Water

1
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).

2

• 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.

3

• 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).

4
Oke (1987)
Bailey et al. (1997)
5

• 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.

6
Oke (1987)
7
Bailey et al. (1997)
8
Oke (1987)
9
Oke (1987)
10
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.

11
http//oceanworld.tamu.edu/
12
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.

13
Evaporation over the oceans
14
Bailey et al. (1997)
15
Oke (1987)
16
Oke (1987)
17

• In lakes the upper layer is called epilimnion and
  the lower one hypolimnion.
• The thermocline is a layer within water where the
  temperature changes rapidly with depth - it
  separates the epilimnion and 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.

18
Boyd (1979)
19

• 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.

20
Convection in lakes
Reference Pearson Publishing