《城市气象学》课件：07-1城市水平衡-湿度-云.ppt

1、Ch7 水分、云、雾和降雨水分、云、雾和降雨一、水平衡一、水平衡二、湿度二、湿度三、云三、云四、雾四、雾五、降水五、降水六、能见度六、能见度一、水平衡一、水平衡Properties of water 1Water 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

2、 with most other natural materials 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 2Wa

3、ter is the only substance that 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 3ice

4、 and water phases (melting 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

5、it is 248, at 20 245 and at 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

6、 Earth-Atmosphere system. Values expressed as percentages of the mean annual global precipitation of 1040 mmOver 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

7、may be written: p=E+r, where, 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+ AConsiderable amounts of water vapour are released when fossil fuels such as natural gas, gasoline

8、, fuel oil and coal are 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 ofvapour for the urban atmosphere (F).222)

9、4(22222222xyOHyxCOOyxHCOHOHyxThe importation of water 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 o

10、f I in a small (mainly residential) community in California. 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

11、lost from the system via evapotranspiration or runoff.Winter and summer patterns of daily water use by the small community of Creekside Acres, CaliforniaWater balanceLet us compare the water balance of an urban building-air volume with that of a corresponding soil or soil-plant-air volume, in the su

12、rroundingcountryside. To simplify matters consider 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 ignor

13、e irrigation). On the other hand, it seems likely 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 som

14、e building materials are quite efficient water 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

15、.g. storm sewers) that accompanies urbanization.降水量未变时城市化前（虚线）和城降水量未变时城市化前（虚线）和城市化后（实线）径流量市化后（实线）径流量径流速度与河流集水面积的关系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径流的影响二、湿度二、湿度1 1、绝对湿度：包括水汽密度、水汽压、比、绝对湿度：包括水汽密度、水汽压、比湿、混合比、露点温度等；湿、混合比、露点温度等；2 2、相对湿度（）、相对湿度（）1、绝对湿度：干岛、绝对湿度：干岛城市干岛：城区的绝

16、对湿度往往小于附近郊区；植物生城市干岛：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。长茂盛的季节和白昼比较明显。城市湿岛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“城市城市湿岛湿岛”。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“凝露湿岛凝露湿岛”。水水汽汽压压水汽压陡壁水汽压陡壁此时有热岛，强度

17、此时有热岛，强度4.4水汽密度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、雾天湿岛结霜湿岛、雾天湿岛热岛热岛湿岛湿岛上海冬季有一霜期，在晴寒无风或小风天气，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

18、岛雨天湿岛上海在降雨时或骤上海在降雨时或骤雨初歇后，伴有城雨初歇后，伴有城市热岛而风速又甚市热岛而风速又甚小时可有城市湿岛小时可有城市湿岛出现，我们称之为出现，我们称之为雨天湿岛。雨天湿岛。绝对湿度日变化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、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。可是在季风气候区，由于夏

19、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

20、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 downwind of St. Louis for asupersaturation of 0.17 percent, and nearly doubled for a supers

21、aturation of 1 percent. The more CCN that are present the smaller will be the cloud drops because of the increased competition for the available water vapor. These same authors were able to obtain a few drop-size distributions upwind and downwind from St. Louis. Their results showed that the average drop diameters were smaller downwind by about 2-3 m and that a flat distribution of diameters changes to a steep, single-mode distribution. This is of considerable importance for initiation of precipitation.CCNThe end of Part one