| Xi'an, being located in the center of our country, is adjacent to Tsinling Mountains which separates the subtropical regions and warm temperate regions, and also separates the North and the the South.It has the vital climatic significance. With the aim of International Metropolis Construction in Xi'an being approached, the urbanization level of this city has being improved and Xi'an's urban climate is changing tremendously.According to a comprehensive urbanization system of evaluated guideline and the observation data of Xi'an meteorological elements form 1961 to 2009,the paper reviewed the urban development achievement of Xi'an in the last 20 years and analyzed the effects of development of Xi'an city on climate. The conclusion can be drawn as follows:1. The assessment of Xi'an urb anization level:(1) Based on 16 urbanization indicators, This essay discussed 4 aspects of urbanization level which are the population urbanization, economic urbanization, society and life quality, landscape urbanization. The level of urbanization has been unceasingly enhanced since 90s,20th century, especially in the period 2005-2009. The urbanization level of Xi'an is 85.9 percent of the 7countries for the United States, japan and the Beijing, Shanghai's urban average. (2) The proportion of secondary industry and the area of urban road for per person in Xi'an are much higher, while the per capita GDP,the number of public buses for ten thousands people and the per capita green area are lower than other indicators. Therefore it will be the key point of Continuous hikes on the per capita GDP, he number of public buses and the green area to fulfill urban construction in the future. But the first two indicators's increasing will bring more negative changes and further impacts for the city.2.The urbanization in Xi'an has influences on temperature:(1) The rising rate of the average temperature in Xi'an is 0.5℃per ten years and larger than that of the whole country. The average temperature rising rate in winter is 0.51℃/10a,and 0.18℃/10a in summer. The minimum temperature rising rate is 0.69℃/10a while the average maximum temperature rising rate is 0.29℃/10a. The trend of ascent had been more marked since the last 1990's. (2) The influence to temperature because of urbanization in Xi'an is mainly manifested that Xi'an has the obvious urban heat island effect. On average, the urban heat island intensity is about 0.5℃generally.The high value of the urban heat island intensity manifests in the lowest air temperature and its extreme may reaches about 6℃.(3) It has shown that the urban heat island intensity's increasing exists in each month. The intensity in spring is the highest, and the winter take second place.There are some features of Xi'an heat island intensity:The intensity of clear days is more marked than cloudy days.During one day, the intensity of the day is lower than the night,and it is the weakest at midday and the strongest at 9:00 pm. (4) Cancelling natural variations in climate, the contribution of urban development to the temperature rising was seen in the 1980s and it kept the rising rate of 0.44℃/10a.Since 1970s, Xi'an temperature has been continually increased about 1.15℃.3. The urbanization in Xi'an has influences on vapour pressure and Relative humidity:(1) The influence to temperature because of urbanization in Xi'an is mainly manifested that Xi'an has the obvious Dry Island Effect and Moisture Island Effect.According to the data of vapour pressure and Relative humidity between the city and the suburbs, it can be seen that Xi'an Dry Island occurs widely in Lintong,Lantian,Xi'an and surrounding area.The center of the urban dry island lies in Xi'an and Lintong. (2) Xi'an dry island intensity is more serious in spring and summer while it's weaker in winter and autumn. It's more frequent that dry island and moisture island appear day alternates with night, and they easily affected by UHI intensity and wind speed. The diurnal variation characteristics of the urban dry island intensity are similar with the urban heat island intensity, and it's also the weakest at midday and the strongest at 2:00 am.4. The urbanization in Xi'an has influences on precipitation:(1) There is a weak tendency, as well as a sensible cycle period of 5years and 20years, toward the Xi'an precipitation. (2)The probability of rainstorm and heavy rain is increasing in this city affected by the urbanization. There is a decreasing trend that the incidence of light rain between the city and the suburbs.5. The urbanization in Xi'an has influences on the wind:(1) According to statistical analysis, the northeastern wind was the leading one all the year round in Xi'an. When the wind blew into the city ditrict, the wind speed reduced to 61%-83% of original speed.After the wind pass trough the city, wind speed had increased 0-13%. As a result, urbanization reduced the prevailing wind speed. (2) There is an increasing trend of the wind speed, especially after 1976. (3) The wind speed of the city and the suburbs were ordered from strong to small as winter, summer, spring, and autumn.6. Wind fever simulation:(1) With the increasing of height, the wind speed first increase, then decrease, and then increase, finally decrease.There are not visible change of wind speed in the for seasons and dramaticlly between day and night. Suburban atmospheric surface layer wind speed was smaller than urban. (2) Temperature inversion often appears in Xi'an, especially in winter. In the vertical direction, urban temperature was higher than Suburban at 2:00 and 8:00, and urban temperature was lower than Suburban at 14:00 and 20:00. |
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