Climates of simple, nonvegetated surfaces

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Climates of simple, non-vegetated surfaces

• The next few lectures involve applications of
  some earlier concepts to investigate the climates
  of various surfaces.
• We will first start with a review of the surface
  radiation and energy balances.
• Review The surface radiation and energy budgets
  are partitioned as

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• Radiation
• Q K L K? - K? L? - L?
• Such that
• Q K? (1 - a) L ? - esT4 - (1 e)L?
• incoming reflected
  incoming emitted outgoing reflected
  outgoing
• Energy
• Q QH QE QG

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Oke (1987)
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• Surface characteristics are very important in
  determining the climates of non-vegetated
  surfaces including surface albedo (a), texture
  (porosity, transmissivity, diffusivity, etc.),
  and soil moisture.
• How do the surface radiation and energy budgets
  vary over different non-vegetated surfaces?
• Albedo and emissivity data are taken from Table
  1.1 (p. 12)

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Oke (1987)
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Peat Soil

• a 0.05, e 0.90 - 0.98, hot and dry, porous,
  low diffusivity.
• Heat tends to be absorbed in a thin layer near
  the surface.
• This leads to large temperature variations.
• Peat soil tends to be dry such that the latent
  heat flux is small, and sensible heat flux is
  large.

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Peat Soil
http//www.dkimages.com/discover/DKIMAGES/Discover
/Home/Science/Earth-Sciences/Geology/Soil/Types/Ty
pes-4.html
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Sandy Desert

• a 0.20 - 0.45, e 0.84 - 0.91, very hot and
  dry.
• This is similar to peat soil except there is
  usually greater incoming solar radiation
  (cloudless sky), offset partly by a higher
  albedo.
• Sands are very dry and hot surfaces such that
  latent heat flux is minimal, large sensible heat
  flux and outgoing longwave radiation.
• The ground heat flux partly offsets L? at night.

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Oke (1987)
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• Large temperature gradients near the surface lead
  to instability and phenomena such as dust
  devils, shimmering, and mirages.
• Large diurnal range in temperatures, from 40-56oC
  at 1.5 m above the surface to 80oC at the
  surface.
• Instability also enhances downward momentum
  transport (windy/gusty) whereas winds are light
  at night.
• Strong daytime winds can transport sand and dust.

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Oke (1987)
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Photos courtesy of NASA
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http//www.theaugeanstables.com/
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Snow and Ice

• Snow and ice - a 0.40 - 0.95, e 0.82 - 0.99,
  allows transmission of solar radiation, cold and
  wet, possibility of melting/freezing, porous
• Much more complicated than bare soil.
• Snow and ice allow partial transmission of solar
  radiation according to Beer's Law
• K?(z) K? (0) exp(-az)

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• where a is an extinction coefficient.
• Although solar radiation decreases exponentially
  with depth, its penetration can reach 1 m in snow
  and 10 m in ice.
• In addition, snow and ice have high albedo values
  and thus low energy status.
• Albedo varies with age of the snow.

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Oke (1987)
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• L? and QE are often small owing to cold surface
  temperatures (limited by 0oC).
• Melting uses much of the energy when the snowpack
  reaches 0oC, i.e. surface energy balance is given
  by
• Q QH QE QG ?QS QM
• Snow and ice are porous such that movement of
  rain and meltwater and phase changes complicate
  the energy budget.

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Oke (1987)
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Oke (1987)
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Oke (1987)
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Oke (1987)
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Oke (1987)
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Water

• a 0.03 - 1.0, e 0.92 - 0.97, allows
  transmission of solar radiation, and it is a warm
  and wet fluid.
• Its thermal and dynamic properties make it an
  important heat store and medium of energy
  transport.
• As for snow and ice, water is translucent and
  allows deep penetration of solar radiation
  (usually down to 10 m, but can reach as far as
  1000 m).

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• Water is a fluid such that there is heat transfer
  by convection, advection, and conduction.
• Surface energy balance thus given by
• Q QH QE QG ?QS QA
• at a depth where there is no vertical heat
  transfer, ?QS 0 on an annual basis.

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• Solar radiation transmitted according to Beer's
  Law with the extinction coefficient dependent on
  a number of properties including the nature of
  the water (chemistry, biology, and turbidity)
• Extinction coefficient increases with wavelength
  so infrared radiation is absorbed more readily.
• Its albedo is not constant, but varies with the
  solar zenith angle (highly reflective at low
  angles) and also depends on roughness (i.e.
  wind-driven waves).

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• Water has relatively high emissivity such that
  all L? is absorbed.
• Nearly constant surface temperatures mean little
  variation in L?.
• High heat capacity of water leads to relatively
  slower warming of the surface.
• Wet surface implies that most of the energy
  consumed as evaporation rather than sensible heat
  such that the Bowen ratio (ß) remains low.

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Climate over water

• Has very little diurnal change in temperature
  (typically lt 0.5oC).
• Has an annual range of about 8oC at mid-latitudes
  and 2oC at Equator.
• Even though water absorbs well, it has little
  response.
• Penetration of solar radiation through a large
  volume of water.
• Convection leads to mixing of water and vertical
  heat transport.

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Climate over water

• Evaporation is always at the potential rate
  because of unlimited source of water
• High thermal capacity of water
• Since water bodies are conservative compared to
  land, shorelines are zones of strong atmospheric
  discontinuities.
• Only the upper 30 or so metres of the ocean
  remain active in heat exchanges.

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• In lakes the upper layer is called epilimnion and
  the lower one hypolimnion.
• In fresh water, the maximum density is at 4oC.
• During summer, surface waters warm above this so
  have warm, less dense water on top, this is a
  stable regime.
• In fall, surface temperatures cool and density
  increases ? instability and mixing so the
  epilimnion cools rapidly.

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• In spring, if temperatures are below 4oC, surface
  warming increases the density ? unstable and
  enhanced mixing.
• Due to the smooth water surface, forced
  convection is weaker ? steep wind gradient near
  the surface because momentum exchange is confined
  to a shallow layer.