1. Urban Moisture, Cloud, and Precipitation GEOG 310 Urban Climatology Dr. Hengchun Ye

2. 1. Difference of Absolute Humidity (gram of water vapor/total kg of air) between urban and rural differences rather small and spatial pattern often complex: a. Urban canopy air usually is drier by day, but more moist by night. (During the day, greater rural evapotranspiration (ET); starts early evening, rural air cools faster, ET decreases, and moisture in lower layer is depleted by dew formation throughout night. Therefore, rural humidity decreases through the night, a vapor inversion forms and. (In the city there is less ET, reduced dewfall, anthropogenic water vapor input and stagnation of airflow, which all combine to maintain more humid atmosphere) (sunrise-evaporation of dewfall and other surfaces, water rapidly replenishes rural moisture as convection slow to develop) b. Nighttime moisture island in the city is similar to temperature. In ideal conditions, humidity island also has a ‘cliff’ at urban/rural boundary. During the day, city drier. Winds can decrease urban/rural differences.

3. 2. In cold climate regions, city in winter has more humidity during day. (Rural source of vapor, ET, is eliminated and vegetation dormant, but city’s anthropogenic releases from combustion, especially space heating, provide significant vapor input). 3. Relative humidity (RH=total water vapor/holding capacity at the given air temp). City’s annual mean is 6% less, winter is 2% less, summer is 8% less than rural. RH is a function of temperature (the holding capacity increases with air temperature; due to heat islands, RH is smaller).

4. 4. Fog. City is more foggy (on average winter has 100% more and summer has 30% more fog in cities). a. Problem of definition, based on visibility to measure fog, which is always less in cities and makes no distinction between pollution or water droplets. b. Dense fog, visibility less than 400 m, often less in cities than suburbs or rural areas. Improvement may be due to both heat island and abundance of condensation nuclei in city. (Increased number of nuclei results in greater competition for vapor and larger number of small droplets, which don’t produce dense fog) c. However, at high latitudes, urbanization leads to ice fogs. (Release of vapor in -30°C or less causes fog of ice crystals as saturation vapor pressure very low. Combustion of fuel for heating, industry, aircraft and especially cars, mainly responsible)

5. 5. Clouds. City averages 5-10% more. a. Clouds are influenced both by increased convection and by enormous production of hygroscopic condensation nuclei (produced by catalytic oxidation of sulfur dioxide and hygroscopic sulfates, oxides of nitrogen transformed to nitric acid; many other small particles). The former is more likely to produce clouds in summer, the latter more apt to cause early condensation at inversion layers in higher winter humidity. b. Summer cumulus clouds form earlier over cities than rural. c. Anomalous cloudiness over industrial plants and cooling towers. (Example: St. Louis, first cumulus formed 3 times expected frequencies over central urban area and refineries to N. of city, which are sources of heat, vapor and nuclei. Cloud condensation nuclei (CCN) increased 54% at low flight levels from upwind to downwind of St. L. Also average droplets diameters are smaller downwind by about 2-3 μ (microns) and more uniformly small due to greater competition for vapor.)

6. 6. Precipitation is the most puzzling of urban meteorological variables. It is complicated by topography and water bodies, which stabilizes air masses in summer, but furnish moisture in winter. Table 8.7 (handout) urban-rural differences of annual precipitation reported in the literature (urban climate)

7. Precipitation-continue a. There is much evidence for increased precipitation in urban areas over rural areas (downwind). b. On average, cities are 5-10% more in precipitation amounts; days with 0.2”+ have 10% more in cities. c. 3 main contributing factors for modification and augmentation of precipitation: i. heat island causes rising motion over cities. ii. obstacle effect-aerodynamic roughness of cities impedes progress of weather systems. Slower movement leads to increased precipitation than when moving fast. iii. pollution-CCN can promote or inhibit precipitation. For ice nuclei, particles might initiate precipitation process in super-cooled clouds. Some evidence shows that pollution effect is secondary to aerodynamic and thermodynamic impacts.

8. Maximum urban-rural differences in summer rainfall and severe weather events (handout)

9. Percent increase in thunderstorm incidences as related to urban population size (handout)

10. d. Network approach. Setting up large network of instruments show upwind vs. downwind differences. Need to make sure there are no upwind controls, such as other urban, topography, and water effects. Ex.1 St. Louis upwind-downwind differences 23%, statistically significant. Ex.2 Tel-Aviv, Israel. With 3 decades of record, shows 5-17% increased rain downwind. Ex.3 Turin, Italy. Between 1952 and 1969, city grew 700,000 to 1.2 million and cars from 70,000 to 390,000. Summer shower events increased in frequency in the city, but also decreased in volume/shower. In winter, there was also influence from the air of Milan, an industrial city. There was a decrease in both volumes and frequency of precipitation in Turin. But adjacent rural zone had a relative increase in events ascribed to nucleation by industrial aerosols. Ex.4 In W. Europe, colder season showed increased days with drizzle in industrial cities.

11. e. Refineries produce sulfates and nitrates. Nitrates are suspected of being more active nuclei, being larger and more hygroscopic than sulfates. Their presence leads to a wide range of cloud drop sizes, which are better for rain. Sulfates are small particles and more apt to stabilize clouds-possibly more fog. This shows that the nature of aerosols are important, not just their numbers. f. Weekdays vs. weekends. Paris showed 24% decrease on weekends. 22 E. US cities also noted weekly cycle, but only in winter, with weekends 8% less. Rainfall pattern followed pollution levels. Recently, some cities showed more pollution and rainfall on week days.

12. g. Slow-moving storms show heavier rains over urban areas. Several cases of floods in cities while rural surroundings had considerably less.

13. Examples: 1. Cloudburst over Berlin on 6/27/64 with little vertical motion, a cumulonimbus developed over the city leading in 30 minutes to electrical discharges, rain and hail. In 2-hrs, 81 mm fell while to the SE and NE almost no rain observed. 2. In Nov., 2004, a storm cloud stalled over central L.A. dumping over 5” rain, lightning and several inches of hail, while very little rain fell elsewhere in the urban area. h. Both summer rain amounts and thunderstorm frequencies seem to be larger just downwind from cities, but not all over city. (In Berlin, Germany, average thunderstorm days number 32 in a SW suburb, only 20 in the inner city and 28 to the NE. Summer maximum rainfall in SW approaching zone for storms has 200 mm, inner city 160 mm, with 190 to NE suburb. No explanation was made for avoidance of inner city by thunderstorms.) i. Fujita noted curious avoidance of central Chicago and central Tokyo by tornadoes. Horseshoe area located over Chicago appears tornado-free during past 20 yrs. Also there’s a horseshoe-shaped tornado-free area around Tokyo. The opinion of Fujita is that heat island so uniform that significant vorticity (spin) mainly generated at interface with cooler country air. Also the urban friction assumed to decrease tornado intensity. j. Changnon showed there is an increase in thunderstorms and hail for cities over rural areas and that the increase is proportional to the population size of the city. (Ex. Laporte, Ind. Anomaly concerned a 100-200% increase in thunderstorms and hail over a small town 35 miles SE of the Chicago-Gary, Ind. Industrial source). k. Snow. There is less snow in cities (heat island) as more falls as rain (in Lund, Sweden, snow depth decreased from 8cm in rural area to 3cm in urban center). But another effect is in cold climate, anthropogenic nuclei (cola-fired power plant produce ice nuclei, some of which are already effective at -5C, can cause snow fall related to the plume) induce more winter snows.

14. Urban Wind Field The changes brought about by urbanization in the local atmospheric boundary layer have a notable effect on the low-level wind. This is caused by the Heat Island and the change in surface roughness.

15. Surface Wind 1.Fig 6.1 change in wind speed classes during rapid growth area of Columbia, MD. Three wind categories, (a) Columbia wind speed was less than 70% of that at airport; (b) Wind speed is 70-99% of airport; (c) wind speed is higher than airport. The frequency of (c) was 25% in 1969 and dropped to 14% in 1974. The frequency of (a) decreases from 43% to 65%. Exampes: Wind speed decreases in Gantsevitchi, Russia from 3.9m/s to 2.5m/s. Also in Partma, Italy. 2. Wind direction of greater speed is less affected by urbanization than weak ones.

16. 3. When regional winds are light and skies are clear, a drift or convergence of surface air towards the urban heat island and a compensating outflow aloft have been recorded empirically in different cities. (This is a logical consequence of the heat island, which creates an unstable vertical lapse rate of temperature and so induces a rising air current). 4. Centripetal urban air flow (into the city) is usually more pronounced at night than during the day because the temperature difference (urban-rural) is greater at night.

17. 5. When air flows from country to city, it encounters a great increase in surface roughness. The wind near the ground therefore slows down except for local accelerations around individual buildings, and becomes more turbulent. Landsberg (1981) states that there is a reduction in speed of 20 to 50% at the standard observing level of 10 m. Changes in Mean Wind Speeds in Parma, Italy in Three Consecutive Decades* IntervalJanuary April JulyOctoberYear 1938-1949 0.5 1.8 1.8 1.0 1.3 1950-1961 0.5 1.4 1.4 0.7 1.0 1962-1973 0.3 1.0 1.3 0.6 0.8 *wind speed (m sec-1); adopted from Zanella (1976).

18. 6. When there are deciduous trees in the city, the reduction in wind speed increases in summer and decreases in winter. 7. A differential day-night wind speed reduction has also been observed in different cities. Lee (1979) clearly showed that if in the daytime the wind speeds in London were reduced on an average of 30 percent, the average nighttime reduction was only 20 percent. In some cases, a strong nocturnal convergence results in greater wind speed in the city than in its surrounding.

19. 8. The flow of air around buildings and in the street canyon is very complex and depends greatly on the design of the city.

20. Vertical Wind 1. One of the common mathematical models describing airflow over a city was proposed by Davenport in 1968: U*/VG = (Z/ZG)α, where: U* is the wind at height Z VG is the gradient wind speed at height ZG α is dependent on surface roughness Measurements of the vertical Wind Profile for Different Surface Roughnesses* Terrain characteristicsExponent of power lawGradient wind level (m) Open country, flat 0.16270 Suburban settlement0.28390 Inner cities0.40420 *Davenport (1965).

21. 2. This formula is good for determining the average wind over a city (but does not help in predicting and calculating turbulence and peak gust winds. Generally the wind speed increases with height. However city friction reduces the rate of increase with height, reaching the non-friction level at a much higher level -gradient wind level). 3. Urban areas have also an affect on the wind direction and turbulence. There may be more turbulence within the urban boundary layer than in the surrounding areas. 4. Narrow streets between tall buildings can have a funneling effect, in which wind is actually speeded up when forced to flow through a narrowing or bottleneck. 5. Edges of buildings and the geometry and spacing of buildings may cause turbulent eddies (small whorls) as wind flows around these obstacles.

22. C. Practical Importance of Knowing the Wind Structure 1. Air pollution control: the important variable is the concentration of pollutants at ground level, which is a function of wind speed, direction and turbulence. 2. Building heating and ventilation. 3. Wind loading on structures (wind stress). 4. Windbreaks to minimize drifting and blowing snow on expressways and around buildings, in cold climates. 5. Coastal cities prone to damaging winds, such as hurricanes.

23. D. Conclusions The urban wind field is rarely simple. Even when the synoptic weather situation is least complicated, small differences in local topography will cause irregular air flows. River valleys, bays, and shore locations on lakes and oceans develop their own secondary circulations, such as land and sea breezes, and mountain and valley breezes. (They interact with the urban-induced circulation and may, at times, dominate it).

24. Land/sea breeze (coastal cities)

25. Mountain/valley breeze (mountainous cities)