| With the accelerated urbanization process, population density become more dense, and urban area is enlarged continuously, which lead to the quality of underlying surface changed. The harmful gases and dust that are released from a large amount of energy consumed in high-intensity economic activity have great influence on human being's production and living. Regional climatic condition are different from the original by variety changes, a unique microclimate characteristic is forming in urban areas-- urban heat island effect. This paper conducts a special research on the reasons, distribution law and hazards of Changsha's urban heat island effect, and studies the function of urban green space in relieving urban heat island effect, using the meteorological data of Changsha City. The purpose is to provide references with scientific value for the urban planning and construction and sustainable economic and social development of the city. Research resluts are summarized as following.1. Internal structure of Changsha city is the main reason for the formation of urban heat island. These complexions such as street direction, geometric shapes and high-density buildings in old city area, not quite reasonable design and too high too close buildings in new city area all make the wind speed low in the city zone and make it difficult to diffuse heat, which result in the local temperature too high. In most cases, underlying surface are paved with cement, concrete and other hard materials with low reflectance, high heat capacity characteristics. On the same day the solar radiation condition, the city underlaying surface can absorb more calories than the suburbs'. In addition, jagged building walls make longwave radiation between wall and wall, wall and ground reflected by many times with absorption, which provide energy base for the formation of Changsha heat island. Sunset, different rate of temperature drop between urban and rural areas is the direct cause of forming urban heat island at night.2. Released urban artificial heat plays an important role in forming urban heat island. Urban factories and enterprises, vehicles, air conditionings and residents cooking consume large amounts of energy, artificial heat released from that is of 3 to 6 times to suburb. Artificial heat emission has dual role to urban heat island effect. On the one hand to increase the city's heat directly lays the foundation for the formation of heat island; On the other hand, large quantities of dust and a variety of polluting gases are emitted when emitting heat, which form a covering over the city's "dust cover" and "hood" and increase the intensity of urban heat island.3. Certain climatic conditions are also external factors in forming the heat island in Changsha City. This research shows that the occurrence of urban heat island has similar weather conditions, i.e. most of the heat island in sunny, less clouds, no wind (or breeze) and the atmospheric temperature structure of a stable layer of weather conditions. 4. The level distribution of Changsha urban heat island is basically consistent with the outline of the city urban construction, and with the development of the city has the tendency to expand outward. During the day, urban heat island is mainly in downtown and of large area, centralized distribution, but also there are distribution characteristics of multi-centre heat island. At night urban heat island has characteristics of smaller range and strong intensity. Industrial areas, densely populated residential areas, and water effects (large lakes) constitute the main heat island scattered. The vertical distribution of Changsha urban heat island is the effect height of the highest temperature of the city about in 50m, the effect height of the lowest temperature of the city over 70m.5. Changsha's heat island intensity has been enhancing year by year in the nearly 20 years, its average degree was 1.46 degrees celsius in 2008. The seasonal variation of heat island intensity manifested as that autumn is stronger than winter than spring than summer. There are two types of Changsha's heat island diurnal variation:one type is that the variation characteristics show obvious changes during the day and its strength is weak, heat-island intensity late at night or before sunrise reaches a maximum, this change process often appears in autumn-winter days with sunny weather conditions; Another type is that diurnal variation period can be clearly seen by diurnal variation characteristics, sometimes also has the obvious peak amplitude,steady overall change and small change range, and the appearing time of largest heat island intensity changes greatly. This kind of heat island mostly appears in the cloudy, windy and more rainfall wet weather.6. Because of the existence of heat island effect, the mean annual temperature soared from 17.2 degree celsius at the beginning of this century to 18.1 degree celsius in 2008. In recent years, the summer temperature extremely reached 39.8 degree celsius to 40.6 degree celsius, there are 85 days'diurnal average temperature above 30 degrees celsius in one year and 56 days above 35 degrees celsius, the continuous high temperature and frequency heat shock caused by heat-island have seriously effected urban residents'normal life and health. Urban heat island effect is helpful to the convergence circulation between urban area and suburban area, forcing the atmospheric pollutants in suburban area gush to urban area. In addition, if the static wind or temperature inversion layer existing, pollutants, suppressed in low-altitude temperature inversion layer, are difficulty to spread, increasing the concentration of near-surface pollutant, making urban often suffered from closed pollution by various high concentration pollution sources. Heat island effect change the habit of some plants.The time of sprouting, blossing and frondescing of some plants in urban area of Changsha are 3-5 days ahead of the same plants in suburban, and the defoliation time postpones 5-7 days.7. Summer heat island effect brought blazing hot, increased the consumption of cooling energy, which increased the emissions of greenhouse gas at the same time. The emissions of greenhouse gas directly accelerated the global warmmer. In turn, temperatures rise aggravated heat island effect, leading to a vicious circle.8. In the hot season, the number of more comfortable days in urban forest area increased 31.18% more than this in urban non-forest area, the number of comfortable days increased 38.57%, slightly uncomfortable days reduced 7.38%, slightly hot days reduced 18.24%, hot days decreased 53.71%. Urban forest hugely alleviated the urban heat island effect.9.Different types of green space have different effects to reduce the urban heat island, Cooling effect and humidifying effect are showed that the arbor forests'is higher than shrubberys' than thicket's than grasslands'. Temperature Changing indices of arbor forest, shrubbery, thicket, and grassland were 7.32,6.59,5.33 and 4.10, humidity index were 16.87,13.23,9.42,7.61 in order. Different greenbelt area and shape, different forest types and its crown density have different effects of reducing the heat island.10. The cooling effect of urban green space system is different along with the different weather, the cooling effect of sunny day is higher than cloudy day, and this of cloudy day higher than rainy day. The diurnal variation regularity of urban green space system is that, the supreme cooling effect period is shortly after the sunrise, the cooling effect gradually declines in the morning, and it reaches to the minimum in the afternoon around 14:00, it picks up gradually again before sunset. |
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