Earth's energy balance

Question 1

What are the different types of surface energy fluxes?

Answer 1

• Radiative fluxes
• Non-radiative fluxes
• Links to surface temp
• Turbulent fluxes

Question 2

What type of raditaion is emitted from sun and earth?

Answer 2

• Sun = short wave raditation
• Earth = long-wave raditaion (infrared)

Question 3

What may mess up the earths energy balance?

Answer 3

Greenhouse effect

Question 4

What is surface albedo?

Answer 4

Measure of the reflectivity of surface
- albedo (r) = S-upward/S-downward

Question 5

How does albedo vary?

Answer 5

• Varies massively depending on the surface - e.g., boreal forest has low albedo, snow has high albedo
• Also vaires throughout year - higher albedo in winter (snow)
• Varies according to land-cover - and throughout year - cropland in snow - higher albedo in winter than a forest as less getting in the way of snowfall

Question 6

How can you measure the amount of longwave radiation being re-emitted by the earth?

Answer 6

Stefan-Boltzmann Law:
- L (reflected) = (epsilon)x(omega)xTs^4
- epsilon = Surface emissivity (generally 0.95-1)
- omega = Stefan-Boltzmann constant
- Ts = Surface temperature
- So very controlled by surface temp

Question 7

Using what we know, how can we then calculate the surface net radation?

Answer 7

Surface net raditation (Rn) = (S-down)(1-r) + (epsilonxL-down) - ((epsilon)x(omega)xTs^4)
- So is: SW absorbed by surface + LW absorbed by surface - LW emitted by surface

Question 8

What are some of the non-radiative fluxes?

Answer 8

• Sensible heat flux (H) - conductive heating - released from land surface raises temp of air above
• Latent heat flux (lambdaE) - change of state
• Evaporative water flux (E) / evapotranspiration - energy (lamda) absorbed from surrounding environment in order to turn a liquid into gas
• Latent heat of vapourisation - lamda
• Ground heat flux (G) - conductive heating - energy going into ground

Question 9

How is the overall surface energy balanced?

Answer 9

Overall surface energy balance (Rn) - radiative fluxes cause an inbalance that then has to be resolved by the non-radiative fluxes

Question 10

What is the monthly surface energy flux like in a tropical rainforest in Brazil?

Answer 10

• Lots of rain all year round - soil remains wet
• This means that lots for here - lots of radiation is coming in that needs to be balanced by the non-radiative fluxes
• Therefore, as there is lots of water in the soil - we see a very high latent heat flux (lamdaxE) - means water is being evaporated from surface and energy is being used to do so - so energy is being taken from the atmosphere to do this

Question 11

What is the monthly surface energy flux like in a grassland in California?

Answer 11

• Wet, mild winters, dry, hot summers
• So shows distinct seasonal cycle with the amount of radiation coming in
• So, here - due to dry summers - there is a lack of soil moisture and so there is instead there is a large sensible heat flux (H)
• That means, instead of taking energy from the atmosphere in evaporation of water - it instead just heats the atmosphere - giving the hot summers experienced

Question 12

What is the Bowen ratio?

Answer 12

Ratio of sensible heat (H) / latent heat (lamdaxE)
- E.g., Bowen ratio is low in rainforests (0.1-0.3) and high in deserts (>10)

Question 13

What are each of the non-radiative fluxes proportional to?

Answer 13

All are a function of the surface temperature:
- Sensible heat flux (H) is proportional to temperature difference between surface and air - energy exchange only
- Latent heat flux (lambdaxE) is proportional to vapour pressure deficit between surface and air - energy and mass exchange
- Ground heat flux (G) is proportional to temperature difference between the surface and the soil - energy only

Question 14

What equation was generated to avoid the non-linearities in solving lamdaE or Ts?

Answer 14

Penman-monteith equation:
- Can calculate latent heat flux of a given environment
- As we can actually measure all elements of it

Question 15

What are turbulent fluxes?

Answer 15

• Vegetation creates rough surface which creates turbulence and enhances the transport of sensible and latent hear away from surface
• Turbulent motion = effective mixing of air parcels due to surface friction or surface heating
• Surface friction - created by rough surfaces - e.g., forests
  ○ Frictional drag slows air nearest the ground
• Surface heating - surface typically warmer than atmosphere during daytime
  ○ Air warmed by surface rises - enhancing mixing

Question 16

How can you measure turbulent fluxes?

Answer 16

Eddy Covariance Technique:
- Look how things in atmosphere covary over time
- Measure fluctuations of potential temp, wind, water vapour, specific humidity, CO2, and vertical velocity over time and determine their covariance with w (vertical velocity)