Introduction to thermal infrared remote sensing - Surface Energy Balance System Basics Z. (Bob) Su International Institute for Geo-Information Science and Earth Observation (ITC), Enschede, The Netherlands [email protected], www.itc.nl/wrs Monday 29 June 2009, Lecture D1Lb3 • Learning Objectives 1. To understand basic concepts of surface radiation budget 2. To understand basic measurements to derive surface radiation components 3. To be able to derive surface radiation components using different information 4. To familiarize with retrieval of surface parameters Biochemical Processes S o l a r R a d i a t i o n - Water,Energy andCarbonCycles Terrestrial Land-Atmosphere Interactions Thermal radiation Sensible Heat (diffusion/convection) Soil Heat (Conduction) Latent Heat (Phase change) Water & vapour Precipitation Gases Wind Advection THERMAL INFRARED RADIATION is a form of electromagnetic radiation with a wavelength between 3 to 14 micrometers (µm). Its flux is much lower than visible flux. VIS SOLAR AND TERRESTRIAL SPECTRUM NIR TIR UV MIR SOLARWavelength (μm) TERRESTRIAL Source: J.-L. Casanova SOME DEFINITIONS RADIANT POWER: The rate of flow of electromagnetic energy, i.e., radiant energy, Φ (watts, i.e., joules per second.) RADIANT EMITTANCE: Radiant power emitted into a full sphere, i.e., 4π (steradians), by a unit area of a source, expressed in watts per square meter. Synonym radiant exitance, flux, radiant flux, E (W m-2) RADIANCE: Radiant power, in a given direction, per unit solid angle per unit of projected area of the source, as viewed from the given direction. Note: Radiance is usually expressed in watts per steradian per square meter, L (W m-2.sr-1) = M/π IRRADIANCE: Radiant power incident per unit area upon a surface. Note: Irradiance is usually expressed in watts per square meter, but may also be expressed in joules per square meter. Synonym: power density, flux, radiant flux, M (watts/m2). Source: J.-L. Casanova GRAPHICAL REPRESENTATION OF E, L AND M RADIANCE, L = E/π (Wm-2.sr-1) IRRADIANCE, M (Wm-2) EMITTANCE or EXITANCE, E (Wm-2) Source: J.-L. Casanova SPECTRAL MAGNITUDES ALL THESE MAGNITUDES ARE USUALLY EXPRESSED AS SPECTRAL MAGNITUDES, THAT IS TO SAY, REFERRED TO A WAVELENGHT -1 -1 RADIANT POWER: Pλ (W.μm ) or (W.cm ) -2 -1 -1 -2 RADIANT EMITTANCE: Eλ (W.m .μm ) or (W.cm m ) -2 -1 -1 -1 -2 -1 RADIANCE: Lλ (W.m .sr . μm ) or (W.cm m .sr ) = Eλ/π -2. -1 -1 -2 IRRADIANCE: Mλ (W.m μm ) or (W.cm m ). cm-1 = number of wavelenghts per cm = 10,000 . (1/ λμm) e.g. 1 μm ~ 10,000 cm-1; 10 μm ~ 1,000 cm-1; 11 μm ~ 909.09 cm-1… Source: J.-L. Casanova Thermal emission- the Planck’s Law: The theory to express the thermal emission is based on the black body concept: a black body is an object that absorbs all electromagnetic radiation at each wavelength that falls onto it. A black body however emits radiation as well, its magnitude depends on its temperature, and is expressed by the Planck’s Law: −5 c1λ B(λ,TS ) = ⎛ c2 ⎞ exp⎜ ⎟ −1 ⎝ λTS ⎠ 8 4 -2 -1 c1 = 1.19104·10 W µm m sr 4 c2 = 1.43877·10 µm K, are the first radiation constant (for spectral radiance) and second radiation constant, respectively, B (λ, Ts) is given in W m-2 sr-1 μm-1 if λthe wavelength is given in µm. Terminology in Thermal Remote Sensing • Thermodynamic (kinetic) temperature • Brightness temperature • Directional radiometric surface temperature & Directional emissivity • Hemispheric broadband radiometric surface temperature & Hemispherical broadband emissivity Source: J. Norman, and F. Becker, 1995 Thermodynamic (kinetic) temperature • Thermodynamic (kinetic) temperature (T) is a macroscopic measure to quantify the thermal dynamic state of a system in thermodynamic equilibrium with its environment (no heat transfer) and can be measured with a infinitesimal thermometer in good contact with it. • By maximizing the total entropy (S) with respect to the energy (E) of the system, T is defined as dS 1 = dE T which is consistent with the definition of the absolute temperature by the ideal gas law PV = RT where P is pressure, V volume, R the gas constant (R=kN0, k is Boltzmann constant, N0 number of molecules in a mole). Brightness temperature • Brightness temperature (Tb,i) is a directional temperature obtained by equating the measured radiance (Rb,i) with the integral over wavelength of the Planck’s black body function multiplied by the sensor response fi. λ 2 f i (λ )C 1 R b ,i ()θ ,φ = R ()Tb ,i ()θ ,φ = dλ ∫λ 1 ⎛ ⎛ C ⎞ ⎞ λ 5 ⎜ exp ⎜ 2 ⎟ − 1⎟ ⎜ ⎜ λ T θ ,φ ⎟ ⎟ ⎝ ⎝ b ,i ()⎠ ⎠ λ 2 f i ()λ d λ = 1 is the detector relative response. ∫λ 1 Directional radiometric surface temperature & Directional emissivity By applying the Kirchhoff law (energy α = A / I = 1 conservation) to a black body and a real (grey) λ λ λ body, the emissivity is defined - The emittance of a ρ = R / I = 0 real body is always lower than that of the black λ λ λ body at the same temperature. In order to measure α + ρ = 1 BLACK BODY its temperature using a radiometer, we need to λ λ introduce a factor called “emissivity”, < 1, to ε A = I = M (black body definition) equate its emittence to that of the block body. λ λ λ α′λ = A′λ / Iλ < 1 ρ′λ = R′λ / Iλ > 0 Iλ: incident flux α +ρ =1 ′λ ′λ αλ: absorptance Aλ: absorbed flux As the body is in thermal equilibrium: A′ = M′ = α′ I λ λ λ λ ρλ: reflectance Rλ: reflected flux M′λ / α′λ = Iλ = Mλ GREY BODY ε: emissivity M′λ = ελ. Mλ = ελ. M′λ / α′λ ελ = α′λ ελ = 1 - ρ′λ Directional radiometric surface temperature & Directional emissivity • The spectral radiance (RR) measured by a directional radiometer is the sum of the emitted black body radiance modified by the emissivity and the reflected radiation within an infinitesimal wavelength band. RR,λ (θ ,φ ) = ε λ (θ ,φ )RB,λ (θ ,φ ) 2π π / 2 + ρb,λ (α, β ,θ ,φ )cos ()α Lλ (α, β )sin ()α dαdβ ∫0 ∫0 Bidirectional reflectance distribution function Sky radiance If the incident sky radiance is isotropic, we have 2π π / 2 ρ b ,λ ()()()()()α , β ,θ ,φ cos α Lλ α , β sin α dαdβ = ρ λ θ ,φ Lλ ∫0 ∫0 Therefore for opaque bodies at thermal equilibrium, ελ (θ,φ)()=1−ρλ θ,φ Hemispheric broadband radiometric surface temperature & Hemispherical broadband emissivity • A hemispheric radiometric surface temperature is used to provide an estimate of the emitted radiance (Rl) over a broad wavelength and the full hemisphere of view 2π π / 2 λ 2 4 Rl = ελ ()()()()θ,φ RB,λ θ,φ sin θ cos θ dλdθdφ = ελσTR ∫0 ∫∫0 λ 1 In-Situ measurements of Surface Energy Balance- EAGLE/SPARC Campaign 2004, Barrax, Spain - Situated in the area of La Mancha, in the west of the province of Albacete, 28 km from Albacete - Geographic coordinates: 39º 3’ N; 2º 6’ W - Altitude (above sea level): 700 m Corn Bare Soil Water Body Garlic Green Grass Vineyard Reforestation The Canopy from Different Perspectives The Fundamental of Earth Observation Sensor Response (Sensor - Object Radiative Relationship) A. How much radiation is detected? B. When does it arrive? Sensor Object Properties: Its range, its combined temperature Nadir & Emissivity (or reflectivity) ° 52° at different times, at different spatial 23 resolution, at different wavelengths, at different direction, at different atmosphere polarization Object A: A Passive Sensor System A+B: An Active Sensor System Factors Influencing the Emissivity ε •The material (minerals, water etc.) • The surface geometry (roughness of the surface) • The wavelength of the radiation •The view angle (Source: Sobrino et al., 2005) Spectral emissivity extracted from MODIS UCSB Emissivity Library 1.0 0.9 0.8 Water: Seawatr1 Vegetation: 106 0.7 108 121 Soil: e3101 0.6 e4643 Spectral emissivity eply3c7 0.5 800 1200 1600 2000 2400 2800 Wave number (cm-1) Surface Radiation Budget - Measurements Rswd Rswu Rlwd Rlwu Solar Radiation Reflected solarradiation thermal radiation Surface thermal Atmospheric radiation R = R − R + R − R Net radiation n swd swu lwd lwu Surface Radiation Budget - Measurements 1 3 CNR1 net radiometer: Spectral response - Pyranometer: 305 to 2800 nm - Pyrgeometer: 5000 to 50000 nm 5μm − 50 μm 2 4 ()1μm = 1000 nm Radiation components: 1. incoming short-wave radiation 2. surface-reflected short-wave radiation 3. incoming long-wave thermal infrared radiation 4. outgoing long-wave TIR radiation Albedo (whiteness): ratio of scattered to incident electromagnetic radiation EAGLE/SPARC Campaign 2004, Barrax, Spain Barrax 2004 Radiation @ Vineyard 1200 1000 SW_inc 800 600 SW_out 400 LW_inc 200 LW_out 0 196 197 198 199 200 201 202 203 -200 W/M^2 1000 1200 -200 200 400 600 800 0 0 50 140 230 EAGLE/SPARC Campaign2004, Barrax,Spain 320 Radiation 15July Components, Barrax, 2004 410 500 550 640 730 820 910 1000 1050 Time 1140 1230 1320 1410 1500 1550 1640 1730 1820 1910 2000 2050 2140 2230 2320 LW_out LW_in SW_out SW_in EAGLE/SPARC Campaign 2004, Barrax, Spain Exercise: 1. Calculate & plot the net radiation and explain what are the influencing factors 2. Explain the pattern of the radiation components 3. Calculate the statistics of the radiation components (mean, minimum, maximum, etc.) 4. Calculate the albedo from the radiation components 5. Check the energy balance and explain what you see How is the net radiation consumed? R n = G 0 + H + LE + Δ S Δ S ≈ 0 Right hand side: 1.