1、第二章第二章 城市生态系统城市生态系统内容一、关于地球系统一、关于地球系统二、全球大城市分布二、全球大城市分布三、城市下垫面组成及描述三、城市下垫面组成及描述四、城市生态系统四、城市生态系统“印痕印痕”（Footprint） 一、地球系统 大气圈、水圈、冰雪圈、土壤岩石圈、 生物圈与区域气候变化相互作用 区域与全球气候系统的研究 区域水循环的研究 区域水循环的研究区域碳循环的研究 冰冻圈生态系统与全球变化研究二、全球大城市分布广州，搜狗三、城市下垫面组成及描述lTowns and cities are the most densely populated areas on Earth and
2、will continue to be the artificial landscapes most widely used by the greater part of the Earths population in the future. In 2030 more than 60% of humans will live in cities. lChanges in urban conditions have often caused deterioration in environmental quality and may result in damage to the health
3、 of city-dwellers. lThe differences between the climate of a city and the climate of its surroundings are referred to as the “urban climate”. The most important features of urban climate include higher air and surface temperatures, changes in radiation balances, lower humidity, and restricted atmosp
4、heric exchange that causes accumulations of pollutants from a variety of sources. lAlthough these changes mainly affect local or regional conditions, persistent substances released into the atmosphere may also affect larger areas or even the global climate.Causes of Urban ClimateThe four main causes
5、 of urban climate, which result from different uses of built-up areas, are:l replacement of natural soil by sealed surfaces, mostly artificial and having a strong 3-D structure;l reduction of the surface area covered by vegetation;l reduction of long-wave emission of the surface by street canyons an
6、dl release of gaseous, solid and liquid atmospheric pollutants, and waste heat.1.Thermal and Hydrological Properties of Urban SurfacesThe thermal behaviour of sealed surfaces is largely determined by the density, heat capacity, thermalconductivity, thermal diffusivity and thermal admittance of the m
7、aterials used10-62. Structure of Urban Atmosphere and Wind ConditionsThe 3-D relief of cities multiplies effective surface area. The aerodynamic properties of surfaces can be characterized by the roughness length (z0) and the zero plane displacement (d0), which together measure the unevenness of a s
8、urface with respect to the surfaces effects on the horizontal and vertical wind vectors.Urban Boundary Layer3. Radiation and Heat BalanceCompared with the surrounding countryside, the radiation and heat balances of an urban area aresubject to a wide variety of effects. These are caused by gaseous, p
9、articulate and liquid air pollutants,which reflect, scatter and absorb radiation, and the type, structure, use and exposure of urban surfaces. The short and long-wave albedo long wave emission, effective radiation and thermal behaviour of the surface are determined by the composition of the urban at
10、mosphere and the citys surface conditions. During low advection conditions without precipitation, the radiation and heat balance of the ground/air boundary layer is:4. Urban Moisture Environment,Precipitation Amount, Fog DensityThe urban atmospheric moisture environment depends on the water balance
11、of the urban area:lPrecipitation patterns are odd in urban areas - most rain falls on the lee side of the city, due to the urban heat island effect, the urban emission of particles which become condensation nuclei and, the higher roughness length which causes rain not to fall vertically.lRelative hu
12、midity is affected by the pattern of rainfall. During the day it will be lower in the city than in the surroundings because of higher runoff, sealed surfaces and loss of vegetation. But at night the city can be more humid because of higher temperatures and less dew-fall than in the countryside . lIn
13、 former days towns and conurbations in industrialized countries had more fog days than did their surroundings. But the number of foggy days is decreasing, due to cleaner air and a stronger urban heat island effect caused by a decrease in the percentage of natural surfaces within the cities.5. Urban
14、Heat Island EffectAir and surface temperatures in cities are normally higher than in the surrounding areas. Urbanexcess heating is the result of the different importance of the various factors in the energy balance and air movement in cities and surrounding areas. The severity of excess heating, kno
15、wn as theUrban Heat Island Intensity (UHII), is normally expressed in terms of the horizontal temperaturedifference (tu-r) between the city (u) and the surrounding area (r). 6. Urban Air PollutionlUrban air quality is mainly determined by road traffic, industry, power and heating plants and househol
16、ds. The most important pollutants in urban atmospheres include NO, NO2, CO, NMVOC (Non-Methane Volatile Organic Compounds), O3, SO2, dust and soot. lMost air quality problems are caused by SO2, NO2 and SST/PM 10. In western industrialised countries classical air pollutants such as SO2 are no longer
17、a serious problem in most areas.四. Urban Ecological Footprints The footprint function f describes the source area Q of a measuring signal (scalar, flux) in relation to its spatial extent and its distribution of intensity, as illustrated in Fig. 3.16, and is given by:How Can Cities be Sustainable?Whats Your Suggestion?.The End
