16 Climate of the city

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16Climate of the city
Air pollution Radiation balance Modification of
the microclimate Ways to mitigate? (Principles
of Climatology GEOG31064 Sheridan)
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Luke Howard (1812)

• London was this day involved, for several hours,
  in palpable darkness. The shops, offices, c
  were necessarily lighted up but the streets not
  being lighted as at night, it required no small
  care in the passenger to find his way and avoid
  accidents. The sky, where light pervaded it,
  showed the aspect of bronze

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Luke Howard (1812)

• Such is, occasionally the effect of the
  accumulation of smoke between two opposite gentle
  currents, or by means of a misty calm. I am
  informed that the fuliginous cloud was visible
  for a distance of forty miles. Were it not for
  the extreme mobility of the atmosphere, this
  volcano of a thousand mouths would, in winter,
  scarcely be habitable.

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Pollution

• Aerosols suspended particulate matter
• Gases
• Primary pollutants
• Secondary pollutants

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Pollution aerosols

• Either put directly into atmosphere or come about
  from a chemical reaction or deposition upon H2O
• PM10 (particulate matter under 10 microns) and
  PM2.5 commonly measured

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Pollution gases

• CO, CO2
• NO, NO2 Nitrates
• SO2 Sulfates
• Hydrocarbons

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Pollution smog

• Smog development
• Wet when aerosols have water vapor condense
  upon them (e.g. London)
• Dry when ultraviolet radiation reacts with NO2
  to produce ozone

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Pollution factors

• Temperature inversion
• Wind speed and direction
• Solar radiation
• Humidity
• Topography

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The radiation balance

• All components of the radiation balance are
  affected by urbanization

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Urban effects on shortwave
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Urban geometry
SHORTWAVE
LONGWAVE
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More urban effects

• What are the effects of atmospheric pollution on
• Longwave energy downward?
• Shortwave energy downward?
• Thermal conductivity

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More urban effects

• Overall solar energy decreases around 5-10
  percent in urban areas.
• Wavelength-dependent Ultraviolet radiation
  decreases up to 90 percent.

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Urban heat fluxes

• Latent heat flux typically much smaller in a city
• Why?
• Sensible heat flux thus increases
• Also increases due to increased turbulence in
  urban environment

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Combustion

• In wintertime in New York City, for each unit of
  solar radiation received, 2.5 times that amount
  is added to the climate system by combustion of
  fossil fuels.
• Nights in Cincinnati, 25 of the net radiation
  loss is added to the climate system via
  combustion.

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End result urban heat island

• Annual temperature 1C warmer
• Most significant heat island
• At night
• Clear
• Calm
• Winter
• During the week
• Longer growing season (3 weeks)

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Heat island variability over season and time of
day
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Heat island varies from place to place
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Heat island varies from city to city

• Heat islands noted in cities of only 1,000 people

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Heat island varies over time
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Heat island varies vertically
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Urban modification of moisture

• Relative humidity 6 lower
• Greater decreases in summer
• Cloud cover increases 10
• Condensation nuclei
• Anthropogenic moisture sources trivial
• 1/500th of summer POTET
• Fog double

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Urban modification of precipitation

• Up to 10 higher in city
• Up to 33 higher within 50 km of city DOWNWIND

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Urban modification of precipitation

• Changes by day of week?
• Snowfall 15 less
• Change in days with precipitation?

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Urban streamflow
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Urban modification of winds

• Local circulation develops
• Wind decreases overall
• Wind tunnels

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Wind speed vertically
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How to mitigate a heat island?

• Highly reflective roofs?
• Why would this help tropical cities more than
  mid-latitude ones?
• Green roofs
• Green parking lots