Distribution of Atmospheric Water Vapor Content over China

CHINESE ASTRONOMY AND ASTROPHYSICS

ELSEVIER

Chinese Astronomy and Astrophysics 37 (2013) 212–229

Distribution of Atmospheric Water Vapor Content over China†  YAO Yong-qiang1 LI Jun-rong1 YIN Jia1 ZHANG QIAN Xuan1† 2 1 1,3 Yong-jing LIU Li-yong WANG Hong-shuai ZHOU Yun-he1 1 1 LI Lin YOU Xian-long MA Jiang-long1 1

National Astronomical Observatories, Chinese Academy of Sciences/Beijing 100012 2 Jinan Meteorological Administration, Jinan 250021 3 Graduate University of Chinese Academy of Sciences, Beijing 100049

Abstract Based on the data of 210 ground meteorological stations over China from 1961 to 2008, an empirical relation between the ground water vapor pressure and precipitation has been discovered. Then the equations used for estimating the ground water vapor pressure based on the precipitation are developed, and thereby to derive the atmospheric water vapor content, which is one of the important parameters for astronomical site survey. The results of this study show that in different seasons, there exists a stable correlative relation between ground water vapor pressure and precipitation, the water vapor pressures calculated from the precipitation data of different areas are consistent with the local practical measurements, and that the calculated atmospheric water vapor contents correspond well to the local long-term averages. Key words: atmospheric effects—methods: statistical— methods: data analysis

1. INTRODUCTION The atmospheric water vapor content is one of the key elements for astronomical site survey, and is one of the important parameters to evaluate an infrared and submillimeter wave † Supported by National Natural Science Foundation and Young Talent Foundation of National Astronomical Observatories, Chinese Academy of Sciences Received 2011-08–23; revised version 2011–11–23  A translation of Acta Astron. Sin. Vol. 53, No. 4, pp. 325–341, 2012  [email protected]

0275-1062/13/$-see frontfront mattermatter © 2013 B.V. All Science rights reserved. cElsevier 0275-1062/01/$-see 2013 Elsevier B. V. All rights reserved. doi:10.1016/j.chinastron.2013.04.009 PII:

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astronomical site. There are many significant absorption bands of water vapor fallen in the extent of electromagnetic radiation spectrum. In the astronomical site survey, an excellent astronomical site should have an integral water vapor content as low as possible, because the water vapor may affect the transmission of electromagnetic wave signals. In the remote sensing detection, it is necessary to consider the influence of water vapor on the echo intensity, while making the atmospheric correction on the remote sensing echo data. And the absorption of water vapor on the solar radiation should be taken into account, while making inversion of atmospheric aerosol optical thickness from the information about the solar radiation of vertical incidence. The water vapor in the atmosphere gathers in the troposphere, the total amount of water vapor in an atmospheric column from the top of the troposphere to the ground is known as the atmospheric water vapor content. Because the most part of water vapor in the atmosphere is gathered in the lower half of the troposphere, the water vapor content near the ground occupies a large proportion of the water vapor content in the whole troposphere. The water vapor content in the whole troposphere, therefore, depends to a great extent on that near the ground, and it is notably correlated with the ground humidity parameter. In the investigation of the time-variation characteristics of Chinese astronomical and meteorological conditions, we found that there exists a stable correlation between the ground water vapor pressure and precipitation: in general, the more is the precipitation in an area, the greater the water vapor pressure is; the more is the precipitation in a season, the greater the water vapor pressure is, too[1] . The relation between the ground water vapor pressure and the precipitation for the 21 stations in China is shown in Figure 1, and it is obvious that the relation between the ground water vapor pressure and the precipitation in each place is nearly linear. Therefore, a systematic investigation on the ground water vapor pressures and precipitations all over the country is made to find the empirical relation, with which the atmospheric water vapor content needed for astronomical site survey can be reckoned from the precipitation. In general, the atmospheric water vapor content is obtained with the calculation of sounding balloon data, but in many areas, particularly in the vast remote areas, the sounding balloon data are in short or even empty. Many potentially excellent astronomical sites are located in the remote areas, which lack the ground meteorological station of regular observations. Thus, for the convenience of astronomical site survey, and the atmospheric radiation and remote sensing measurements, it will be of practical significance to sum up an empirical relation between the precipitation and atmospheric water vapor content, and to utilize this relation to investigate the water vapor conditions of an area. At present, there is still no systematic investigation of the relation between the measured precipitation and the ground water vapor pressure in China. As far as the study of atmospheric water vapor content, WANG Jichang et al.[2] estimated the atmospheric water vapor content for the two candidate sites at Wuma and Kalasu, using the measured ground water vapor pressure; by fitting the ground and high-altitude meteorological data of 20 stations in China during 1992∼1993, YANG Jingmei et al.[3−4] summed up the empirical relations between the atmospheric water vapor content in the whole troposphere, or the effective water vapor content, and the ground water vapor pressure in the areas, where the stations in question are located; ZHANG Xuewen [5] confirmed the existence of the linear relation between the atmospheric water vapor content and the ground water vapor pressure, using 308 groups of climatic data of variety of areas and seasons; by using the data of 3

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meteorological sounding stations in Zhangjiakou, Xingtai and Beijing at 8h and 20h Beijing Time during 2004∼2005, LI Guocui et al. [6] calculated the total amount of water vapor for the corresponding places and moments, and studied the features of the total amount of water vapor and its relation with the ground water vapor pressure in North China. These empirical relations have been well applied to some practical work. In the remote study of astronomical site survey, the atmospheric water vapor content of a site can be measured in situ with an instrument, while in the investigation of water vapor conditions of a relatively large area, it is more convenient in practice to use an empirical relation to calculate the atmospheric water vapor content, and to provide an effective scientific support to the estimation of water vapor conditions of an area. Hence, it is pointed out in our study that the empirical relation and the distribution of atmospheric water vapor content over China are of practical significance in the fields, such as the astronomical site survey, atmospheric radiation and transmission, meteorology, and so on.

25 Aletai Baluntai Beitashan Changchun Chengde Chengdu Delingha Deqin Fuzhou Gaize Geermu Jiamusi Jiangzi Lasa Lijiang Mohe Shiquanhe Tashikuergan Wulatezhongqi Xuyi Zhengzhou

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The relations between ground water vapor pressures and precipitations from 1961 to 2008 at different places

In this paper, the data of meteorological elements of 210 ground meteorological stations of a variety of areas over China are chosen to be statistically analyzed for studying the relation between precipitation and ground water vapor pressure. In this work, emphasis is laid on the northern and western China areas where the climate is relatively dry, while few stations are chosen in the southern China area where the climate is relatively humid and less typical, according the requirement of work, such as astronomical survey, atmospheric radiation, remote sensing and so on. In general, these stations, however, are representative in the geographic distribution, altitude, aridity and humidity. This study will offer a feasible means to obtain the ground water vapor pressures of all areas over China, and the means that may be widely used in the investigation relevant to atmospheric water vapor content.

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2. METEOROLOGICAL DATA 210 regional stations have been chosen to make the study and analysis, from the 756 ground stations all over China with their monthly mean data and annual data of ground meteorological measurements during 1961∼2008 provided by the Meteorological Reference Room of China Meteorological Administration. The spacious extent of the selected stations and the long duration of their data are favorable to high reliability of the analyzed results. The selected stations are representative in the geographic distribution, altitude, aridity and humidity. Figure 2 shows the distribution of these 210 stations.

Fig. 2

The distribution of meteorological stations selected in this paper

3. EMPIRICAL RELATION BETWEEN GROUND WATER VAPOR PRESSURE AND PRECIPITATION China is a country with a vast territory and a complex terrain, the precipitation is very nonuniformly distributed in various regions, and has a very large spatial variation. It depends mainly on the synthetical effect of various factors, such as the geographic location, general atmospheric circulation, climatic system, the conditions of underlying earth surface, and so on. Such synthetical conditions differ with areas, and therefore the empirical coefficients differ with stations to a certain extent. The precipitation in southeastern China is greater than that in northwestern China, namely it decreases from the southeastern coastal region to the northwestern inland. The precipitation is relatively large in Hainan Island and in Guangdong, Guangxi, Fujian, southern part of Zhejiang of the southeastern coastal region in China, where the annual precipitation is about 2000mm; and next in Yangtze River Basin, about 1200mm; in YunnanGuizhou Plateau, about 1000mm; in Huanghe River lower reaches, southern part of Shaanxi

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and Gansu, North China plain, and Northeast China plain, the precipitation is reduced to about 600mm; in the inland of Northwest China below 200mm; in the northwestern part of Qinghai-Tibet plateau under 50mm, while in the desert of South Xinjiang only 10mm. The precipitation changes not only with the area, but also with the season. The rainfall is mainly concentrated in summer in most areas, where a year is clearly divided into the rainy season and the dry one. The start and the end of the rainy season differ with the area. In the plateau of West China the rainy and dry seasons transit from each other more clearly than in the eastern China. The rainy season in Yunnan-Guizhou Plateau begins generally in the last ten-day of May, and ends in the last ten-day of October. The precipitation in rainy season is greater than that in the dry season for 9 folds. The rainy season in the northern part of Qinghai-Tibet plateau begins generally in the middle ten-day of June, and ends in the last ten-day of October. As far as the whole plateau is concerned, in the northeastern part it begins earlier but ends later than the southwestern and northwestern parts. The precipitation in Xinjiang features a relatively uniform distribution all the year round, and there is no clear distinction between rainy and dry seasons. As for the eastern China, in the southern part the rainy season begins usually earlier, and ends later than the northern part. In the coastal region of South China the rainy season begins in April, and ends in October. In Yangtze River Basin it begins in the first ten-day of June, and ends at the beginning of September. In North and Northeast China the rainy season begins in the middle ten-day of July, and ends at the end of August. As the precipitation distribution in a rainy season is also nonuniform, a relatively dry period still emerges in many areas. For example, the relatively dry period shows itself in the plateau of Northwest China from the middle ten-day of July to that of August, and in the eastern part of Yangtze River Basin in the same period, too, while in South China (on the south of 27◦ N) it begins almost in the last ten-day of June, and ends in that of July, but in North and Northeast China it is not conspicuous. In the severely dry areas, it is prone to summer drought. It follows, therefore, that in the plateau of Northwest China, South China and Yangtze River Basin, the precipitation in the rainy season is concentrated in two periods, hence the rainy season in these areas is divided into two phases. According to the beginning and end of rainy season, the whole country, therefore, is divided into 6 regions: Xinjiang and Loess plateau area, North and Northwest China, Qinghai-Tibet plateau, Yunnan-Guizhou plateau, Yangtze River Basin, and South China. The numerical relations between ground water vapor pressure and precipitation will be statistically analyzed for each area, respectively. Because the annual precipitation in Xinjiang and Loess plateau area is on the little side, and no clear distinction between the rainy and dry seasons, the annual data of meteorological elements will be used for the analysis, while for the other areas, where a clear distinction between the rainy and dry seasons shows itself in precipitation, the monthly data are applied to the analysis for the rainy season and dry season, respectively. An empirical relation between ground water vapor pressure E and the corresponding precipitation P is developed with the least square fitting. After a regression analysis, it is found that there exists a functional relation between the ground water vapor pressure E and the corresponding precipitation P , which can be expressed as an empirical relation as follows: E = a×P +b,

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in which a and b are empirical coefficients. For the most areas in China where has the distinct dry and rainy seasons, then E and P are the seasonal mean value of dry season and that of rainy season, respectively, while for Xinjiang and Loess plateau area, E and P are the annual mean values, respectively. Because of the difference among climatic conditions of various areas, the empirical coefficients of various stations show themselves with a certain dissimilarity. Figure 3 demonstrates the relation between ground water vapor pressure and precipitation for the stations in the 6 areas of Beijing, Mangkang of Tibet, Lijiang of Yunnan, Xuzhou of Jiangsu, Haikou of Hainan, and Baluntai of Xinjiang. In this figure the straight lines indicate the fitted correlation between ground water vapor pressure and precipitation. In the diagrams of Beijing, Mangkang of Tibet, Lijiang of Yunnan, Xuzhou of Jiangsu, and Haikou of Hainan, each point means a monthly average, while in the diagram of Baluntai of Xinjiang, each point is an annual one, and in each diagram the straight line is the fitted result on the above-mentioned empirical relation. From the figure it can be seen that corresponding functional relations between E and P in the concerned seasons exist, and the results calculated with the empirical equation are consistent with the actual circumstance for most areas, this confirms further the correlation between E and P . As far as the degree of correlation is concerned, the Pearson’s definition is invoked here: the bigger the absolute value of correlation coefficient, the stronger the correlation, and the nearer to zero the correlation coefficient, the weaker the correlation. In general, the utmost strong correlation is considered with a correlation coefficient within 0.8∼1.0, the strong one within 0.6∼0.8, the medium one within 0.4∼0.6, the weak one within 0.2∼0.4, and the most weak one or even no correlation within 0.0∼0.2. Beijing is located in the northwestern part of North China plain with Yan Mountain Range at the back, the northern section of Taihang Mountain Range in the west, and a plain in the southwest. Its climatic feature is rather continental, and its climatic division belongs to the temperate and semi-humid region. The annual mean precipitation of this area during 1961∼2008 is 55.46 cm, and the corresponding ground water vapor pressure is 10.6 hPa. Mangkang of Tibet is located in the Qinghai-Tibet plateau, and is in a semi-arid area. The annual mean precipitation of this area during 1961∼2008 is 51.49 cm, and the corresponding ground water vapor pressure is 5.4 hPa, both are less than those in plain area. Lijiang of Yunnan is located in Yunnan-Guizhou plateau, is featured with a plateau climate of southwest monsoon, namely with clear-cut four seasons, mild air temperature, moderate rainfall, obvious Meiyu and concentrated summer rainfall. The annual mean precipitation of this area during 1961∼2008 is 96.67 cm, and the corresponding ground water vapor pressure is 9.8 hPa. Xuzhou of Jiangsu is located in the Yangtze River Basin, is featured with a moist monsoon climate, namely with clear-cut four seasons, hot and rainy summer, cold and dry winter. The annual mean precipitation of this area during 1961∼2008 is 85.01 cm, and the corresponding ground water vapor pressure is 13.6 hPa. Haikou of Hainan is located in the seashore of South China, on the edge of low-latitude torrid zone, and is featured with a maritime tropical climate, namely with rich rainfall, warm and dry spring, hot and rainy summer, autumn of frequent typhoons and rainstorms, winter of cold spells during cold air current invading. The annual mean precipitation of this area during 1961∼2008 is 163.63 cm, and the corresponding ground water vapor pressure is

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25.6 hPa. Baluntai of Xinjiang is located in Zhungaer Basin of Northwest China, in the inland surrounded by high mountains. It is difficult for the southeast and southwest monsoons with abundant water vapor to arrive here. Its climatic feature is intensively continental, and it is classified as a temperate and dry area. The annual mean precipitation of this area during 1961∼2008 is 18.44 cm, namely the annual precipitation is relatively small, and the corresponding ground water vapor pressure is 4.8 hPa.

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Although the 6 areas mentioned above are very diverse in geographic location, and so are their aridity and humidity, and underlying earth surface, too, there exists a corresponding linear correlation between ground water vapor pressure and precipitation for each area and for various seasons. Statistically analyzed results are given for the 6 areas below, and our empirical equations can rather well express the correlations between them. Table 1 gives the empirical coefficients and related information of the functional relations between ground water vapor pressure and precipitation for the stations in the representative areas, such as North and Northwest China, Qinghai-Tibet plateau, Yunnan-Guizhou plateau, Yangtze River Basin and South China, in which COE indicates correlation coefficients, M RE mean relative errors, RM E root-mean-square errors, and it is the same in the following.

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3.1 North and Northeast China Area It is indicated in Table 1 that from the statistical results of E-P relations of the 10 regional stations during 1961∼2008 in North and Northeast China area, where July and August of the every year are in rainy season, and other months in dry season, in dry seasons the correlation between ground water vapor pressure and precipitation is rather strong with all the correlation coefficients greater than 0.62, while in rainy seasons the correlation is relatively weak, with nearly half of the correlation coefficients being less than 0.4. In North and Northeast China the rainy season is distributed in July and August with intensive but short-duration rainfall. The precipitation shows its strong localization, large annual variation, the concentration in time, and the close relation with terrain. The rainstorms emerge mainly in the upwind face of mountain range and in mountain areas. The slope of the straight line fitted for rainy seasons is very small, nearly zero, the water vapor pressure tends to be saturated. Generally speaking, in rainy seasons the correlation between precipitation and ground water vapor pressure is not conspicuous with some correlation coefficients less than 0.4, while in other months the precipitation is distributed evenly, and there exists a good correlation between precipitation and water vapor pressure with the most correlation coefficients greater than 0.6 and mean relative errors less than 35%. 3.2 Qinghai-Tibet Area It can be seen from Table 1 that from the statistical results of E-P relations of the 10 regional stations during 1961∼2008 in Qinghai-Tibet area, where June∼October of every year are in the rainy season, and other months in the dry season, and there exists a good correlation between ground water vapor pressure and precipitation, in various seasons the correlation coefficients are mostly greater than 0.5. Overall, the precipitation in Qinghai-Tibet plateau decreases progressively from the southeast to the northwest. In the southeastern part there is much precipitation, because of the effect of southwest monsoon from the Indian Ocean. The northwestern part, however, is relatively dry, because of the temperate continental climate with almost no effect of monsoon as it is located in remote inland. The rainy season finds itself in June∼October of every year, and the correlation coefficients between precipitation and ground water vapor pressure are mostly greater than 0.6 in this period. The correlation coefficient of Deqin region of Yunan, the southeastern part of this area, is relatively small, about 0.5 with a mean relative error less than 25%. In other seasons, the correlation coefficients between precipitation and ground water vapor pressure are mostly greater than 0.6, too, with the mean relative errors less than 35%. The correlation of Shiquan River region is inferior, with a correlation coefficient of about 0.36.

Table 1

Latitude N Mohe 52◦ 58 Daxinganling 50◦ 24 Hailaer 49◦ 13 North Aershan 47◦ 10 and Yichun 47◦ 44 NorthErlianhaote 43◦ 39 east Chifeng 42◦ 16 China Fuxin 42◦ 05 Dandong 40◦ 03 Beijng 39◦ 48 Shiquanhe 32◦ 30 Gaize 32◦ 09 Naqu 31◦ 29 Rikaze 29◦ 15 Qinghai-Nimu 29◦ 26 Tibet Jiangzi 28◦ 55 31◦ 09 plateau Changdu Linzhi 29◦ 40 Mangkang 29◦ 41 Geermu 36◦ 25

Region Station

Longitude E 122◦ 31 124◦ 07 119◦ 45 119◦ 56 128◦ 55 111◦ 58 118◦ 56 121◦ 43 124◦ 20 116◦ 28 80◦ 05 84◦ 25 92◦ 04 88◦ 53 90◦ 10 89◦ 36 97◦ 10 94◦ 20 98◦ 36 94◦ 54

Altitude m 433 371.7 610.2 997.2 240.9 964.7 568 167.8 13.8 31.3 4278.6 4414.9 4507 3836 3809.4 4040 3306 2991.8 3870 2807.6 0.005 0.139 0.191 0.166 0.045 0.35 0.153 0.075 0.028 0.065 0.758 0.547 0.304 0.245 0.332 0.384 0.334 0.23 0.208 1.39

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Wet season COE M RE (%) 14.73 0.13 9.4 14.53 0.5 7.4 13.36 0.48 7.8 12.24 0.55 6.2 17.42 0.22 6.3 10.94 0.61 7.1 16.27 0.58 5.2 21.05 0.35 5.7 24.07 0.25 25.8 23.32 0.41 4.5 3.382 0.66 19.4 3.061 0.79 18.9 3.955 0.8 15 6.409 0.77 17.2 5.939 0.75 17.3 5.559 0.74 17.7 6.688 0.69 14.3 9.035 0.62 12.4 6.159 0.84 10.1 4.328 0.61 23.5 RM S (hPa) 1.71 10.17 1.46 1.11 1.44 1.09 1.15 1.54 1.46 1.34 1.32 1.24 1.01 1.51 1.43 1.61 1.69 1.813 0.926 1.46 0.107 0.875 1.225 0.947 0.995 1.816 1.179 1.159 0.794 1.155 0.941 1.161 0.687 1.033 0.909 1.045 0.824 0.577 0.743 2.112

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Dry season COE M RE RM S (%) (hPa) 1.236 0.79 23 2.3 1.999 0.78 23.8 2.35 1.973 0.78 24.3 2.13 1.405 0.82 25.8 1.75 1.833 0.84 25 2.3 2.282 0.7 31.1 1.81 2.761 0.77 27.8 2.48 2.359 0.77 34.7 3.27 4.421 0.7 30.7 4.01 5.217 0.68 28 4.04 1.14 0.36 34.1 0.6 0.939 0.68 30.2 0.5 1.133 0.79 26.4 0.58 1.908 0.71 30.6 0.96 1.954 0.67 29.5 1.11 1.903 0.66 25.1 0.97 2.301 0.84 21.9 0.86 3.769 0.88 15.5 1.91 2.422 0.88 20.2 0.8 1.505 0.66 28.2 0.66 b

The empirical coefficients of the functional relations and the correlations between ground water vapor pressure and precipitation in most parts of China

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Lijiang Tengchong Baoshan Kunming YunnanLincang Guizhou Lancang plateau Guiyang Zunyi Anshun Dushan Chongqing Yichang Jingzhou Wuhan Yangtze Changde river Xuzhou basin Xuyi Nanjing Hefei Nanchang Guilin Liuzhou Shaoguan Zhangzhou South Baise China Guangzhou Nanning Yulin Qinzhou Haikou

Region Station

Latitude N 26◦ 52 25◦ 01 25◦ 07 25◦ 00 23◦ 53 22◦ 34 26◦ 35 27◦ 42 26◦ 15 25◦ 50 29◦ 31 30◦ 42 30◦ 21 30◦ 37 29◦ 03 34◦ 17 32◦ 59 32◦ 00 31◦ 47 28◦ 36 25◦ 19 24◦ 21 24◦ 41 24◦ 30 23◦ 54 23◦ 10 22◦ 38 22◦ 39 21◦ 57 20◦ 00

Longitude E 100◦ 13 98◦ 30 99◦ 11 102◦ 39 100◦ 05 99◦ 56 106◦ 44 106◦ 53 105◦ 54 107◦ 33 106◦ 29 111◦ 18 112◦ 09 114◦ 08 111◦ 41 117◦ 09 118◦ 31 118◦ 48 117◦ 18 115◦ 55 110◦ 18 109◦ 24 113◦ 36 117◦ 39 106◦ 36 113◦ 20 108◦ 13 110◦ 10 108◦ 37 110◦ 15

Altitude m 2392.4 1654.6 1652.2 1886.5 1502.4 1054.8 1223.8 843.9 1431.1 113.3 3511 133.1 32.2 23.1 35 41.2 40.8 7.1 27 46.9 164.4 96.8 61 28.9 173.5 41 121.6 81.8 4.5 63.5 0.206 0.121 0.147 0.153 0.185 0.119 0.121 0.071 0.06 0.087 0.232 0.157 0.102 0.069 0.067 0.223 0.138 0.096 0.15 0.021 0.07 0.136 0.066 0.134 0.24 0.12 0.212 0.118 0.122 0.055

a

Table 1 b

Wet season COE M RE RM S (%) (hPa) 10.81 0.79 9.3 0.08 15.91 0.58 8 1.68 16.21 0.46 9.7 1.98 14.31 0.62 8.7 1.66 15.98 0.69 6.8 1.53 19.28 0.66 5.7 1.47 17.18 0.35 13 2.76 20.92 0.28 8.1 0.59 18.28 0.39 7.1 0.67 20.52 0.43 7.2 0.86 19.26 0.38 13.3 4.25 23.26 0.43 10.8 3.16 25.72 0.24 12.1 3.64 26.11 0.24 11.9 3.54 26.4 0.2 11.2 3.35 20.85 0.54 13.7 2.67 23.84 0.42 13.1 3.77 24.95 0.27 13.1 3.73 24.69 0.32 2 3.7 27.61 0.09 9.9 3.09 22.42 0.22 19 4.76 22.8 0.38 15.2 4.06 24.2 0.17 15.5 4.18 23.37 0.38 14.1 4.11 22.77 0.6 10.2 3.01 24.91 0.4 12.3 3.72 23.71 0.58 10.2 3.15 25.22 0.41 10.8 3.26 25.19 0.63 9.7 3.13 28.62 0.32 6.5 2.26

(continued)

0.623 0.278 0.254 0.295 0.362 0.365 0.455 0.559 0.32 0.407 0.596 0.613 0.604 0.443 0.375 0.548 0.464 0.436 0.264 0.274 0.173 0.187 0.116 0.074 0.397 0.17 0.184 0.127 0.186 0.155

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Dry season COE M RE RM S (%) (hPa) 4.977 0.58 17.8 0.82 8.081 0.59 13.7 1.5 8.386 0.46 14.5 1.56 7.847 0.45 13.6 0.95 8.995 0.59 12.1 1.47 11.45 0.65 10.5 1.63 7.606 0.69 16 1.78 7.843 0.8 17.9 3.03 8.617 0.78 17.7 2.64 8.533 0.75 20.6 3.03 9.235 0.81 15.6 2.41 7.632 0.71 24.8 3.16 7.686 0.66 27.9 4.16 7.78 0.6 29.8 0.03 8.433 0.57 28.1 3.93 6.547 0.52 34.9 1 7.336 0.44 35.7 3.84 7.603 0.45 33.8 3.88 8.83 0.24 13.7 0.02 9.086 0.53 31.6 0.03 8.612 0.45 18 2.05 10.73 0.35 17.4 2.31 10.79 0.36 17.9 2.33 13.02 0.19 14.2 2.32 12.95 0.42 14.4 2.28 12.98 0.31 17.3 2.79 13.4 0.24 15.8 0.03 13.85 0.21 16.3 2.73 13.96 0.29 16.5 2.82 19.09 0.33 11.1 2.59 b

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3.3 Yunnan-Guizhou Plateau Area It is demonstrated from Table 1 that from the statistical results of E-P relations of the 10 regional stations during 1961∼2008 in Yunnan-Guizhou plateau area, the correlation coefficients between precipitation and ground water vapor pressure are mostly greater than 0.45 in various seasons of a year. The correlation coefficient of Guiyang in rainy season is relatively small, about 0.35. In Yunnan-Guizhou plateau area the rainy season is distributed in June∼September. A subtropical humidness features this area. In spite of its plateau terrain, the climate is notably diverse in different places because of the variety of their altitude and atmospheric circulation conditions. For example, in Kunming of Yunnan, with the altitude of about 1900m, and the rather low latitude (25◦ N), in winter it does not be generally affected by cold waves, on the contrary, it is controlled frequently by southwest warm currents, so it is often clear. In winter and in spring it is quite dry and warm. In the summer half year, it is mainly affected by southwest monsoons with abundant precipitation and many rainy days. Because of the high altitude, its temperature is a bit low in summer. In a year, the distinction between dry and wet seasons are clear-cut. In Yunnan area, in rainy seasons, the correlation coefficient between precipitation and ground water vapor pressure is greater than 0.4, and its mean relative error less than 10%. Within the Guizhou area, where the altitude is at large about 1000m, in the winter half year, it is frequently affected by north cold front, and a phenomenon known as “Kunming quasi-stationary front” in climatology is formed. In the winter half year, Guizhou is often shrouded in the stationary front, there are many overcast and rainy days, while in the summer half year, there is more precipitation with the influence of southeast monsoon. In the rainy season of this area, the correlation between precipitation and ground water vapor pressure is relatively weak, less than 0.4, but greater than 0.2, with a mean relative error of 20%. In other seasons, in Yunnan-Guizhou plateau area, the linear correlation between precipitation and ground water vapor pressure is relatively strong with the most correlation coefficients greater than 0.45 and their mean relative errors are less than 25%. 3.4 Yangtze River Basin Area It is indicated in Table 1 that from the statistical results of E-P relations of the 10 regional stations during 1961∼2008 in Yangtze River Basin area, where June∼September of a year are in the rainy season, and other months in the dry season, and in dry seasons the correlation coefficients between ground water vapor pressure and precipitation are greater than 0.4, while those of the northern parts of Jiangxi and Zhejiang are on the little side, within 0.3∼0.4. In rainy seasons the correlation is relatively weak with the most of correlation coefficients less than 0.4. Yangtze River Basin area is along the River with some parts in seaside. Belonging to a kind of subtropical climate, the precipitation is rich in the whole year. Every year, at the beginning of summer, Meiyu weather appears. In the Yangtze River and Huaihe River basins to the east of Yichang of Hubei between the latitudes 28∼34 ◦ N, overcast sky and rainy days dominate the weather with plentiful rainfall and great relative humidity. The rainfall is at large continuous, and it is alternated with shower or thunder even thunderstorm. When Meiyu is finished, precipitation decreases obviously and relative humidity becomes lower. The beginning and end times of the Meiyu weather, the duration and amount of

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precipitation in every year vary greatly, so great is the yearly variation. And there exist evident differences in the precipitation and regional distribution of Meiyu in each year. In Yangtze River Basin area, the rainy season finds itself mainly in June∼September, when the correlation between precipitation and ground water vapor pressure is weak with the most of correlation coefficients less than 0.4, while in other seasons there exist some correlations with the correlation coefficients mostly greater than 0.4 and the mean relative errors less than 30%. 3.5 South China Area It can be seen from Table 1 that from the statistical results of E-P relations of the 10 regional stations during 1961∼2008 in South China area, where the correlation coefficients between ground water vapor pressure and precipitation are rather small, most of them less than 0.4. South China area is located in the low-latitude zone bordering the Pacific Ocean and the South China Sea. The enhancement of the southwest monsoon of East Asia is advantageous to water vapor transportation from ocean to land, which results in abundant rainfall. On the other hand, in the seaside of South China there is an obvious effect of land and sea breeze, and convergence centers can be formed in some particular coastal areas to enhance precipitation, and to give rise to local torrential rain and to bring about a diurnal variation of precipitation. If a convergence center caused by land breeze in April∼June coincides just with the center of coastal torrential rain, it implies that the land breeze may intensify precipitation to give rise to torrential rain and to form nocturnal rain. That the convergence centers caused by the sea breeze are all in inland leads to the formation of daytime torrential rain in inland. South China area features the tropical climate with relative rich precipitation in the whole year. The rainy season finds itself from April to October, in which from April to June it is the preceding flood period of South China. In early April the precipitation begins to slowly increase. In the middle ten-day of May the precipitation increases quickly to put South China in the prevailing stage of preceding flood period. Earlier than the middle ten-day of May a heavy rain belt is situated in the northern part of South China, appears the frontal precipitation caused mainly by the incursion of northern cold air, while after the middle ten-day of May the heavy rain belt moves to the seaside of South China, and the precipitation increases, because of the influence of East-Asian monsoon. In summer, typhoons, which formed above the tropical ocean surface of the northwestern Pacific Ocean, move northwestwards and hit the southeast seaside of China to increase evidently the precipitation. The correlation between annual precipitation and ground water vapor pressure is relatively poor, most correlation coefficients are less than 0.4 with the mean relative errors less than 25%. 3.6 Xinjiang and Loess Plateau Area Table 2 gives the statistical result of E-P relations of the 10 regional stations during 1961∼2008 in Xinjiang and Loess plateau area, where there is no clear-cut distinction between dry and wet seasons owing to the shortage of precipitate water, the annual mean data, therefore, are used for the analysis. The correlation between ground water vapor pressure and precipitation is rather good, the correlation coefficients are all greater than 0.5.

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The climate in Xinjiang and Loess plateau area is dry, in the whole year the precipitation is relatively small, and there is no clear-cut distinction between the dry and wet seasons in a year. The correlation coefficients between precipitation and ground water vapor pressure obtained with annual data are all greater than 0.53 with the mean relative errors less than 10%. Table 2 The empirical coefficients of the functional relations and the correlations between ground water vapor pressure and precipitation in Xinjiang and Loess Plateau Station Aletai Beitashan Shihezi Yining Baluntai Yinchuan Yulin Haiyuan Tianshui Lanzhou

Latitude N 47◦ 44 45◦ 22 44◦ 19 43◦ 57 42◦ 44 38◦ 29 38◦ 16 36◦ 34 34◦ 35 36◦ 03

Longitude E 88◦ 05 90◦ 23 86◦ 03 81◦ 20 86◦ 18 106◦ 13 109◦ 47 105◦ 39 105◦ 45 103◦ 53

Altitude (m) 735.3 1218.2 442.9 662.5 1739 1111.4 1157 1854.2 1141.7 1517.2

a

b

COE

0.044 0.033 0.048 0.027 0.033 0.028 0.025 0.022 0.017 0.027

5.036 3.5 6.618 7.536 4.121 7.535 6.506 5.467 9.033 6.908

0.61 0.56 0.73 0.67 0.79 0.56 0.56 0.64 0.68 0.6

M RE (%) 4.6 5 2.4 2.3 2.6 3.6 4.3 3.4 2 2.7

RM S (hPa) 0.23 0.16 0.25 0.25 0.2 0.26 0.27 0.22 0.28 0.25

4. DISCUSSION 4.1 General Relation between Ground Water Vapor Pressure and Precipitation It can be seen from Tables 1 ∼ 2 that in some areas the similar E-P relations exist among various stations. For example, Suihua, Jiamusi, Jixi, Huhehaote and Xilinhaote, which are located in North China and Northeast China, are more or less equal in their empirical coefficients. If the mean values of these coefficients are calculated, an empirical expression may be obtained to express the general E-P relation of these areas. With this expression the ground water vapor pressure in these areas can be proximately calculated, the resultant errors of calculation are shown in Table 3, from which it is considered that the results are rather accurate with mean relative errors less than 35%. Table 3

The empirical E−P relations and their errors in North and Northeast China

RM S(hPa) M RE(%)

Sui hua 2.5 24.7

Jia musi 2.92 26.6

Dry season E=1.1P +2.436 Ji Chang Haer xi chun bin 2.77 2.79 2.56 28.6 32.3 28.9

Fu xin 3.29 34.4

Sui hua 1.55 6.5

Rainy season E=0.081P +19.255 Jia Ji Chang Haer musi xi chun bin 1.53 1.7 1.6 1.48 6.5 7.4 5.9 5.8

Fu xin 2.32 8.5

Tacheng, Yining, Yinchuan, Yulin and Lanzhou, which are located in the dry area of Northwest China, have similar E-P correlations. Using the same procedure an empirical expression indicating the general E-P relation of this area can be approximately calculated, and the ground water vapor of these places, too. The resultant errors of calculation are shown in Table 4, from which it is considered that the results are correct and reliable.

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Table 4

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The empirical E−P relations and their errors in Northwest China

RM S(hPa) M RE(%)

Tacheng 0.94 13.3

E=0.026P +6.907 Yining Yinchuan Yulin 0.72 0.78 0.59 8.1 8.4 6.7

Lanzhou 0.27 2.7

Because of the joint effect of geographic location, climatic conditions, and atmospheric circulation, the correlations between ground water vapor pressure and precipitation are similar for some regional stations, and there are nearly same regressive coefficients of their empirical equations. In this case, it is possible for us to calculate the ground water vapor pressure with the E-P empirical expression of another station. This will be valuable for the study of water vapor distributions in those remote areas and plateau areas which are short of observations. 4.2 Calculation Pattern of Precipitation– Ground Water Vapor Pressure–Atmospheric Water Vapor Content According to the water vapor distribution and the geographic feature of high altitudes in west and low altitudes in east of China, by taking the geographic latitude and altitude as two parameters in the parameterized equations of empirical coefficients, YANG Jingmei et al.[3−4] have derived a relation between ground water vapor pressure and atmospheric water vapor content for each regional station. A further fitting gives the empirical relations of these empirical coefficients with the geographic latitude and altitude, then an empirical calculation pattern for the general relation can be obtained. If the ground water vapor pressure E of a regional station has been calculated from its precipitation P , then the atmospheric water vapor content W of the corresponding area can be obtained by substituting the calculated ground water vapor pressure E into the empirical equation of YANG Jingmei et al.[3−4] . Table 5 lists for the representative stations in Northwest China the annual mean precipitation P during 1961∼2008, the ground water vapor pressure E calculated with the empirical relation given by this paper, and the atmospheric water vapor content W obtained by substituting E into the equation of YANG Jingmei et al.[3−4] , and its relative error. Because the annual precipitation in this area is on the little side, and there is no apparent difference between the dry and wet seasons, the analysis is carried out with the annual data. Table 5

The statistics of precipitation and atmospheric water vapor content in Northwest China Station Aletai Beitashan Shihezi Yining Baluntai Yinchuan Yulin Haiyuan Tianshui Lanzhou

P (cm) 1.42 1.46 1.74 2.05 1.8 1.41 3.3 3.14 4.31 2.69

E(hPa) 0.43 0.34 0.64 0.69 0.4 0.67 0.62 0.53 0.83 0.65

W (cm) 1.1 0.81 1.39 1.51 0.88 1.49 1.4 1.12 1.91 1.41

M RE(%) 4.1 4.2 2.2 2.1 2.4 3.3 3.9 3.2 1.9 2.5

In Table 6, for the representative stations in other areas over China, the statistics relevant to water vapor are listed. In these areas, there is a clear difference of precipitation

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between the dry and wet seasons, and the analysis, therefore, is carried out with monthly data. Table 6

The statistics of precipitation and atmospheric water vapor content over China Station Mohe Daxinganling Hailaer Aershan Yichun Erlianhaote Chifeng Fuxin Dandong Beijing Shiquanhe Gaize Naqu Rikaze Nimu Jiangzi Changdu Linzhi Mangkang Geermu Lijiang Tengchong Baoshan Kunming Lincang Lancang Guiyang Zunyi Anshun Dushan Chongqing Yichang Jingzhou Wuhan Changde Xuzhou Xuyi Nanjing Hefei Nanchang Guilin Liuzhou Shaoguan Zhangzhou Baise Guangzhou Nanning Yulin Qinzhou Haikou

P (cm) 2.38 2.47 1.63 2.36 3.15 0.64 1.79 2.42 4.61 2.25 0.14 0.19 0.83 0.33 0.48 0.33 1.15 2.15 1.12 0.13 0.99 3.84 2.93 2.05 2.44 3.15 4.01 6.22 6.97 7.92 6.45 6.05 7.24 8.18 9.8 3.37 4.77 5.95 6.07 11.96 8.18 5.71 8.26 5.9 2.5 5.01 4.06 5.44 5.02 4.38

Dry season E W (hPa) (cm) 3.71 0.72 4.16 0.79 3.98 0.79 3.64 0.75 4.97 0.91 3.44 0.72 4.87 0.94 5.16 0.93 8.08 1.41 7.82 1.37 1.27 0.2 1.16 0.18 1.7 0.27 2.25 0.36 2.39 0.39 2.25 0.36 3.25 0.6 5.01 1.03 3.25 0.53 1.79 0.37 5.59 1.27 9.15 1.73 9.13 1.75 8.45 1.61 9.88 2.05 12.6 2.59 9.43 1.96 11.32 2.3 10.85 2.22 11.76 2.35 13.08 2.95 11.34 2.2 12.06 2.33 11.4 2.21 12.11 2.46 8.39 1.64 9.55 1.85 10.2 1.98 10.43 2.02 12.36 2.51 10.03 1.91 11.8 2.76 11.75 2.68 13.45 3.13 13.94 3.25 13.83 3.24 14.15 3.31 14.54 3.39 14.89 3.46 19.77 3.09

M RE (%) 29.09 29.28 27.56 32.11 27.9 15.39 33.6 34.11 31.36 29.12 32.87 29.29 25.72 29.28 27.33 23.66 22.93 15.84 21.18 30.28 15.5 14.51 15.21 14.29 11.71 10.19 15.37 17.53 17.23 20.57 13.58 24.17 27.23 29.82 27.51 34.44 35.69 32.89 34.9 30.88 19.08 15.89 16.77 12.61 15.96 15.06 14.1 14.12 14.21 14.19

P (cm) 9.88 13.28 9.19 9.95 15.37 3.58 9.19 12.65 25.26 16.31 1.2 3.22 7.62 8.34 6.34 5.29 8.03 10.56 10.79 0.65 15.27 21.08 13.52 14.84 16.98 23.64 14.59 14.51 20.4 17.39 15.23 16.58 12.84 14.84 14.02 14.51 16.06 14.44 12.32 15.48 21.13 16.62 16.36 18.26 13.95 21.31 15.78 18.85 27.37 20.25

Wet season E W (hPa) (cm) 15.23 2.68 6.38 1.16 15.12 2.68 13.89 2.49 17.81 3.09 12.19 2.2 17.68 3.12 22 3.79 24.78 4.25 24.38 4.19 4.29 0.71 4.82 0.8 6.27 1.05 8.45 1.42 8.04 1.35 7.59 1.27 9.37 1.76 11.46 2.39 8.4 1.41 5.24 1.13 13.96 2.94 18.46 3.6 18.2 3.56 16.58 3.24 19.12 3.89 22.09 4.85 18.95 3.86 21.95 4.41 19.5 3.94 22.03 4.4 24.5 5.23 25.86 4.95 27.03 5.18 27.04 5.18 27.34 5.51 24.09 4.62 26.06 5 26.34 5.04 26.54 5.08 27.93 5.63 23.9 4.68 25.06 5.22 25.28 5.19 25.82 5.37 26.12 5.41 27.47 5.66 27.06 5.59 27.44 5.66 28.53 5.81 29.73 5.09

M RE (%) 9.11 59.21 7.51 5.88 6.22 0.69 4.98 5.59 4.78 4.42 18.71 19.56 15.43 17.52 17.63 17.71 14.53 12.47 10.26 22.79 8.77 8.19 9.96 8.9 6.67 5.57 12.71 7.94 6.94 7.12 8 10.7 12.02 11.81 11.08 13.6 12.98 13.02 12.64 9.85 19.51 11.73 12.12 10.71 11.92 9.04 7.52 7.92 6.99 7.7

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Figure 4 shows the distributions of W , the atmospheric water vapor contents over China in the dry seasons and rainy seasons of 1961∼2008, respectively. In dry seasons, the atmospheric water vapor content in South China is a little large, with the seasonal mean value greater mostly than 2.5 cm; that in the southern part and western part of QinghaiTibet plateau is a bit small, with the seasonal mean value mostly less than 0.5 cm; that in the area norther than 35◦ N, too, with the seasonal mean value less than 1.5 cm. In rainy seasons, the atmospheric water vapor content in the southern and eastern China is a bit large, with the mean value generally greater than 4 cm; secondarily, that in the southern part of Northeast China , 3∼4 cm, namely, the water vapor content is relatively large; that in a small area of the western part and middle part of Xinjiang is relatively large with seasonal value of 3∼3.5 cm, while in other areas relatively small; in particular, that in the southwestern part of Qinghai-Tibet plateau and in a partial region of the middle part of Xinjiang is the least, with seasonal mean value less than 1 cm. For most areas of China, regardless of the dry or rainy seasons, the atmospheric water vapor content derived from the precipitation is consistent well with that derived with the ground water vapor pressure by YANG Jingmei et al.[3−4] .

Fig. 4

The distributions of atmospheric water vapor content over China in dry season (a) and rainy season (b)

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5. CONCLUSION In this paper the empirical relations of E-P have been set up to calculate the ground water vapor pressures, by means of the meteorological element data of 210 stations over China. Through verifications and comparisons, it is confirmed that this procedure is rather precise and is valuable to practical applications. The conclusions of this study are as follows: (1) There exists a linear correlation between ground water vapor pressure and precipitation, the precipitation affects the ground water vapor pressure at the place in question to a great extent. In spite of the considerable variations of precipitation in time and space, there exists a generally regular and stable correlation between ground water vapor pressure and precipitation. In the rainy seasons of North and Northeast China, the water vapor pressure tends to be saturated, the linear correlation between precipitation and ground water vapor pressure is not so good that most correlation coefficients are less than 0.4, while in dry seasons there exists a strong correlation with the correlation coefficient generally greater than 0.6, and the mean relative error generally less than 35%. The precipitation in Qinghai-Tibet plateau decreases progressively from southeast to northwest. In rainy seasons the correlation coefficients between precipitation and ground water vapor pressure are mostly greater than 0.6 with the mean relative errors less than 25%. In other seasons, there exists a strong correlation, and the correlation coefficients are mostly greater than 0.6, too, with the mean relative errors less than 35%. In Yunnan-Guizhou area, because of the variety of their altitudes and atmospheric circulation conditions, the climate is remarkably diverse in different places. In Yunnan area, in rainy seasons the correlation coefficients between precipitation and ground water vapor pressure are greater than 0.4 with the mean relative errors less than 10%. In Guizhou area the correlation is relatively weak, and the correlation coefficients are within 0.2∼0.4 with the mean relative errors less than 20%. In other seasons, in Yunnan-Guizhou plateau, the linear correlation between precipitation and ground water vapor pressure is relatively strong with the most correlation coefficients greater than 0.45 and their mean relative errors less than 25%. In Yangtze River Basin, in rainy seasons the correlation between precipitation and ground water vapor pressure is weak with the most of correlation coefficients less than 0.4, while in other seasons there exists a certain correlation with the correlation coefficients mostly greater than 0.4, and the mean relative errors less than 30%. In South China the correlation between annual precipitation and ground water vapor pressure is relatively weak, the most of correlation coefficients are less than 0.4 with the mean relative errors less than 25%. The climate in Xinjiang and Loess plateau area is dry, because the precipitation is relatively small, there is no clear difference between the dry and wet seasons in a year. The correlation coefficients between precipitation and ground water vapor pressure obtained with annual data are all greater than 0.53 with the mean relative errors less than 10%. (2) The functional relations between ground water vapor pressure and precipitation are similar among the stations of some areas, it is possible, therefore, to calculate the ground water vapor pressure of these stations by means of an average E-P empirical relation, and also to calculate the ground water vapor of a certain station according to the empirical

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relation of another station. This procedure is of practical significance for investigating the atmospheric water vapor content above a station which is short of sounding data. (3) In this paper, the results are obtained from the analysis of the meteorological data of 210 stations which are distributed in various altitudes and different longitudes and latitudes over China, and they are rather precise. The atmospheric water vapor content derived with the procedure of this paper is consistent well with that obtained by YANG Jingmei et al.[3−4] . Thereby, it will bring conveniences to the study of water vapor contents of some areas, in particular those of remote areas, and will have important applications in some fields, such as the astronomical site survey, atmospheric radiation and transmission, remote sensing, meteorology and so on. ACKNOWLEDGEMENT The authors thank the Data Sharing Service of the Meteorological Reference Room of the National Meteorological Information Center for providing a wealth of filed meteorological data. References 1

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