Inadvertent climate modifications

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Inadvertent climate modifications

• Climatic effects of human activities can be
  divided into urban and non-urban modifications.
• Non-urban Modifications
• The removal of vegetation from an area will alter
  its surface properties, and hence the energy and
  mass balances.
• If the area involved is large, these energy and
  mass changes may give rise to local, mesoscale or
  even larger scale changes to climate and
  hydrology.

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• Some examples are
• Removal of vegetation ? leads to adjustment in
  local water balance (canopy no longer there to
  intercept water), evapotranspiration is reduced,
  snow cover distribution and duration is changed,
  and runoff may be increased.
• In addition, the radiation budget is modified
  because of the new geometry and albedo - thus the
  energy balance partitioning is likely to change.

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• Large scale irrigation - leads to the oasis
  effect, with cooler air temperatures and
  increased humidity.
• Dam construction and large scale flooding - may
  result in lake freezing over, cooling,
  stabilization, fogs and ice fogs.

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Cotton and Pielke (1998)
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Modification by Buildings

• This gives rise to radiative, thermal, moisture
  and aerodynamic modifications of the surrounding
  environment
• Radiative - an increase in K? locally on building
  sides and a decrease in shadows. There is a
  reduction in L ? increase in L? from warm
  buildings.
• Thermal - temperatures usually higher due to
  buildings and nature of materials.

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• Moisture - upset because of spatial variability
  in precipitation receipt, drainage and
  evaporation.
• Aerodynamic - significant airflow changes around
  buildings.

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Oke (1987)
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Oke (1987)
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Airflow around buildings

• Distinct flow zones are found around buildings.
• A undisturbed B displacement
• C cavity D wake

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Oke (1987)
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Oke (1987)
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Oke (1987)
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• There is high pressure on windward wall, a
  stagnation point, and a separation of the flow
  occurs around sharp edges of the building.
• A reverse flow in low pressure zone can develop
  in lee of building.
• The basic pattern is modified by building shape.
• This knowledge can be applied in the designs of
  buildings to minimize discomfort and wind related
  maintenance and dispersion of pollutants.

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• When designing buildings and towers it is
  necessary to consider the wind load, or the
  force of wind on the building.
• This will depend on whether or not the building
  is clad or just a frame allowing air to pass
  through.
• Wind load depends on pressure differences across
  the wall or roof.

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Oke (1987)
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Taipei 101
An 800-ton tuned mass damper helps stabilize the
tower in high winds and earthquakes. This damper
is an enormous ball of welded steel plates
hanging inside the top of the building, and is
visible from the restaurant and bar which
encircles the space around the ball. (Source
emporis.com)
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Shanghai World Financial Center
The trapezoidal notch at the top of the building
reduces wind loading. (Source emporis.com)
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• Roofs are prone to being ripped off in strong
  winds because of the pressure difference across
  the roof (high inside, low outside in the suction
  zone as air flows over the roof ? augmented by
  lift under eaves.
• Opening windward windows increases ?P, opening
  leeward windows decreases it.
• Areas most prone to damage are where flow
  separation occurs.

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• Wind can effect access to and comfort around
  buildings.
• Wind speed, suction and positive pressure
  (ripping doors off - difficult to open doors).
• Extreme wind environments around buildings can be
  hazardous.
• Wind can easily increase four-fold around a
  corner, thus 16-fold in force (force is
  proportional to u2).

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Modifications in Urban Areas

• Urbanization radically changes the nature of
  surface and atmospheric processes (radiation,
  thermal, moisture, aerodynamic).
• A few examples are

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• Pollution ? reduces K?, CCN and clouds.
• Materials ? heat storage increases, and
  waterproof surface leads to increased runoff and
  decreased evaporation.
• Geometry ? radiation trapping, air stagnation,
  rough surface.
• Energy and Moisture ? extra heat and moisture
  from human activities supplement natural
  amounts.

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Oke (1987)
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Bailey et al. (1997)
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Oke (1987)
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Radiation effects

• K? can even be reduced by pollution from 2-30.
• There is greater scattering by larger particles
  (scatter all wavelengths), which means an
  increase in diffuse fraction of radiation.
• This accounts for a pale blue or white sky over
  urban areas.
• a (0.15) is generally less over urban areas than
  typical rural areas -- especially in
  high-latitude cities in winter.
• This partially offsets reduced K?.

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• Urban canyon geometry results in enhanced L? from
  buildings also polluted atmosphere will
  increase L?.
• Cities are therefore warmer especially during
  nighttime.
• Reduced evaporation ? channeling of energy into
  sensible forms (?QS, QH).

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Bailey et al. (1997)
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Oke (1987)
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Oke (1987)
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Urban Heat Island

• The altered energy balance in urban areas leads
  to a warmer city compared with neighbouring rural
  areas on most occasions.
• This heat island is present at the surface and in
  the boundary layer.
• It is most prominent on cloudless days, and a
  muted version occurs at night.
• The spatial distribution of the warming shows a
  marked correspondence with patterns of land use
  and building density.
• These thermal features include relatively sharp
  gradients at the rural-urban border, warmer and
  cooler patches in industrial or commercial areas
  and parks, and a maximum in or near the downtown
  core.

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• In the absence of synoptic weather systems, there
  is a daily cycle, with the peak heat island
  effect occurring about 3 to 4 hours after sunset,
  and a minimum in the early afternoon.
• The heat island is usually approximated by the
  difference between the air temperature at urban
  versus rural sites (?Tu-r).
• Therefore the heat island depends not only on the
  properties of the urban area but also on rural
  conditions.

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Oke (1987)
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Oke (1987)
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• The magnitude of the heat island effect is
  strongly influenced by weather controls and is
  inversely related to wind speed and cloud cover
  (type and amount).
• Wind speed is a surrogate for turbulence and
  advection effects that smear thermal differences,
  and cloudiness stands for the role of longwave
  exchange, which is a control on radiative cooling
  potential.

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• As might be expected, the magnitude of the heat
  island positively correlates with city size.
• Estimates of (?Tu-r) show good correlations with
  the logarithm of the city's population and the
  geometry of its downtown street canyons - both
  the ratio of street height to width (H/W) and sky
  view factor (SVF), for a point in the middle of
  the canyon's floor.

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Oke (1987)
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Bailey et al. (1997)
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Oke (1987)
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• For the largest Canadian cities, maximum heat
  islands on clear, calm nights are as large as
  12oC.
• A city of one million inhabitants will
  experience, on an annual average, temperatures of
  about 1-2oC warmer than in surrounding rural
  areas.
• Given the impacts of urban areas on air
  temperatures, are they directly responsible to
  global warming, especially considering that many
  meteorological stations are located in urban
  areas?

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• Indeed, the measured global temperature record is
  based on historical observations from weather
  stations, most of which have been sited near
  settlements and airports.
• It is possible that the global temperature record
  has been contaminated by overrepresentation of
  urban-scale effects.
• However, urban areas cover only 0.25 of the
  Earth's surface and these data generally have
  been corrected (or even removed) from the global
  temperature trend analyses.
• The consensus of global climatologists is that
  urban effects in the global temperature signal
  are now small.

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Oke (1987)
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Bailey et al. (1997)
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Bailey et al. (1997)
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