Urbanization, Meteorology, and Local Climate

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Urbanization, Meteorology, and Local Climate

• Justin Cox
• Department of Meteorology
• University of Utah
• 13 October 2004

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This is why we care!
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Urban meteorology

• Luke Howard (1833) documented the urban heat
  island (UHI) in London
• Urban effects on meteorology have been observed
  and documented for a long time
• Research on urban meteorology has increased lately

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Scope and Goals of Seminar

• Focus on the effects of urbanization on local
  meteorology
• Introduce surface characteristics that are
  meteorologically important
• Discuss the cases in which the characterization
  of land cover is important
• Discuss the direction of future research
• Solicit ideas and advice from attentive audience
  members

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Surface Budgets

• Surface radiation, heat, momentum, and moisture
  budgets are affected by land cover change
• Different aspects of urbanization have different
  effects on these budgets
• Effects can vary widely depending on the climate
  regime of a particular locale

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Surface energy b udgets
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Characteristics of urban surfaces

• Altered albedo can be higher or lower
• Higher heat capacity
• Lower moisture flux to atmosphere
• Larger roughness elements
• Increased surface area
• Source of anthropogenic heat and emissions
• Impermeable to water
• Decreased net longwave raditaion loss

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The atmospheric boundary layer

• Lowest layer of the atmosphere
• Interactions with the earths surface are
  important
• Diurnal evolution is complicated
• Turbulence generation by shear and buoyancy is
  important
• Fluxes of energy, momentum, and moisture to/from
  the surface

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Problems in defining the boundary layer

• Complicated vertical structure
• Sub-layers grow and decay over the diurnal cycle
• Turbulence is often intermittent, complicating
  the classification of stability
• Boundary layer top is not necessarily at inversion

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Diurnal evolution of the PBL
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Boundary layer characteristics

• Daytime
• Deep mixed layer from surface heating
• Turbulent eddies on the scale of BL depth
• Thermally driven flows can develop from spatial
  variations in surface heating
• Nighttime
• Surface inversion develops from radiational
  cooling
• Mixed layer can persist above inversion
• Turbulence can be intermittent and mix down
  faster and warmer air

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Examples of land cover impacts on meteorology

• Lake/sea/vegetation breezes
• Urban heat island and related circulations
• Altered distribution and frequency of
  precipitation
• Increased emissions that act as CCNs
• Local convergence zones
• Oasis effects in arid regions

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The Urban Heat Island

• Positive temperature anomaly compared to
  surrounding nonurban land
• Magnitude is greatest at night and with low wind
  speeds
• Caused by thermal inertia of urban materials and
  anthropogenic heating

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Urban heat island
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Urban heat island

• Is often calculated relative to surrounding
  countryside
• Representativeness of observations is an
  important issue
• Local anomalies related to street canyon
  orientation, ventilation, proximity to heat
  sources/sinks, etc. can affect observations
• Rural variability is also important (Hawkins et
  al. 2004)

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Urban circulations

• Represent a type of thermally driven flow
• Similar to lake and sea breezes
• Interact in complex ways with other thermally
  driven flow systems during weak synoptic forcing
  (clear skies, weak winds)

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Urban circulation interacting with sea breeze
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Urban weather modification

• Dixon and Mote (2003), Atlanta
• Urban in-up-out circulation and surface moisture
  important on borderline days

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Urban canopy parameterizations

• Modification to the drag term of the momentum
  equation
• Anthropogenic heat added to temperature equation
• Radiation attenuation and trapping due to
  building geometry
• Urban land cover properties added to surface
  parameterizations

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What now?

• In the presence of local wind systems, how might
  the urban landscape affect sensible weather?
• Surface temperature
• Mixing depth
• Precipitation distribution, intensity

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RAMS simulations

• Case studies of VTMX IOP6
• Idealized run with valley/plain topography
• Control simulations simulate diurnal cycle of
  thermally driven winds
• Experiments simulate various hypothetical
  configurations of urban, rural, and undeveloped
  land cover

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Important points to remember

• Weather events still dictated by synoptic
  patterns and large-scale forcing
• Local effects like UHI and related circulations
  come into play during quiescent periods with low
  wind speeds
• Urban weather modification (thunderstorm
  initiation, frost, stable layer
  formation/destruction) is important in a small
  range of conditions that are near a threshold