1. Introduction

• 1.1 Definition
• 1.2 Key Equations

2. Related Material

• 2.1 Surface Energy Balance
• 2.2 Night time Energy Balance
• 2.3 Radiative Energy
• 2.4 Simplified Radiation Budget
• 2.5 Albedo
• 2.6 Factors that Modify Albedo
• 2.7 Emissivity
• 2.8 The Role of Clouds
• 2.9 Clear Sky Atmospheric Emissivity
• 2.10 Temperature Inversion

3. Instrumentation

• 3.1 NR-LITE and CNR1

4. Local Energy Balance Charts

• 4.1 Energy Balance Chart Example
  

Radiation usually refers to "electromagnetic radiation," which is a form of energy that moves at the speed of light and is made of photons, which are tiny particles of energy that have wave-like properties such as wavelength, period, and frequency.

Shortwave radiation is radiation with short wavelengths (typically around 0.4 to 2.5μm), usually referring to the part of the electromagnetic spectrum dominated by incoming solar radiation (Roberts 2007), which is also known as insolation (Ahrens 2007). Radiation with longer wavelengths (generally around 2.5 to 14μm) is known as longwave radiation, typically corresponding to the part of the spectrum dominated by radiation emitted by the Earth's surface and atmosphere (Roberts 2007).

• Exitance (Surface emission)
  • M = ε σT⁴ (Wm-2)
    • T = (M/(ε σ))^1/4
• σ = Stefan-Boltzmann's Constant
  • 5.67x10-8 Wm^-2K^-4
• ε = emissivity (efficiency of emission relative to a blackbody)
  • Kirchhoff’s Law
    • absorption = e at a specific wavelength
    • (1-albedo) = e
• Surface Energy Balance
  • Rnet = Incoming – Outgoing
• Measurements of Exitance
  • M = (1-ε)*σ Tenv⁴+ε*σ Ts⁴

Surface Energy Balance:

• Describes the balance between radiation, conduction and convective heat flow
• By convention flow towards the surface is positive, flow away is negative

Night Time Energy Balance:

• Net Radiation may be positive or negative depending on surface and air temperatures
• Soil heat flow is typically positive
• Sensible and latent heat flow may be positive or negative depending on air temperature, surface temperature and condensation

Radiative Energy:

• Wein's Displacement Law
• Blackbody Emission, Planck's Equation

• Describes the balance between outgoing and incoming radiant flux density
  • R=Esolar-Msolar+Elongwave-Mlongwave

Albedo:

• Defined as the ratio of reflected radiation (exitance) to incident radiation (irradiance) at a specific angle of incidence
  • Includes a weighted spectral average
    • Solar albedo is weighted by the solar spectrum
  • Integrated across all view geometries
• Includes two key components
  • Directional hemispherical reflectance (Direct Beam)
    • Direct (or specular) Irradiance (E_s,λ )
  • Bi-hemispherical reflectance (Diffuse Beam)
    • Diffuse irradiance (E_s,λ ): Skylight

   

• water surfaces: .06-.21, sea ice: .3-.4, fresh snow: .75-.95, dry sand: .35-.45, wet sand: .20-.30, chaparral: .15-.20

• Forest Albedo: Old Growth Forest has lower albedo than Second Growth Forest
• Albedo Old = 0.079, Albedo SG = 0.12 (up to 0.20)
• Old Growth Forest absorbs more radiation, absorbs more carbon and produces more water

• Importance:

• Critical for energy balance calculations
• Higher albedo surfaces absorb less radiation
• Lower absorption leads to surface cooling, reduced evaporation
• Ice-Ocean albedo feedback: warming melts ice, lowering albedo. Lowered albedo increases absorption, promotes melting.

Factors that Modify Albedo:

• Incidence Angle
• Amount of diffuse radiation (hazy, clear, cloudy days)
• Surface roughness (in the case of water)
  

For Water:
Albedo rises as ω increases. Albedo is nearly flat on overcast days

For Vegetation:
Albedo increases at higher solar incidence. Conifers are darker than grass.

Albedo at COP:
Albedo increases with solar zenith; surface change is evident for solar zenith > 70 ° across the different dates.

• Emissivity: Measure of the efficiency at which an object emits radiation relative to a perfect emitter (a blackbody)
• Emissivity can be all-wave, or spectral
• ε(T) = M(T)/M(T)_bb or ε_λ = L_λ(T)/L_λbb(T)
  • Spectral emissivity varies with view angle but not temperature
    • Common in remote sensing
  • All wave emissivity varies with temperature, but not view angle
    • Common in energy budget calculations
• L_λ(T)=ε_λ*L_λbb(T)

Typical Emissivities

Sample materials from Sellers, W. D., 1965: Physical Climatology. University of Chicago Press, 272 pp..

• Water and Soil
• water= .92-.96
• snow=.95-.998
• sand,wet=.95
• ground,moist,bare=.95-.98
• ground, dry plowed=.90
• Natural Surfaces
• desert=.69-.91
• grassland=.90
• fields & shrubs=.90
• oak woodland=.90
• Miscellaneous
• white paper=.89-.95
• red bricks=.92
• aluminum foil=.01-.05
• white paint=.91-.95
• black pain=.88-.95
• polished silver=.02
• human skin=.95

• Emissivity reduces emission, causes an object to cool slower and look colder
• M = εσT⁴ or Lλ = ε_λL_λ(T)
• Emissivity modifies absorption of long wave
• Absorbed longwave = ε_s*E_lwd
• Emissivity is required to determine kinetic temperature from radiance or exitance

THE ROLE OF CLOUDS:

• Clouds act to block shortwave radiation but reemit long wave
• The SW and LW impact of a cloud will depend on its thickness, effective droplet radius and height

An example of clould influence at Coal Oil Point

• Clouds generally have high emissivities
• Emissivity varies, depending on cloud thickness and particle radius
  &epsilon = 1-e^-Βτ_abs
  Βτ_abs is a function of: LWP = Liquid Water Path, ρ = liquid water density, and re = effective droplet radius
  Cloud thickness can be expressed as “Liquid Water Path”, in general cloud emissivity decreases for larger droplets and increases for LWP.

• Chen et al., 2000, J. Climate, 13, 264-286
• Garrett et al., 2002, Aerosol Effects on Cloud Emissivity and Surface Longwave Heating in the Arctic, AMS Journal, 59, 769-778

• What controls downwelling longwave?
  • Atmospheric Temperature
  • Atmospheric Emissivity
  • Cloud Cover (Clouds act like blackbodies)

• Humidity and temperature
  Brutsaert's eqn:    ε=1.24*(e_a/T_a)^1/7, where
  • ε_a=atmospheric emissivity
  • T_a=air temperature (Kelvin)
  • e_a=vapor pressure (mb)
• Atmospheric emissivity increases with increased moisture
• Key Assumption: Clear skies and no inversion

Temperature Inversion:

• Process of retrieving the temperature of an object based on its radiance or exitance
• Expressions of Temperature
  • Tk: Kinetic temperature. Temperature of an object due to molecular motion. The same temperature you would get by sticking a thermometer into something
  • Tr: Radiant temperature. Temperature retrieved based on total exitance
• Radiant Temperature
  • Tr = (M/σ)^1/4
• Kinetic Temperature
  • Tk = [M/(εσ)]^1/4

NR-Lite Photo by Dar Roberts 9/21/07
CNR1 Photo by Eliza Bradley; 3/3/08

Kipp & Zonen CNR1(COPR): Four Channel
Output: SWdown, SWup, LWdown, LWup
Shortwave: Pair of Pyranometers, 305 to 2800 nm, Thermopile
Longwave: Pair of Pyrgeometers, 5 to 50 mm, Thermopile
    SW cutoff filter

To learn more about net radiometers, see Brotzge and Duchon (2000)

Time: UTC
Units: W m-2