Biometeorology Lecture 2: Surface Energy Balance Professor Noah Molotch September 5, 2012

Energy Budget by Latitude Figure 4.13

Surface Energy Balance Radiative Fluxes Shortwave Radiation Longwave Radiation Turbulent Fluxes Latent heat flux (e.g. evaporation) Sensible heat flux (heating surface)

Earth Energy Balance Figure 4.12 Turbulent fluxes Radiative Fluxes

Surface Energy Balance Two major fluxes are “radiative” flux and “turbulent” flux. Radiative fluxes are associated with shortwave radiation incoming from the sun and reflected by Earth’s surface and longwave radiation emitted by Earth’s surface and radiated toward the surface by the atmosphere. Turbulent fluxes are associated with heating of the Earth’s surface and phase changes of water (e.g. evaporation) – these are driven by wind and hence the word “turbulent”.

Surface Energy Balance We can express this mathematically: Radiative Fluxes = Turbulent Fluxes or R net - G = H + LE Here: R net is the net radiation G is the ground heat flux H is the sensible heat flux LE is the latent heat flux

Earth Energy Balance Figure 4.12 Turbulent fluxes Radiative Fluxes

Radiative flux is summarized using the term “net radiation”. It is equal to the balance of incoming and outgoing shortwave and longwave radiation: R net = S ↓ - S ↑ + L ↓ - L ↑ Where S is shortwave from the sun and L is longwave radiation emitted by earth

Radiative Flux Figure 4.1 R net = S ↓ - S ↑ + L ↓ - L ↑

Insolation at Earth’s Surface Figure 4.2

Albedo Figure 4.5

Figure 4.1 Longwave Radiation

13 Longwave Radiation Longwave emitted by Earth

Longwave Radiation Emission of radiation from a perfect emitter (i.e. black body) at a given wavelenght is given by Planck’s Law. All of the energy emitted across all wavelengths is the longwave emission. 14 Longwave All energy is the area under this curve

Longwave Radiation We estimate the total emission from a black body using Stefan-Boltzmann’s Law: L = σT 4 T = temperature in Kelvin (Kelvin is degrees Celsius (thus 0 *C = K)

Longwave Radiation In reality objects are not perfect black bodies. Meaning they are not perfect emitters. Thus we introduce the term emissivity (ε). L = εσT 4 T = temperature in Kelvin (Kelvin is degrees Celsius (thus 0 *C = K)

Longwave Radiation and Emissivity The emissivity of a material (ε) is the relative ability of its surface to emit heat by radiation. ε is the ratio of energy radiated by an object and the energy radiated by a black body at the same temperature. A true black body would have an emissivity of 1 while any real object would have an emissivity less than 1. Aluminum emissivity = 0.04; Cast iron = 0.65; water = 0.95.

Longwave Radiation Earth’s surface emits longwave radiation as a function of the surface temperature and emissivity. The atmosphere emits longwave radiation back toward the Earth surface (e.g. Greenhouse effect). This also varies with atmospheric emissivity and temperature.

Net Longwave Radiation

Clouds and Albedo Figure 4.7

Earth Energy Balance: Turbulent Fluxes Figure 4.12 Turbulent fluxes

Turbulent Fluxes As implied by word “turbulent”, these fluxes are largely driven by wind. Sensible heat fluxes (associated with convection) driven by difference in temperature between surface and the atmosphere. Latent heat fluxes (e.g. evaporation) driven by difference in vapor pressure between surface and atmosphere. Both sensible and latent heat fluxes are driven by turbulence as air at the surface-atmosphere interface is replenished by wind.

Turbulent Fluxes Recall our energy balance equation: R net - G = H + LE The left side of the equation is often referred to as “available energy”. If surface moisture is limiting then we can not expend energy to evaporate water and LE will be low. As a result we heat the surface and H must be high. Remember the natural grass VS Astroturf example.

Turbulent Fluxes R net - G = H + LE ↓ LE then ↑ H ↑ H = heating Also note available energy increases because: - asphalt has low albedo - atmosphere heats up and re- radiates longwave energy back to surface

Turbulent Fluxes R net - G = H + LE ↑ LE = ↓ H ↓ H = cooling

Turbulent Fluxes R net - G = H + LE ↓ LE then ↑ H ↑ H = heating

Simplified Surface Energy Balance NET R = +SW (insolation) –SW (reflection) +LW (infrared) –LW (infrared) Figure 4.16

Daily Radiation Curves Figure 4.14

Radiation Budgets Figure 4.20 El Mirage, CA Pitt Meadows, BC

Summary Radiative Fluxes Shortwave Radiation (controlled by sun and albedo) Longwave Radiation (controlled by emissivity and temperature of surface and atmosphere) Turbulent Fluxes Latent heat flux (controlled by moisture availability and wind speed) Sensible heat flux (controlled by temperature and wind speed) Partitioning between sensible and latent heat fluxes controls surface temperature – largely driven by land cover (e.g. urbanization).