\(R_n\) is the flux density of ALL radiation. ie. the sum of component flux densities: Net short-wave radiation (\(SW^*\)) and net long-wave radiation (\(LW^*\)):
\[
R_n = SW^* + LW^*
\qquad(2)\]
A net radiometer
Component Fluxes
Recall that \(SW\) is not emitted by Earth’s surface, but \(LW\) is!
Absorbed : available energy for other processes
The Surface Energy Balance
We can define \(R_n\) from the perspective of the Earth’s surface in term’s of it’s energy balance. How much energy is absorbed vs. how much is emitted?
\(R_n\) is the main energy source driving near-surface climates. It can be positive (usually day) or negative (usually night).
Test your knowledge (iClicker)
\(R_n\) is the energy available for other processes such as (select any valid answer):
Photosynthesis
Evaporation
Convection
Connectivity of energy and mass
The radiation budget impacts all other “budgets” in Earth’s climate system.
Water & carbon balances are directly influenced by the flux of energy into our out of the surface.
Diurnal Variation
In the following slides, we’ll look at examples of \(R_n\) and its four component fluxes over the single day periods that show the contrasting effects of weather conditions on \(R_n\).
Clear skies during summer
Rainy conditions during fall
Snow-covered ground in winter
Snowmelt
Hot & Sunny
Figure 1: Radiation data from the Burns Bog Flux Station.
Total for the day:
\(SW^* =\) 0 \(MJ m^{-2}\)
\(LW^* =\) 0 \(MJ m^{-2}\)
\(R_n =\) 0 \(MJ m^{-2}\)
Cloudy & Rainy
Figure 2: Radiation data from the Burns Bog Flux Station.
Total for the day:
\(SW^* =\) 0 \(MJ m^{-2}\)
\(LW^* =\) 0 \(MJ m^{-2}\)
\(R_n =\) 0 \(MJ m^{-2}\)
Cold and Snowy
Figure 3: Radiation data from the Burns Bog Flux Station.
Total for the day:
\(SW^* =\) 0.5 \(MJ m^{-2}\)
\(LW^* =\) -0.6 \(MJ m^{-2}\)
\(R_n =\) -0.2 \(MJ m^{-2}\)
During Snowmelt
Figure 4: Radiation data from the Burns Bog Flux Station.
Total for the day:
\(SW^* =\) 0.8 \(MJ m^{-2}\)
\(LW^* =\) 0.4 \(MJ m^{-2}\)
\(R_n =\) 1.2 \(MJ m^{-2}\)
Units (iClicker)
The measured radiation data are presented in W m-2, so why are daily totals presented in MJ m-2?
W = J s-1
W = MJ d-1
W = MJ h-1
A Full Year?
Figure 5: Total radiative fluxes at the Burns Bog Flux Station, 2022.
Total flues over one year:
\(SW^* =\) 3.16 \(GJ m^{-2}\)
\(LW^* =\) -1.08 \(GJ m^{-2}\)
\(R_n =\) 2.07 \(GJ m^{-2}\)
Does it vary between years?
Figure 6: Total radiative fluxes at the Burns Bog Flux Station by year.
Summary of our interpretation
Strong correlation between \(R_n\) and \(SW\downarrow\)
Daily \(R_n\) variation is mainly driven by the solar cycle
Daytime \(SW^*\) is only partly offset by \(LW^*\)
\(LW^*\) is more negative when ground is warmer than the air
\(LW^*\)can be positive if the air is warmer than the ground
Effect of Landcover
Mature Forest
Clearcut
Landcover Effects
Measurements show that \(R_n\) in a clearcut is \(\approx\) 24% less than in uncut forest.
Why? \(SW \downarrow\) and \(LW \downarrow\) didn’t change drastically, but \(SW \uparrow\) and \(LW \uparrow\) did.
Figure 7: Radiation balance of a forest and a clearcut.
Diurnal Course
\(SW \uparrow\) higher in the clearcut
Larger albedo
\(LW \uparrow\) higher in the clearcut
Higher T
Figure 8: Diurnal course of outgoing radiation for a forest and clearcut
Daily Totals
Figure 9: The clear-cut receives less radiative energy than the forest.
iClicker
If the forest has higher \(R_n\) than the clearcut, where is that extra energy going? (Note there are two correct answers, you only need to select one)
A. Photosynthesis
B. Albedo
C. Longwave emissions
D. Evapotranspiration
E. Heating the soil
Nighttime Cooling
Nighttime surface cooling is only controlled by \(LW\):
