Climates of nonuniform terrain

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Climates of non-uniform terrain

• In our previous lectures we have assumed that
  surfaces have been extensive, flat and
  homogeneous.
• In this case the response of the surface to
  energy and mass exchanges is equal everywhere ?
  all gradients are perpendicular to the surface,
  and all fluxes are vertical.
• This is not reality -- the surface is a patchwork
  of different topography and surface types.

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• This results in horizontal variations in the
  available radiation and the way that the
  available energy and water is partitioned giving
  horizontal variations in climate.
• In turn there are horizontal gradients of
  properties ? horizontal fluxes will occur.
• Thus we will talk about the effects of spatial
  inhomogeneity and topography.

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Spatial Heterogeneity

• There are two main cases
• 1) modification of atmospheric properties as air
  moves to a different surface type (advective
  effects)
• and 2) circulation induced by contrasting surface
  properties when regional winds are light
  (thermal circulations).

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Advective Effects

• There is a clothesline effect as flow of air
  through a vegetative canopy is restricted.
• As air flows from warmer, drier ground it will
  increase heat supply (QH) and water vapour
  gradient, such that there is enhanced
  evapotranspiration.
• Plants near the edge of stand will be dessicated,
  buffeted by wind, and more prone to pests and so
  are frequently stunted.

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Leading Edge or Fetch Effect

• As air passes from one surface type to a new one,
  it must adjust to a new set of boundary
  conditions.
• Only in the lower 10 of the internal boundary
  layer is the air fully adjusted to the surface.
• As an illustration of the way adjustment takes
  place, consider the advection of moisture in the
  box model
• A u?V

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• The height of the fully adjusted boundary layer
  is the level at which the vertical flux equals
  the surface value.
• This grows slowly -- a fetch of 100-300 m for
  every 1 m increase in height.
• This has implications for the setting of
  meteorological instruments.
• There would also be differences in heat flux and
  momentum transport downwind of a discontinuity.

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• It is not possible to separate changes in energy,
  mass and momentum.
• Changes in momentum will alter K's over rough
  surface and therefore enhance QH and QE.

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Oasis Effect

• In hot, arid regions are areas of freely
  available water.
• Thus QE can exceed Q as QH due to advection of
  warm air can supply extra energy ? cool
  temperatures, high evaporation.
• What other oasis effect situations can be
  imagined?
• Lake in a dry area, glacier, isolated snow patch,
  urban park, isolated tree.
• In each case, evaporation will be large.

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Thermal Circulation Systems

• These arise because when surfaces of different
  thermal environments lie next to each other,
  horizontal pressure gradients develop.
• Land/Sea Breezes
• These different thermal properties produce
  corresponding pressure differences resulting in a
  circulation that reverses at night.

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• The daytime sea breeze has greater intensity,
  depth and range (?T is greatest and instability).
• The sea breeze can extend up to 30 km inland, 2-5
  m s-1 in speed and circulation of 2 km in
  height.
• Late in the day, the Coriolis force may have an
  effect so winds cross the coast at an angle.
• Land breeze is generally much less intense.

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• Advancing sea breeze front produces uplift, and
  potentially clouds and thunderstorms (Florida is
  a good example).
• Lowest layer of air advected across the coastline
  is modified by a leading edge effect.
• A city can also generate a country breeze
  because under light synoptic conditions they are
  often warmer than the surrounding countryside.
• However there is no diurnal reversal - city is
  almost always warmer than surrounding region.

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Effects of Topography

• First let us discuss the effect of radiation
  loading.
• The angle at which radiation strikes a surface is
  important in determining the amount of radiation
  it receives that in part determines its climate.
• The effects of aspect and slope are most dramatic
  at higher latitudes where the sun is at a lower
  angle anyway.
• South facing slopes get more radiant energy than
  horizontal and north facing slopes.
• Note this effect is only important for direct
  beam radiation.

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• Net radiation can be 2-4 times higher by day on a
  south facing slope ? results in higher QH (if
  water is limiting).
• This results in distinctly different
  micro-climates and vegetation.

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Topographically-generated winds

• These arise due to thermal differences because of
  slope/aspect.
• Tend to occur under cloudless skies with little
  or no synoptic-scale winds.
• The strength and nature of flows depend on
  orientation and configuration of the valley.
• By day, air near valley sides will be heated more
  than air at same elevation in centre of valley ?
  anabatic cloud formation along ridges.

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• This circulation warms the entire valley so that
  it is warmer than neighbouring plains and an up
  valley (valley wind) develops, whereas an
  anti-valley wind develops above the ridges.
• This system will tend to reverse at night.

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