1. Surface Energy Budget, Part I
COMET Planetary Boundary Layer Symposium
Matt Haugland

2. Introduction Why is the Surface Energy Budget so important?
Air is largely (80%) transparent to solar radiation.
Nature of the earth’s surface determines way in which the atmosphere is heated.
Differences over land surfaces result from contrasts in soil moisture, vegetation type & status, presence of snow/ice
Surface energy budget is the key to understanding local climates

3. What is the Surface? Can be defined as almost anything
Often defined as an infinitesimally thin massless plane separating the air and ground
More helpful to define surface as a layer (with mass and volume), which can include air and soil

4. What is the Surface Energy Balance?
Based on Conservation of Energy
Energy is supplied by the sun
Energy going into the surface equals energy leaving the surface
If defined as a layer/volume, energy into the “surface” can also be stored inside the surface (as heat)

5. Surface Energy Balance Equation
Energy IN Energy OUT Heating
Rn = H + LE + G (+ P) (+DQ)
where:
Rn = Net Radiation
H = Sensible Heat Flux
LE = Latent Heat Flux
G = Ground Heat Flux
P = Photosynthetic Heat*
DQ = Net Heating/Storage**
* P term is very small, usually ignored in meteorological applications
* DQ term only included if surface is defined as a volume – a massless plane can’t be heated

6. Surface Energy BalanceRn
Physical processes… H LE
Advection &
Convection
Evaporation &
Condensation
Mixing
Radiation
Conduction /
Heat storage G

7. Rn = (SW↓ + SW↑ ) + (LW↓ + LW↑ )
Net Radiation
Rn = (SW↓ + SW↑ ) + (LW↓ + LW↑ )
where fluxes toward (away from) the surface are positive (negative)
SW↓ is the sum of the direct (from the sun) and diffuse (from the sky including clouds) shortwave radiation.
SW↑ is the shortwave radiation reflected by the surface.
LW↑ is the longwave radiation emitted by the surface - proportional to T4
LW↓ is the longwave radiation received by the surface from the atmosphere – proportional to T4

8. Net Radiation What makes net radiation high/positive?
What makes net radiation low (near zero)?
What makes net radiation negative?

9. Net Radiation What makes net radiation high/positive?
Clear/Sunny conditions during the day (shortwave)
Low surface albedo (shortwave)
Atmosphere warmer than ground (longwave)
What makes net radiation low?
Clouds, low sun angle (shortwave)
Atmosphere similar temperature as ground (longwave)
What makes net radiation negative?
Atmosphere cooler than ground
(usually negative at night, most negative around sunset)

10. Fluxes: Positive or Negative?
Positive if energy is transferred FROM the surface
(surface acts as a source of energy)
Negative if energy is transferred TO the surface
(surface acts as a sink of energy)

11. Sensible Heat Flux
Daytime: heating of the air (cooling of the surface) by turbulence/mixing/convection
Nighttime: cooling of the air (heating of the surface) by turbulence/mixing
Heating/cooling of the surface (indirectly) by advection
Laminar or calm-air conduction usually ignored
Determines how much heat is transferred above the surface -- important factor when predicting above-surface temperature and boundary layer depth

12. Sensible Heat Flux What makes sensible heat flux high?
What makes sensible heat flux low?
What makes sensible heat flux negative?

13. Sensible Heat Flux What makes sensible heat flux high?
Wind/turbulence (strong vertical mixing)
Less latent heat and ground heat flux
Warm surface, cool air
What makes sensible heat flux low?
Calm/weak wind
High latent and ground heat fluxes
What makes sensible heat flux negative?
Air warmer than surface

14. Sensible Heat Flux A few more notes…
Sensible heat flux affects boundary layer mixing, which can affect...
Dewpoint
Wind speed
Wind direction
Air pressure
Daytime evolution of near-surface parameters
Areas with high sensible heat flux tend to take on characteristics of higher boundary layer top, especially toward the afternoon (stronger wind, more veering of wind, lower dewpoint, etc.)

15. Latent Heat Flux Results from: evaporation (↑flux; moist surface)
transpiraton (↑flux; leaves)
evaporation + transpiration = evapotranspiration
condensation (↓flux; dew deposition)
Actual energy is consumed/released by phase changes

16. Latent Heat Flux What makes latent heat flux high?
What makes latent heat flux low?
What makes latent heat flux negative?

17. Latent Heat Flux What makes latent heat flux high?
Water on the ground (rainfall, lakes, oceans)
Plants
What makes latent heat flux low?
Dry ground
Lack of plants/vegetation
What makes latent heat flux negative?
Condensation (dew)

18. Latent Heat Flux A few more notes…
More green vegetation doesn’t always mean higher LE. Transpiration is limited by water availability.
Higher LE doesn’t always mean higher dewpoint. A small moist/vegetated area in a dry (mesoscale) region could have a higher LE than a moist region, but a lower dewpoint.

19. Ground Heat Flux
Thermal energy exchange through conduction (rock, soil, concrete)
Thermal energy exchange through diffusion and mechanical mixing (water)
Conduction into plants can be considered part of ground heat flux
Depends on surface cover and soil characteristics (including soil moisture)

20. Ground Heat Flux Ground Heat Flux: can consume 5-15% of energy from Rn
can be used to evaluate H and LE
easier/cheaper to measure than Rn, S, LE
sometimes ignored if “surface” includes deep enough layer of soil
annual average near zero

21. Ground Heat Flux What makes ground heat flux high?
What makes ground heat flux low?
What makes ground heat flux negative?

22. Ground Heat Flux What makes ground heat flux high?
Surface warmer than deeper soil (conduction related to temperature gradient)
High thermal conductivity & heat capacity of soil
What makes ground heat flux low?
Surface temperature close to soil temperature
High latent/sensible heat flux
What makes ground heat flux negative?
Surface cooler than deeper soil (esp. at night and during fall)

23. Conclusions
Weather and climate “start” at the ground (surface) and are driven by solar radiation.
Nature of the earth’s surface determines way in which the atmosphere is heated/cooled.
Differences over land surfaces result from contrasts in soil moisture, vegetation type & status, etc.
Local boundary layer conditions strongly depend on local surface characteristics

24. Questions
Any Questions??