1. ENSC 312 – Week 9: 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:

2. Q* = K* + L* = K↓ + K↑ + L↓ + L↑
Such that:
Q* = K↓ (1 - α) + L ↓ + εσT4 + (1 – ε)L↑
Q* = QH + QE + QG

3. Surface characteristics are very important in determining the climates of non-vegetated surfaces including surface albedo (α), 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)

4. Oke (1987)

5. Peat Soil
α = 0.05, ε = , 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.

6. Peat Soil

7. Sandy Desert α = 0.20 - 0.45, ε = 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.

8. Oke (1987)

9. 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.

10. Oke (1987)

11. Photos courtesy of NASA

14. Snow and Ice
Snow and ice - α = , ε = , 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)

15. 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.

16. Oke (1987)

17. 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 + ΔQS + QM
Snow and ice are porous such that movement of rain and meltwater and phase changes complicate the energy budget.

18. Oke (1987)

19. Oke (1987)

20. Oke (1987)

21. Oke (1987)

22. Oke (1987)

23. Water
α = , ε = , 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).

24. 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 + ΔQS + QA
at a depth where there is no vertical heat transfer, ΔQS = 0 on an annual basis.

25. 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).

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

27. 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 (β) remains low.

28. Oke (1987)

29. Bailey et al. (1997)

30. Oke (1987)

31. Oke (1987)

32. Oke (1987)

33. Oke (1987)

34. Climate over water
Has very little diurnal change in temperature (typically < 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.

36. 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.

37. Evaporation over the oceans

38. Bailey et al. (1997)

39. Oke (1987)

40. Oke (1987)

41. 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.

42. Boyd (1979)

43. 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.

44. Reference: Pearson Publishing

45. Petticrew et al. (2015)

46. Petticrew et al. (2015)

47. Quesnel Lake Temperatures
Petticrew et al. (2015)

48. Petticrew et al. (2015)