest China.ishouse et al., 1990), and far-infrared method (Ottle2.2 Data .et al, 1997; Sobrino et al, 1994). According to theIn the research, MODIS data from Terra and characters of data, the near-infrared method is usedAqua salites are used, and time series are from May in the study to retrieve water-vapor content over the2002 to April 2005. Meanwhile, hourly GMS data and Qilian Mountains.conventional observed data are also used, including ra-Under cloud-free conditions, in order to reduce re-diosonde data at seven stations around Qilian Moun- trieval errors infuenced by albedo, one absorbed bandtains from 2002 to 2005 (Li et al, 2005), precipita- and two atmospheric window bands of MODIS aretion of weather stations and hydrological stations from used to retrieve water-vapor content, in which the at-May 2002 to April 2005. The stations are Lanzhou, mospheric transmittance Tw is defined as (Gao andDunhuang, Jiuquan, Minqin, Zhangye, Dulan, Xining，Kaufman, 1998)Lenghu, and so on. By combining with above data, the .P9401)water-vapor content and precipitation are analyzed toC1P1240 + C2P865validate and test retrieval errors.where ρ940， P1240， and P865 are the reflectance reMODIS sensor onboard Terra and Aqua satellitesspectively at 940-， 1240-， and 865-nm bands, andis one of important components of Earth Observingc1(=0.2) and c2(=0.8) are constants， respectively.System. There are 36 naro bands, the spetrum is When C1p1240 + CpP85≠p940, we asume 7240 = 785between 0.42 and 14.24 um, and the highest resolution(Wang et al,, 2005),is 250 m. The MODIS has 3 water-vapor absorbedρ940.194bands and 2 atmospheric window bands, the wave-Tw=-C1ρ1240 + C2P865 T86length centers of water-vapor are 0.905, 0.936, andexp[a +β√W]， (2)0.94 pum, and those of atmospheric window are 0.865C1P1240 + C2P865 P865and 1.24 μm. NASA have retrieved water-vapor con-W* = mQv=--Qv,3)cos0T cos0。tent interm of near-infrared specific value algorithm by using where a(=0.02) and B(=0.65) are constants, respec-the data of 5 bands, and issue the water-vapor content tively (Kaufman and Gao, 1992). T1240 and T865 areproducts. The present study uses the 5 bands to re- the transmittance at 1240- and 865-nm bands, θ andtrieve water-vapor content under cloud and cloud-free 0。 are the zenith angles of sensor and the sun. Qv andconditions, but the retrieval errors are large, which is W* are the water-vapor content along the direction oftested by observed data of GPS at Qinghai Stations，zenith and sensor, then the relation of Tw and Qv cantherefore, the water-vapor content is retrieved by using be obtained.中国煤化工MHCNM HG110ACTA METEOROLOGICA SINICAVOL.22 .Under cloud conditions, the information of water- In Eqs.(4)- (8), V is the visibility (m), C is the coeffi-vapor absorption will exist in the ratio value of radia- cient (assuming as 2.6 according to scattering theory),tion at two channels because of water-vapor molecules and Qv is the liquid water content (g m- 3). If thereappearing along the route of sn-cloud- sensor, there- are the same size of all cloud droplets, then k is equalfore, no matter how thick or thin of cloud optical thick- to 1, and k is larger than 1 if the size distribution isness, aerosols should be considered when water-vapor in a wide range.content is retrieved. Assuming that water-vapor con- 2.3.2 The method of calculating water-vapor by usingtent on the top of cloud is neglected, then, the rela- radiosonde datation of water- vapor content with cloud optical thick-According to the definition of water-vapor con-ness and cloud effective radius can be formed (Zhang tent, the formula to calculate water-vapor content us-et al, 2006).ing radiosonde data can be expressed as below (Liu etBased on the above parameters, the relationship al, 2005)of visility V, particle size r, and liquid water contentQv may be formulated by definition:Qv=ρwX (H2- H1),9)CV=*2 n3/2 n,r2=kre.(4) where Qv is water-vapor content, Pw is water-vapordensity, and H2 and H1 are the top and bottomAs a rule, the optical thickness is taken as that at 550heights of water vapor column volume, respectively.nm, which is another mode to rflet visibility. Ac- The water- vapor content in the research is the totalcording to aerosol and cloud patterns, we havemass content in a column from surface to 100-hPa3.912level, which will be used for testing the retrieval re-V= n-=β(5)sults of MODIS data.where e(=0.02) is the contrast threshold, β is the ex-2.3.3 The method of calculating cloud motion windAir-flow motion over Qilian Mountains is com-tinction cofficient of molecule and aerosol at 550 nm,plicated under the effect of circulation system andwhich is correlated to the transmittance Tw of atmo-terrain. .Because of sparse weather stations in Qil-sphere, and the relationship of Tw and T can be ex-ian Mountains, it is difficult to understand the mo-pressed bytion of air-flow and cloud according to wind property,Tw = exp(-r).(6)and cloud characters observed by weather stations. AAccording to the attenuation coefficient in the modelpresent, some new researches show that small scale andand Eqs.(4)- (6), the relationship of visibility with op-mesoscale air fAow can be quantitatively estimated bycloud motion wind retrieved from remote sensing datatical thickness can be described. In order to directly(Xu et al., 1997; Steven et al., 1997).gain the correlation of visibility with optical thicknessBy statistics of the relative humidity of air at dif-T at 550 nm, a serial of optical thickness values areferent pressure levels of 7 radiosonde stations in dif-calculated by using 6S module under the atmosphericferent seasons, the result shows that there are 3 highmodule state of middle summer and middle winter, thevalue levels, i.e., 300, 500, and 700 hPa or near sur-relationship of visibility V and optical thickness T isface. As for Dunhuang, Jinquan, Zhangye, and Min-built as follows:qin Stations on the north of the mountains, the level4.5254x τ-1.0971of maximum humidity is at 300 hPa except for sum-1000mer at 500 hPa. The high value of Lenghu StationThen, the liquid water content of cloud may be ex- on the south of the mountains is at 300 hPa exceptfor summer at 400 hPa. As for the east and south ofmountains, the level of maximum humidity is at 400-2.6x reQv= 4525.4x T-1.097.the basis of8) 50 and 500 hParVL J中国煤化工THCNM HGNO.1ZHANG Qiang, ZHANG Jie, SUN Guowu and DI Xiaohong111the vertical distribution of relative humidity, we can region covers 37°- 40°N, 92°- 104°E, which representsinfer that high, middle, and low clouds are situated at the whole region of Qilian Mountains.about 300, 500, and 700 hPa or near surface.The research shows that most of precipitation3. Test of retrieval value by using radiosondenear Qilian Mountains comes from stratus precipita-observed valuetion. In addition, the surface pressure of Qilian Moun-tains is from 600 to 700 hPa, therefore, when analyzingMost of researches show that the water-vapor con-cloud motion winds, we divide clouds into two types tent calculated from radiosonde data is accurate. Inaccording to whether it can bring on rainfall or not,Table 1, observed values of four stations are calculatedi.e., high cloud and middle-low cloud. The cloud is from radiosonde data. At the same time, the retrievalconsidered as high cloud if it is above the level of 400 values of four stations are also done, and the contrasthPa, and cloud below 400 hPa is considered as middle- results are given. The table shows that the largest ab-low cloud.solute error between retrieval value and observed valueMiddle-low cloud is researched because it may is 1.8 kg m-2, most of absolute errors are less than 1produce rainfall. Infrared and water-vapor band meth- kg m-2, the largest relative error is 14%, and mostods are used for appointing the height of cloud mo- of relative errors are in士10%. Because spatial andtion wind (Xu et al, 1997), and based on the inter- temporal variation of water-vapor content over Qiliancross correlation of infrared and brightness tempera-Mountains are remarkable, with a range of more thanture method, the vector of cloud motion wind is eval-10 times, therefore, the above 1 kg m- -2 errors areuated. In the research, the GNS data of 30 min tolerable, and the results also show that retrieval val-and 1 h are used for analyzing cloud motion wind, the ues from MODIS can reflect the actual distribution ofwind vector is expressed by 16 directions, the research water-vapor content.Table 1. Contrast of retrieval value from satellite with observed value of water-vapor contentSpringSummerObserved Retrieval Absolute error Relative error Observed Retrieval Absolute error Relative error(kgm-2)_ (kg m- 2)(kg m-2)(%)(kg m-2) (kg m-2)9.4-0.80.73.6Jiuquan-0.4-6.0Zhangye10.19.-4.021.622.4.87.4Minqin).3-1.3- 1420.31.4. Results and analysesobviously infuenced by westerly belts. By analyzing4.1 Characters of cloud motion wind and cir-air-flow direction and frequency deviated from west-ward air-flow, the climate of overall region can be di-culation 8ystemvided into three parts according to air-flow direction.In sub- area I, the west and northwest of the moun-In order to discuss the distribution of atmosphericwater-vapor content and precipitation, climate char-40'Nacters of air-flow and cloud motion wind are analyzedbelow, which are important to interpret the efect of38-circulation system on water-vapor distribution. Here,705monthly air-Alow and cloud motion wind are analyzed./m-231455180Figure 2 depicts sub-area of yearly distribution of9498100102° Ecloud motion wind divided by wind vector. Sub-areasshow that the direction of cloud motion is mainly con-Fig.2. The sub-areas of yearly cloud- motiontrolled by westerly winds, which means that climate is .wind over t" 中国煤化工TTHCNM HG112ACTA METEOROLOGICA SINICAVOL.22 .tains, there is no other frequency wind besides west ure 2 only shows a climate region affected by circu-and northwest winds, which means that the region is lation system in a normal climate year, however, wecompletely influenced by westerly belts, therefore, it can infer that the range of each climate region willis defined as the westerly belt climate region.extend and shrink with monsoon action, and corre-In sub-area II, the middle and south of the moun- spondingly a drier year or wetter year appears (Zhangtains, though high frequency wind is from west and et al, 2006). Therefore, climate in the Qilian Moun-northwest, the south wind is also high, whose fre- tains is sensitively and strongly relies on the action ofquency reaches 37.7%, especially in summer, and wind circulation system (Song and Zhang, 2003). Statisti-frequency is more than 55.9%. This air-flow is formed cal data of 10-yr precipitation show that precipitationunder the effect of South Asian monsoon from the In-anomaly at Tuole Station can reach 65.4%, and its av-dian and Plateau monsoon (Huang et al, 2003; Tao erage up to 39.6% in the mountains, which also provedand Chen, 1985), which is called the southerly mon- above conclusions.soon. Water-vapor content from south is much richer4.2 Spatial distribution of water- vapor contentthan west and northwest， therefore, the southerlyof atmospheremonsoon is more important to the climate of the re-gion, and the region is defined as the southerly mon-Above analyses have shown effects of the circula-soon region.tion system on the Qilian Mountains, different circu-In sub-area III, the east part of the mountains，lation systems will result in diferent water-vapor dis-besides northwest winds, the east wind occurs more tributions. Spatial distribution is drawn by retrievalremarkable, especially in summer, it reaches 27.1%. at 48 grids with 1°x1° resolution. Figure 3 givesAlthough the east wind is not high, it is influenced by water-vapor content distributions, and the tendencythe East Asian monsoon (Huang et al, 2003; Tao and of water-vapor content is increasing from northwest toChen, 1985), and results in a strong precipitation pro- southeast. It is considered that the water-vapor con-cess (Steven et al, 1997), which means that it is more tent is high on the east and low on the west, which isnotable summer monsoon of East Asia, therefore, it is decided by the summer monsoon of East Asia. Mean-defined as the East Asian monsoon region.while, water-vapor is also high on the south and lowAbove analysis shows that the Qilian Mountains on the north, and decided by southerly monsoon. Fig-are a connecting region infuenced by westerly belt，ure 3 also shows that the gradient tendency from westsoutherly monsoon, and the East Asian monsoon. Fig- to east is stronger than that from north to south,40°N⑥3813)46100102°E101214161820222426Fig.3. Distributions of annual mean water-vapor content (kg m~ 2) in the Qilian Mountains and its sur-roundings.中国煤化工MHCNM HGNO. IZHANG Qiang, ZHANG Jie, SUN Guowu and DI Xiaohong113indicating that the East Asian monsoon and southerly tion with diference of climate factors.monsoon contribute much water-vapor to the moun-4.3 Variations of water- vapor content with al-tains, and the contribution of westerly belt is less thantitude and slope directionthe former two in summer.At the same time, there are good consistent rela-In order to interpret effects of altitude and slopetions between water-vapor content and terrain, which direction on water-vapor distribution and the relationmay be embodied by the disturbance of terrain to with circulation system, on the basis of sub- areas fromwater-vapor distribution. Figure 3 shows that the cloud motion wind, the variations of water-vapor con-highest and lowest value centers of water-vapor con- tent in three regions with altitude and slope direc-tent are related with complex terrain regions, for ex-tion are given in Fig.4, in which Regions I, II and IIIample, there is a high value center near 38°- 39°N，represent westerly belt (Fig.4a), southerly monsoon98° E which is influenced by Shulenan Mountains, and (Fig.4b), and the East Asian monsoon (Fig.4c)， re-there is a low value center of water-vapor content near spectively, and the relations are also shown in Fig.4.37°- 38°N, 100°- 101°E on the north of Qinghai Lake. On the windward slope direction, no matter which cir-The reason is that there is no altitude diference and culation system affects the region, water-vapor contentthe terrain is wide plain.is well correlated with altitude, and the correlation CO-Meanwhile, it is obvious that the large gradient efficients of Regions I, II, and II reach 0.931, 0.925,region of water-vapor content is related with the tran- and 0.880, respectively. However, variable characterssitional belt of circulation system, which reflects that are diferent in three regions, the peak values happenit is a typical character of transitional belt of circula- at altitude 4608， 3855， and 3476 m, and the peak0厂0r(a)b)0个20 F六。200030004000 .50002004000h (m)3Cc)201Fig.4. Variations of water-vapor content on the windward slope with height under the circulation systemof westerly belt (a), southerly monsoon (b), and the East Asian monsoon (c)中国煤化工MYHCNM HG114 .ACTA METEOROLOGICA SINICAVOL.22 .values of water-vapor content are 21.72, 25.75, and that water-vapor content and temperature in Region29.58 kg m- -2, respectively. The relationships between III are high, so that the cloud forming and develop-atmospheric water-vapor content and height in the ment need less terrain forcing. However, in Regionthree regions can be expressed asI, the water-vapor content of atmosphere is low, andQv-(n)=-2x 10-7h2 + 0.0034h + 5.5922，(10)the region is influenced by cold air, therefore, cloudQv-(m)=-6x 10-6h2 + 0.0424h- 54.318，(11)cannot be formed easily, and it needs strong terrainforcing to form cloud. Water-vapor content in RegionQw-(am)=-7x10 -0h2 +0.0486h -61.146，(12) II is betwee Regions I1 and I It is string that thewhere Qv- a.1.m) are atmospheric water-vapor con- distribution of water-vapor content in this research istent in three regions (units: kg m- -2), and h is height consistent with the distribution of precipitation in the(unit: m).previous researches.In comparison of water-vapor contents in theGenerally speaking, it is different that water-three regions, the water-vapor content in the East vapor content changes with altitude on the windwardAsian monsoon region is the highest, and intensity is and leeward slopes. In order to compare difference,the largest, but the altitude of peak value is the lowest. the relationships of water-vapor content with altitudeOn the contrary, water-vapor contents in Regions I and on the leeward slope are given in Fig5, where RegionsII are lower, peak values are lower but the altitudeof I (Fig.5a), II (Fig.5b), and II (Fig.5c) are the sameoccurrence is higher. The difference of peak value be- as above. It is obvious that the relationships of water-tween Regions I and II is about 8 kg m-2, and the al- vapor content with altitude are as well as those ontitude diference is more than 1100 m. All those mean the windward side, and the correlation cofficients in0r0厂(a(b20 -20003000400050002000 .h (m)(c)。2(40westerly belt (a), southerly monsoon (b), and the East Asian monsoon (中国煤化工"YHCNM HGNO. IZHANG Qiang, ZHANG Jie, SUN Guowu and DI Xiaohong115Regions I, II, and III are 0.869, 0.863, and 0.650, re- the moving of cloud toward leeward .spectively. .4.4 Relationship between water- vapor contentQv-(1) = 0.0036h - 0.8668,(13)and precipitationQo-(u) = 0.0031h + 6.8532,(14)Water-vapor content is the basis of precipitation.Qo-(m) = 0.0051h + 3.5661.(15)In order to analyze the relationship of water-vaporFrom Figs.4 and 5， some conclusions can be with precipitation, three typical regions are takendrawn: Firstly, no matter on the windward or lee- into account. Figure 6 gives the comparison of pre-ward slope, the water-vapor content is increasing with cipitation with water-vapor content in Regions I, II,altitude, and there is no peak value generally. Sec- and II. It is obvious that precipitation is well cor-ondly, water-vapor content on the leeward is less than related with water-vapor content, and is increasingthat on the windward, especially in Region I, water- with water-vapor content, with correlation coefficientsvapor content on the leeward is 0.5- 4.49 kg m" -2 less 0.973, 0.617, and 0.973, respectively. Compared withthan that on the windward. Therefore, the vegeta- the other two regions, the correlation of precipitationtion distribution will be different on the two sides with water-vapor content in Region II is lower. One isof mountains. Above characters may be easily inter-that precipitation is affected by many complex fac-preted by the mechanism that when terrain forcing is tors, and the other is that there are no observed dataup to the largest, the precipitation reaches the high- above 4000 m level to contrast. The relationships be-est, and water-vapor content gradually decreases with tween precipitation and water-vapor content can be60000厂(ab)00 F40l 40000一200010120Q,(kgm~)2、(kg m3)00pc)00 t昌400002003(Q、(kg m7)Fig.6. Relationships of annual precipitation with water-vapor content influenced by circulation system ofwesterly belt (a), southerly monsoon (b), and the East Asian monsoon (c).中国煤化工MHCNM HG116 .ACTA METEOROLOGICA SINICAVOL.22 .expressed asunit %. In Region II, the relationship of precipitationwith altitude is not obvious, and the conversion rateP=39.748XQu-327.03,.(16)stabilizes at some degree, meaning that it has reachedPi= 28.651 x Q。- 82.533,(17) a high degree in rich precipitation Region II, and itPr= 21.193x Q。- 11.664,(18) is influenced little by the change of altitude.By comparing conversion rates with each otherwhere variables P,,III are the annual average precip-in three regions infuenced by different circulation sys-itation in Regions I, II, and III (unit: mm).tems, it can be seen that the conversion rate of precipi-Figure 7 shows the conversion rate of water-vaportation is different, about 0.175% in Region II, 0.160%of atmosphere in Regions I, II, and II with altitude.in Region II, and 0.145% in Region I, which meanIt is obvious that the relation of conversion rate withthat Region III has the highest conversion rate, Re-altitude is very well, and precipitation is increasinggion I has the lowest conversion rate. Above resultswith altitude, the correlation coefficient in Regions Iare more related to precipitation distribution in theand II reach 0.984 and 0.929, respectively, and can beeast than in the west (Song and Zhang, 2003).written as5. Conclusions and discussionsfr=-1 x 10-8h2 + 0.0001h - 0.079,The research shows that it is possible to retrievefu=-3x 10-8h2 + 0.0002h- 0.181,(20)water-vapor content by MODIS data, the retrievalwhere variables f,1 are the conversion rate of water- value of water-vapcontent over Qilian Mountainsvapor content to precipitation in Regions I and II is consistent with the observed value from radiosondeP/Qv(Qu is transformed from unit kg m-2 to mm)，(Sun, 1981), and can reflect the actual distribution of0.25 r(a.25 rb)0.2.2 F0.15-。0.).1 t.1-0.051000200300040000002000h(m).h (m)(c).2 t0.15 F).1-.05 F5000Fig.7. Conversion rates of water- vapor content to precipitation with altitude in Regions I (a), II (b), andIII (c).中国煤化工MHCNM HGNO.1ZHANG Qiang, ZHANG Jie, SUN Guowu and DI Xiaohong117water-vapor content.retrieval value and analyze the mechanism deeply. WeQilian Mountains are infuenced by many climate hope to further study water-vapor question based onsystems such as westerly belt, southerly monsoon, and the mesoscale monitoring net which will be built inthe East Asian monsoon. Although the frequency of the future.westerly belt is the highest in the mountains, the rich-est water-vapor is formed because of the contributionREFERENCESof the East Asian monsoon and southerly monsoon.Moreover, Qilian Mountains are divided into three cli-Alishouse, J. C., S. A. Snyder, J. Vongsathorn, et al,1990 : Determination of oceanic total precipitationmate regions, i.e., westerly belt, southerly monsoon,water from the SSM/I. 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