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Biometeorology Lecture 2: Surface Energy Balance Professor Noah Molotch September 5, 2012 Slide 2 Energy Budget by Latitude Figure 4.13 Slide 3 Surface Energy Balance Radiative Fluxes Shortwave Radiation Longwave Radiation Turbulent Fluxes Latent heat flux (e.g. evaporation) Sensible heat flux (heating surface) Slide 4 Earth Energy Balance Figure 4.12 Turbulent fluxes Radiative Fluxes Slide 5 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 Earths surface and longwave radiation emitted by Earths surface and radiated toward the surface by the atmosphere. Turbulent fluxes are associated with heating of the Earths surface and phase changes of water (e.g. evaporation) these are driven by wind and hence the word turbulent. Slide 6 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 Slide 7 Earth Energy Balance Figure 4.12 Turbulent fluxes Radiative Fluxes Slide 8 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 Slide 9 Radiative Flux Figure 4.1 R net = S - S + L - L Slide 10 Insolation at Earths Surface Figure 4.2 Slide 11 Albedo Figure 4.5 Slide 12 Figure 4.1 Longwave Radiation Slide 13 13 Longwave Radiation Longwave emitted by Earth Slide 14 Longwave Radiation Emission of radiation from a perfect emitter (i.e. black body) at a given wavelenght is given by Plancks Law. All of the energy emitted across all wavelengths is the longwave emission. 14 Longwave All energy is the area under this curve Slide 15 Longwave Radiation We estimate the total emission from a black body using Stefan-Boltzmanns Law: L = T 4 T = temperature in Kelvin (Kelvin is degrees Celsius + 273.15 (thus 0 *C = 273.15 K) Slide 16 Longwave Radiation In reality objects are not perfect black bodies. Meaning they are not perfect emitters. Thus we introduce the term emissivity (). L = T 4 T = temperature in Kelvin (Kelvin is degrees Celsius + 273.15 (thus 0 *C = 273.15 K) Slide 17 Longwave Radiation and Emissivity The emissivity of a material () is the relative ability of its surface to emit heat by radiation. 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. Slide 18 Longwave Radiation Earths 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. Slide 19 Net Longwave Radiation Slide 20 Clouds and Albedo Figure 4.7 Slide 21 Earth Energy Balance: Turbulent Fluxes Figure 4.12 Turbulent fluxes Slide 22 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. Slide 23 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. Slide 24 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 Slide 25 Turbulent Fluxes R net - G = H + LE LE = H H = cooling Slide 26 Turbulent Fluxes R net - G = H + LE LE then H H = heating Slide 27 Simplified Surface Energy Balance NET R = +SW (insolation) SW (reflection) +LW (infrared) LW (infrared) Figure 4.16 Slide 28 Daily Radiation Curves Figure 4.14 Slide 29 Radiation Budgets Figure 4.20 El Mirage, CA Pitt Meadows, BC Slide 30 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).