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Urban Meteorology II

Urban Meteorology II. October 15, 2007. Urban areas represent vast tracts of impervious surfaces: roads, sidewalks, driveways, parking lots, roofs, and walls These alter cycles of energy, water, and chemicals between the land and atmosphere. Urban Heat Island.

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Urban Meteorology II

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  1. Urban Meteorology II October 15, 2007

  2. Urban areas represent vast tracts of impervious surfaces: roads, sidewalks, driveways, parking lots, roofs, and walls • These alter cycles of energy, water, and chemicals between the land and atmosphere

  3. Urban Heat Island • Temperature differences between urban and rural areas are attributed to • Urban geometry • Size, shape, and orientation of buildings and streets • The nature of urban surfaces • Albedo, heat capacity, thermal conductivity, and wetness • Suburbs release more sensible heat, while rural areas release more latent heat making them cooler

  4. Urban Heat Island • The urban-rural difference in surface temperature is generally greatest during the day whereas the air temperature difference is greatest at night • Within urban canyons, canyon walls may absorb reflected radiation • Canyon width • Small width means more longwave radiation • So milder temperatures • Larger widths means more shortwave radiation • So larger diurnal temperature differences

  5. Forested vs Urban • Forested areas hold water and have more evaporative cooling • Urban areas have higher runoff rates and lower evaporative cooling and more sensible heating • Deforestation leads to warming and dehumidifying of the atmosphere

  6. Urban Surface Temperatures * Suburban Colorado during summer

  7. Urban Heat Island • North American instantaneous temperature difference • ΔT = 5.21 log10 (population) – 11.24 • Atlanta: 18.32ºC difference • Carrollton: 11.37ºC

  8. Urban Heat Island • In addition to radiative heating, heating from: • Automobiles • Industrial processes • Power plants • Heating systems • Solutions

  9. Increasing Evaporative Cooling • Urban parks • The larger the urban park, the spatially larger the cooling effect • Water sources • Rice paddies outside Tokyo • The urban-rural temperature difference for a city increases with greater impervious surface area and less vegetated area

  10. Urban Soil • Urban soils more polluted, compacted, and eroded than rural soils • Less able to absorb water • Kudzu decreases latent heating provided by trees • Impermeable surfaces

  11. Planning • To minimize climate stress in hot, humid regions • Reduce solar radiation gain • Ventilation • Widely-spaced buildings • Wind-permeable roofs • Minimize flooding hazards • Structures on stilts • Well-built overhanging roofs

  12. Planning • In hot, arid climates • Minimize solar gain in the summer and maximize it in the winter • High-mass construction materials that absorb heat during the day and release it at night • Minimize wind exposure during both seasons • Use of wind towers for cooling • Build within smallest possible space • Narrow streets to provide shade

  13. Planning • Cold regions • Maximize solar gain • Minimal external surface area • Use of high-mass materials • Dark outer surfaces • South-facing windows • Minimize wind exposure

  14. Planning • Midlatitudes (2-season challenge) • Coniferous trees (dense) on the N, NW side to protect against winter winds • Deciduous trees (widely spaced) on the South side to protect against the summer sun, but allow winter sun • Consider shade on East side depending on latitude

  15. Cloud Seeding • Cloud seeding is an attempt to modify the amount or type of precipitation produced by clouds by increasing the amounts of condensation nuclei in the sky • Silver iodide and dry ice most popular • Is effective in altering cloud structure and shape – not certain in ppt amounts altered

  16. Cloud Seeding • China uses silver iodide rockets to produce ppt over cities • China plans to use cloud seeding before the Beijing Olympics to reduce atmospheric pollution

  17. Cloud seeding • In the US • Used in drought-stricken areas • To reduce hail size • Increase snow fall • Reduce fog around airports

  18. GEC • Increased global temperatures will raise the base temperature on which the heat island effect is superimposed • Areas of increasing anticyclonic activity would experience heat island effect • More people die in heatwaves in US tan hurricanes • UV radiation reduced over cities with ozone and aerosols

  19. GEC • In future… • Increased air pollution will decrease air quality, while increasing haze and respiratory diseases • Unorganized city growth will lead to landslides, flooding, dust storms • At street level, temperatures will rise and ventilation will increase

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