《城市气象学》课件：07-2城市雾-雨.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、能见度四、雾四、雾 湿雾是城市中最常见的雾类，它是由大量微小水滴或冰晶所构成。这些微粒浮游在贴近地面一定范围内的低空，使空气混浊，障碍视程。当其使水平能见距离小于lkm时，就称为雾；使水平能见距离大于1k m小于10 km时，称为轻雾，这种雾的形成所要求的相对湿度比较大，这里称之为“湿雾”，以区别于 “光化学烟雾”，那是在相对湿度较小时才能形成的。在伦敦市中心水平能见距离小于1000 m的雾时数最多，约相当于英格兰东南部7个郊县站平均值的2倍。但水平能见距离小于

2、40 m、200m和400m的浓雾出现时数却比近郊、远郊都小。这主要是由于城市中有热岛效应，特别是在夜晚城中心气温比郊区高，热力湍流比郊区稍强，因此浓雾形成的几率比郊区小。可是在一定的气象条件下，伦敦城中心也曾山现过灾难性的浓雾，1952年12月上旬闻名世界的“伦敦大雾”就是其中一例。在1962年12月5日有一个移动性高压中心正位于伦敦附近，当时风力微弱，相对湿度甚高，高压中心有下沉逆温，地面则因冬夜无云辐射冷却甚快，形成辐射雾，一直到12月8日高压脊仍停滞在伦敦上空。高空逆温层不新发展，逐渐与地面辐射逆温层联接在一起，致使大气层结十分稳定。加之冬季太阳高度低，日射不强，辐射逆温不易破坏，因此

3、使雾的浓度增大，经久不散。另外燃煤量大，污染重。辐射雾辐射雾是在天气形势稳定，夜晚无云或少云(无低云)无风或风速很小时，由于辐射冷却作用，使近地而空气气温下降，达到饱和或接近饱和时而形成的雾。它大都出现在大陆弱高压中心。这是上海城市最常见的雾类，在春、秋、冬三季均有，出现时间多集中在子夜至清晨，一般在上午10时前因日射增温而消散。平流雾平流雾是由于来自海上的暖湿空气，平流输入大量的水汽，从而使上海城区的水汽压不断升高，卒至接近或达到饱和时所形成的雾。出现平流雾的天气形势多属东部海面为高压所盘踞，大陆上出现低压或低压槽，气压梯度出东向西，盛行东南气流，风速在3ms 上下。这种雾在长江口区甚多。上

4、海城区因建筑群密集，下垫面粗糙度大，只有在夜间和清晨大气层结十分稳定而水汽的平流输入特丰时才能形成平流雾，出现季节以春季和初夏为多。当风速增大，湍流加强时平流雾即抬升为低云；或当风向改变，水汽的平流输入减少时雾即消散。 平流辐射雾平流辐射雾是在稳定的天气下，既有水汽的平流输入又有夜晚的辐射冷却作用，导致近地面空气达到或接近饱和时所形成的雾。这种雾多出现在入海变性高压后部和大陆弱高压控制下，白天吹海风使空气露点温度不断升高，夜晚又风小无云，辐射冷却剧烈，使气温与露点接近。在上海各季均可有平流辐射雾出现，一般夜间形成，早晨因日射增温或因风向改变而消散。锋面雾锋面雾是当移动滞缓的锋面经过上海或其附近

5、地区时，如果锋面风速小，锋面逆温层距地面的高度低，锋面两侧气温差异大时所形成的雾。从雾的形成机制而言，又可分为锋面降水雾与锋面混合雾两种：锋面有降水时，锋上暖雨滴落入下层冷空气中，如果暖雨滴的水温高于锋下冷空气的湿球温度，暖雨滴在下落过程中蒸发，使下层冷空气变得更冷更湿，达到过饱和状态，多余的水汽在冷空气中的凝结核上凝结成许多小雾滴，形成降水雾。实质是暖雨滴的体积变小转化为众多的雾滴，雨雾并存，能见度恶劣，这种雾称为锋面降水雾锋面降水雾。当锋面两侧冷暖空气本来就十分潮湿，气温差别又大，在移动滞缓的锋面发生混合作用，近地面空气达到饱和而成雾，称为锋面混合雾锋面混合雾。锋面雾出现的时间因锋面移至上

6、海时间而定，从子夜至中午，午前午后均可出现，雾持续时间长短亦颇不一致长者可达数小时，短者仅仅十几分钟，不象前述三种气团内部的辐射雾、平流雾和平流辐射雾那样有规则。锋面雾随锋面降水的停止和锋面移去而消散，与前述气团雾的生消规律亦不同。 上海城区的上述四种雾类，在形成原因、天气形势和有关气象要素的变化上虽各不相同，但其基本的成雾条件是相似的，即有丰富的凝结核，风速小，相对湿度高和空气层结稳定(常有逆温)。就城市的影响而言，因空气混浊凝结核多，风速又较郊区为小，这是有利于城区雾的生成的。但上海城市有干岛效应，相对湿度经常比郊区低，再加上有热岛效应，使低空的大气稳定度减弱，湍流加强，这二者是不利于城区

7、雾生成的。重庆城区雾日数多于郊区的主要原因是： (1)市区人口密度、交通运输量、能源消耗量和大汽中凝结核浓度均远较郊区为大。 (2)城区风速适中(平均风速1.4ms)有利于辐射雾的形成。 (3)城中区三面环水，南北两岸山峦对峙形成屏障，自然地理条件有利于水汽集结。因此重庆虽有城市热岛效应但在1966-1970年中仍是城区雾日数多于郊区。雾：总结雾：总结 综上所述，可见城市既有有利于湿雾生成的因素(凝结核多、风速小)又有不利于雾生成的因素(热岛效应和干岛效应)。在同一区域气候条件下，城市和郊区对比，何者雾多，要看何种要素占主导地位，更要视城、郊自然地理情况的差异而定，城市对雾的生消影响比较复杂，

8、对具体城市应作具体分析。五、降水五、降水城市对局地降水量的影响及其影响的物理机制，在城市气候学界存在着不少争论。有人认为城市对降水有明显的影响。也有人认为城市会使降水减少的效应，多数人认为城市有使城区及其下风方向降水增多的效应。为了解决这一争论问题，通过1968年在比利时首都布鲁塞尔举行的城市气候和建筑气候学讨论会决定在1971-1975年于美国中部平原圣路易斯进行大城市气象观测试验计划(METROMEX)，设立了稠密的气象观测网，经过大量观测、试验和研究，证实了城市对降水量分布是有影响的，在城区及其下风方向有使降水增多的效应。Urban-induced changes in precipit

9、ation and weatherWILLIAM R. COTTONColorado State UniversityandROGER A. PIELKE Sr.University of Colorado at BoulderHuman Impacts on Weather and ClimateSecond EditionThere is considerable evidence which suggests that major urban areas are causing changes in surface rainfall, increased occurrences of s

10、evere weather, especiallyhail falls, and alterations to surface temperatures. Some of the hypothesized causes of those changes include:lurban increases in CCN concentrations and spectra, and IN concentrations;lchanges in surface roughness and low-level convergence;lchanges in the atmospheric boundar

11、y layer and low- level convergence caused by urban heating and land- use changes; andladdition of moisture from industrial sources.First of all METROMEX and related studies showed that St. Louis exhibits a major summertime precipitation anomaly relative to the surrounding rural area. The area-averag

12、e urban-related increase is about 25%. Much of the enhanced rainfall occurs during the afternoon (1500 to 2100 local daylight time (LDT),over the city and the close-in area east and northeast. The clouds producing those changes are deep convective clouds and thunderstorms. In fact the frequency of t

13、hunderstorms is enhanced in that region by 45% and hailstorms increased by 31%. Not only is the hailstorm frequency higher, but hailstones are larger and of greater number.The rainfall observations also indicated a maximum around midnight extending from approximately 2100 to 0330 LDT located northea

14、st of the city. Changnon and Huff (1986) estimated that the area experienced a 58%increase in nocturnal rainfall relative to the surrounding countryside. The storms responsible for the nocturnal maxima were well-organized storms such as squallline thunderstorms that swept across the urban area and m

15、oved across the affected region.How does an urban area cause those changes? Let us examine each of the hypothesized mechanisms and see how well each fits the METROMEX observations.(1) Urban increases in CCN and IN concentrations and spectraAnthropogenic activity in the St. Louis urban area caused ma

16、jor increases in CCN concentrations; as much as 94%. Droplet size distributions as a result were found to be narrower with larger concentrations of droplets in the clouds downwind of the city compared to upwind. Large numbers of large, wettable particles, having radii greater than 10m with many as l

17、arge as 30 m were found over the city. These “ultra-giant” particles can serve as embryos (晶胚) for initiation of collision and coalescence. (碰并)This is consistent with the finding that clouds over the city did have a greater number of larger droplets.The METROMEX scientists cautioned, however, that

18、they had less confidence in those observations compared to the observed higher concentrations of small cloud droplets.the METROMEX modeling studies revealed that the time required to initiate precipitation in upwind and downwind clouds was only different by a few minutes. It was therefore concluded

19、that the anthropogenic CCN do not play a major role in the creation of the urban rainfall anomaly.It was also found that the concentrations of IN were not greatly altered over and downwind of the urban area. If anything, it was found in the winter months that the IN concentrations were actually less

20、 over the urban region. This suggested that the coagulation of the few IN with the more numerous anthropogenic aerosol actually deactivated or “poisoned” the IN.In summary it does not appear that the anthropogenic emissions of aerosols can by themselves cause the observed increases in rainfall. It i

21、s possible that changesin the cloud and raindrop spectra can have an impact on the rate of glaciation of a cloud and thereby the subsequent cloud behavior. We will examine thishypothesis next as the glaciation mechanism.（2） The glaciation mechanism it is generally accepted that cumuli containing sup

22、ercooled raindrops glaciate more readily than more continental, cold-based cumuli that do not contain supercooled raindrops. There are several reasons for this. First of all, larger drops freeze more readily than smaller drops . More importantly, the coexistence of large, supercooled drops and small

23、 ice crystals, nucleated by some mechanism of primary nucleation, favors the rapid conversion of a cloud from a liquid cloud to an ice cloud. Thus the ultra-giant particles observed over St. Louis could produce more supercooled raindrops which would accelerate the glaciation process. This process do

24、es not require any change in IN concentrations. A second factor potentially affecting the rapid glaciation of urban clouds is that the altered drop-size spectra could initiate secondary production of ice crystals. Laboratory studies have indicated that copious (丰富的) quantities of ice splinters (碎片)

25、are produced when an ice particle collects supercooled cloud droplets when cloud temperatures are within the range of 3 to 8 , and when the cloud is composed of a mixture of large drops (greater than 12.5m radius) and small droplets (less than 7 m). All these criteria were met in the clouds observed

26、 over St. Louis during METROMEX.(3) Impact of urban land use on precipitation and weatherThis rough surfacecreates surface drag which slows the winds near the ground. Air approaching the city would slow down and tend to divert around the citysomething like flow around an isolated rock in a stream. O

27、n the downwind side of the city, air streaming around the city would tend to converge, causing upward motion in that region.There are documented cases where changes in surface roughness have led to a slowing down of cold fronts upwind of New York City, and acceleration of the front downwind (Loose a

28、nd Bornstein, 1977). Bornstein and Leroy (1990) found that moving thunderstorms split upon experiencing a barrier induced divergence around the New York City complex, resulting in enhanced precipitation along the lateral edges of the city and downwind of the city. Analysis of winds and precipitation

29、 over Atlanta, Georgia by Bornstein and Lin (2000) suggests that a similar barrier-induced effect is present there as well.Even more important are the changes in the heat and water budget at the surface caused by the presence of a city.All heating leads to what is called an urban heat island.Such lo

30、w-level convergence has been found to be favorable for producing deep, precipitating cumulus clouds and also increases the likelihood that those clouds will merge in this low-level convergence zone to produce bigger, heavier raining clouds. The maximum convergence would occur somewhat downwind ofthe

31、 urban area as the heated boundary layer is advected in that directionObserved cloud morphology and frequencyClouds over the St. Louis urban area were found to have bases 600 to 700m higher than rural clouds. This is consistent with the observation that the air over the city is warmer and drier. The

32、 exception was clouds downwind of refineries , where it is believed that moisture injections by the refineries caused lower cloud bases. Air motion into the bases of the clouds were stronger which is consistent with the expected more vigorous thermals due to the heat island.Cloudiness (defined as th

33、e percent coverage of clouds over an area) was found to be greater over the urban area in the later afternoon (1600 LDT) consistent with the observed convergence and upward motion due to the heat island.Clouds and precipitation deduced from radar studiesThe first detectable radar echoes is a measure

34、 of the initiation of precipitation. Echoes were found to be more frequent over the urban area during the late morning, about 1400 LDT, and after 1930 LDT. This suggests that the heat island-induced convergence field played a major role in creating precipitating cumuli. Moreover, individual cumulus

35、cells over the urban area were found to grow deeper and have slightly longer durations than over the rural areas. Again, this is consistent with stronger convergence over the urban heat island favoring deeper, longer-lasting precipitating cumuli.Clouds over the urban area were also found to merge mo

36、re frequently with cells over the city, grew taller, and lasted longer than did merged cells over rural areas.As noted previously, this is consistent with observations and modeling studies which suggest that cloud merger is enhanced by low-level convergence, such as that caused by the urban heat isl

37、and effect. Because it is generally found that taller and longer-lasting cells create more rain and a greater likelihood for hail, these findings are consistent with the hypothesis that the urban heat island enhances convective rain systems over and downwind of the city.It is our opinion that enhanc

38、ed mesoscale ascent associated with the urban heat island could have intensified the nocturnal storms. The fact that nocturnal storms are typically less severe than afternoon convection means that they could strengthen without exhibiting any increase in severe weather. Further studies are needed (pr

39、obably with multiple Doppler radar) to determine if the storms contributing to the nocturnal urban rainfall anomaly were actuallyweakening or strengthening, on average.In summary, there is considerable evidence indicating that the St. Louis urban area enhances rainfall and possibly the occurrence of

40、 severe weather. The actualphysical processes responsible for those effects, however, have not been fully identified. Both the glaciation mechanism and urban heat-island-induced mesoscale changes are leading contenders. Further observational and modeling studies are required to identify the actual c

41、ausal mechanisms.One may ask: is it really necessary to identify the actual mechanisms responsible for an urban precipitation anomaly? Cant we be satisfied that the rainfall analysisshows a strong rainfall anomaly downwind of the urban area? The answer is clearly no! For one thing we cannot be sure

42、that the statistical analysis of the rainfall records did not produce an urban “signal” purely by chance. Another reason for establishing a cause and effect is that St. Louis, like many major urban areas, is situated in a river valley.A third reason for isolating the causes of the urban precipitatio

43、n anomaly is that it may become necessary to reduce the rain anomaly and enhanced severe weather occurrences. Without a clear identification of the causal factors, one cannot decideif reductions in emission of gases and aerosols contributing to CCN, or alterations in land-use patterning, is required

44、 to reduce the anomaly. At this time it has not been determined how strong the influence of gases and aerosol emissions are onprecipitation relative to land-use changes.Urbanization Signatures in Strong vs. Weak Precipitation over the Pearl River Delta Metropolitan Regions of China Weibiao LI1*, She

45、ng CHEN1, Guixing CHEN2, Weimin SHA2, Cong LUO3, Yerong FENG3, Zhiping WEN1 and Baomin WANG11 Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, 510275, China2 Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan 3 Guangzhou Central Meteorologica

46、l observatory, Guangzhou, 510080, ChinaEnviron. Res. Lett. 6 (2011) 034020 (9pp)AbstractWe assess the issues of urban effects on the precipitation over the Pearl River Delta (PRD)metropolitan regions of China. The spatial and temporal variations of strong versus weak precipitation over the PRD and s

47、urrounding nonurban areas are investigated. The results show that the urbanization signatures in strong precipitation are significantly different from those in weak precipitation over the urban areas. The PRD experiences more strong precipitation but less weak precipitation compared to surrounding n

48、onurban regions. In addition, the strongprecipitation over the PRD displays a pronounced seasonal variation. The seasonality of weak precipitation, however, is much weaker over the PRD compared to the surrounding nonurban regions. Moreover, a strengthening in the precipitation intensity, a reduction

49、 in the rainfall frequency and an increase in the convective precipitation as well as the afternoon precipitation are found over the urban areas, which are probably associated with the abundance in strong precipitation and the deficit in weak precipitation over the PRD.Keywords: urbanization effects, precipitation, Pearl River Delta of ChinaThe end of part two