城市气象学课件：07_1城市水平衡_湿度_云

1、Ch7 水分、云、雾和降雨,一、水平衡 二、湿度 三、云 四、雾 五、降水 六、能见度,一、水平衡Properties of water 1,Water possesses a number of unusual properties which make it an important climatological substance. One important thermal property is its high heat capacity. This effectively means that in comparison with most other natural mater

2、ials it takes much more energy input to cause a similar rise in the temperature of water. Equally, subtraction of energy does not cause water to cool as rapidly. This property makes water a good energy storer, and a conservative thermal influence.,Properties of water 2,Water is the only substance th

3、at exists in all of its states at temperatures normally encountered in the Earth-Atmosphere system. In changing between ice, water and water vapour, latent heat is taken up or liberated and as a result the energy and water balances become enmeshed.,Properties of water 3,ice and water phases (melting

4、 or freezing) : 0334 MJkg-1 at 0 , and is called the latent heat of fusion (Lf). liquid water and water vapour (evaporation or condensation) at 0 requires 250 MJkg-1, which is almost 75 times more energy. This is the value of the latent heat of vaporization (Lv), and at 10 it is 248, at 20 245 and a

5、t 30 243 MJkg-1. In the event that the water changes directly between the ice and vapour phases (i.e. sublimates) the latent heat of sublimation (Ls) is the algebraic sum of Lf and Lv, and at 0 it is 283 MJkg-1.,Schematic diagram of the average annual hydrologic cycle of the Earth-Atmosphere system.

6、 Values expressed as percentages of the mean annual global precipitation of 1040 mm,Over land areas p is greater than E, and the excess is transported as streamflow to the oceans where E is greater than p. So that for the Land and Ocean sub-systems their annual water balance may be written: p=E+r, w

7、here, rnet runoff (i.e. the net change in runoff over a distance). This term may have a positive or negative sign.,the water balance of an urban building-air volume. P+F+I=E+r+ S+ A,Considerable amounts of water vapour are released when fossil fuels such as natural gas, gasoline, fuel oil and coal a

8、re burnt. The use of water to absorb waste heat from power plants and other industrial processes also greatly enhances vaporization from cooling towers, cooling ponds, rivers and lakes. In combination these provide a preferential source of vapour for the urban atmosphere (F).,The importation of wate

9、r to the city (I) is necessary to meet demands from residential, industrial and other users. This mass input to the city system (Figure 8.7b) can be fairly easily monitored, and Figure 8.8 is an illustration of the seasonal and diurnal variations of I in a small (mainly residential) community in Cal

10、ifornia. The strong seasonal difference is due to the summer use of water for lawn and garden sprinkling, swimming pools, car washing, etc. Peak-use is concentrated during the day, with subpeaks in the morning and evening. Ultimately this water is lost from the system via evapotranspiration or runof

11、f.,Winter and summer patterns of daily water use by the small community of Creekside Acres, California,Water balance Let us compare the water balance of an urban building-air volume with that of a corresponding soil or soil-plant-air volume, in the surrounding countryside. To simplify matters consid

12、er both to exist in an extensive area of similar composition, so that we may neglect A for both.,The water input to the urban system is greater because its precipitation (p) is augmented by F and I, for which there are no rural counterparts (if we ignore irrigation). On the other hand, it seems like

13、ly that urban evapotranspiration (E) and sub-surface storage (S) are less than in the rural situation. Evapotranspiration is expected to be reduced because of the removal of vegetation and its replacement by relatively impervious materials (although some building materials are quite efficient water

14、stores).,the urban runoff (r), is greater than in rural areas. Part of this is simply due to the disposal of a portion of I as waste water (via sanitary sewers). The remaining increases are due to the surface waterproofing and artificial runoff routing (e.g. storm sewers) that accompanies urbanizati

15、on.,降水量未变时城市化前（虚线）和城市化后（实线）径流量,径流速度与河流集水面积的关系,城市化对一个流域径流量速度的影响胜过其集水面积对径流的影响,二、湿度,1、绝对湿度：包括水汽密度、水汽压、比湿、混合比、露点温度等； 2、相对湿度（）,1、绝对湿度：干岛,城市干岛：城区的绝对湿度往往小于附近郊区；植物生长茂盛的季节和白昼比较明显。,城市湿岛,在夜晚郊区下垫面温度和近地面气温的下降速度比城区快在风速小，空气层结稳定的情况下，有大量露水凝结，致使其近地面空气层中的水汽压锐减。城区因热岛效应，气温比郊区高，凝露量远比郊区小，且有人为水汽量的补充。夜晚湍流强度又比白天减弱，由下向上输送的水汽量

16、少。因此这时城市近地面空气层的水汽压反比郊区为大，形成“城市湿岛”。这种湿岛主要是由于夜间城、郊凝露量不同而形成的，可称之为“凝露湿岛”。,水汽压,水汽压陡壁,此时有热岛，强度4.4,水汽密度,在城区形成一个弯隆形的湿岛。当时天气：风速在3ms左右，有热岛存在(2.0)，亦系由于城区凝露量小于郊区而形成的城市湿岛，湿岛所及的厚度在500m以上。,湿岛分类,周淑贞对上海的分析表明，湿岛成因可分四类； 凝露湿岛、结霜湿岛、雾天湿岛、雨天湿岛。其中以凝露湿岛为最多，全年各月都有出现。这四种湿岛出现时均伴有城市热岛，且都在风速微弱时存在。,结霜湿岛、雾天湿岛,热岛,湿岛,上海冬季有一霜期，在晴寒无风或

17、小风天气，城乡地面有冰冻和结霜现象出现，城市有热岛存在，其结霜量比郊区小，城区近地面空气层中的水汽压大于郊区，形成城市结霜湿岛。,雨天湿岛,上海在降雨时或骤雨初歇后，伴有城市热岛而风速又甚小时可有城市湿岛出现，我们称之为雨天湿岛。,绝对湿度日变化,绝对湿度日变化与大气层结、湍流状况、人为水汽量、地面蒸散量等都有关系。,2、相对湿度,城市因平均绝对湿度一般要比郊区小，气温又比郊区高，这就使得其相对湿度与郊区差值比绝对湿度更为明显。特别是在城市热岛强度大的时间，其城市干岛效应更为突出。,相对湿度季节变化,城市相对湿度的季节变化比较复杂，在一般情况下，因为冬冷夏热，相对湿度是冬高夏低。 可是在季风气

18、候区，由于夏季盛行海洋季风，冬季盛行大陆季风，相对湿度反而是夏季大冬季小，至于城市与郊区相对湿度差值的季节变化，更是因地而异，各不相同。,三、云,根据大量观测事实和理论分析，城市有使云量增多的效应，尤其是对低云量影响更大。几个原因： 1、热岛效应。城乡之间产生的局部热岛环流，城区有垂直上升气流，更有利于对流云的形成。 2、城市污染，凝结核，对低云的形成起着重要的作用。 3、城市下垫面粗糙度。摩擦阻力，使系统在城区停留的时间长。,这一天汉堡吹东南偏东风，风速3米秒，在汉堡城区迎风顶，受机械湍流作用，湿热空气上升，约在150200米低空出现碎积云，云界正好与城市东部建成区轮廓相重合。在郊区小山丘和森林上空亦出现同样的云，而在其他邻近地区则无云。这说明城市建筑群的机械摩擦作用对低云形成的影响与小山丘和森林相类似。,In the METROMEX observations of Fitzgerald and Spyers-Duran (1973) at low-flight levels the cloud condensation nuclei (CCN) increased 54 percent from upwind to d