Atmospheric InstrumentationM. D. Eastin Measurement of Radiation

Atmospheric InstrumentationM. D. Eastin Outline Measurement of Radiation Review of Atmospheric Radiation Review of Solar Geometry Radiometers Thermopile Radiometer Pyranometer Pyrheliometer Albedometer Pyrgeometer Pyrradiometer Pyrometer Exposure Errors

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Radiation:Energy transmitted (or emitted) from a given “body” or “system” The spectrum of wavelengths over which the total emitted energy originates is a function of the body’s temperature → Planck’s Law The integral of this energy (the area under each curve below) defines the total temperature-dependent “black body” energy where: E = radiant energy (W) λ=wavelength (m) ε=emissivity (0 → 1) σ=Stefan-Boltzmann constant (W m -2 K -4 ) T=temperature (K) Review of Atmospheric Radiation

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Radiation:Energy transmitted (or emitted) from a given “body” or “system” The spectrum of wavelengths over which the total emitted energy originates is a function of the body’s temperature → Planck’s Law Since the black-body temperatures for the Sun (T BB = K) and Earth (T BB = K) are significantly different, atmospheric radiation is divided into two distinct broadband spectrums Solar Radiation →Shortwave Radiation Terrestrial Radiation→Longwave Radiation Review of Atmospheric Radiation

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Radiation:The amount of shortwave (longwave) radiation reaching the base (top) of the atmosphere depends on complex interactions (such as scattering, reflection, and absorption) by clouds, aerosols, atmospheric gases, as well as and the emission of longwave radiation Review of Atmospheric Radiation

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: The net radiation (R n ) observed at the surface consists of all combined incoming and outgoing longwave and shortwave radiation Review of Atmospheric Radiation Direct beam Shortwave (S B ) Top of Atmosphere irradiance (S TOA ) Upwards Reflected Shortwave (S U ) Diffuse Shortwave (S D ) Downwelling Longwave (L D ) Upwelling Longwave (L U )

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Radiant Flux:Amount of radiation coming from a source per unit time (W) Radiant Intensity:Radiant flux leaving a point on the source per unit solid angle of space (W sr -1 ) Radiance: Radiant flux emitted / scattered per unit area from a source (W m -2 sr -1 ) Irradiance:Radiant flux incident on a receiving surface from all directions (W m -2 ) Absorptance:Fraction of irradiance that is absorbed by a medium Reflectance:Fraction of irradiance that is reflected by a medium Transmittance:Fraction of irradiance that is transmitted by a medium SI units and W =watts Meteorology: sr=steradian (solid angle unit) m=meters Instrument: Radiometer Solid Angle Review of Atmospheric Radiation

Atmospheric InstrumentationM. D. Eastin Global Mean Energy Flows (W m -2 ) Review of Atmospheric Radiation

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Up to 90% of the total top of the atmosphere shortwave irradiance (~1370 W m -2 ) reaches the Earth’s surface Longwave emissions can reach 800 W m -2 Surface radiometers should exhibit a dynamic range → 0 – 1500 W m -2 Definitions and Concepts: Review of Atmospheric Radiation

Orbital Variations: Earth’s orbit is elliptical (with slight eccentricity) Seasonal variations in shortwave radiation arise due a 23.5º tilt in the axis or rotation relative to the orbital (elliptical) plane Declination angle ( δ ) describes this tilt as a function of the day of the year (d) Local Solar Time: Solar angle (h) defines the fraction of local solar time (t) the Earth has rotated since local solar noon (t 0 ) (when the Sun is directly overhead) Local solar time (measured on a sundial) differs from a standardized clock by up to 1 hour Atmospheric InstrumentationM. D. Eastin Review of Solar Geometry

Local Day Length ( L day ): The length of a given day defines the duration of solar heating at a given location where: φ =latitude (degrees) δ =declination angle (degrees) Local Irradiance ( S TOA ): The daily amount of solar radiation received over a given location at the top of the atmosphere where: S 0 =total solar irradiance (W m -2 ) h 0 =hour angle between noon and sunset (degrees) φ =latitude (degrees) δ =declination angle (degrees) Atmospheric InstrumentationM. D. Eastin Review of Solar Geometry

Atmospheric InstrumentationM. D. Eastin Summary List of Radiometer Types WavelengthInstrumentPurpose ShortwavePyranometerMeasures global solar radiation (S G ≈ S B + S D ) over a hemispheric field of view PyrheliometerMeasures direct beam solar radiation (S B ) AlbedometerMeasures solar radiation received by and reflected from a surface (S U ) LongwavePyrgeometerMeasures terrestrial radiation of the upward or downward hemisphere (L U or L D ) Pyrometer **Estimates an object’s temperature through measurement of the longwave radiance emitted by the object BothPyrradiometerMeasures total radiative flux of solar and terrestrial radiation (R N ) Radiometers

Atmospheric InstrumentationM. D. Eastin Generic Thermopile Radiometer – Basic Concept Measures hemispheric irradiance by detecting the temperature difference between (1) a black thermopile (an array of thermistors) and (2) a white thermopile housed beneath a dome that protects the sensors from (a) thermal loss due to air motions and/or precipitation wetting, and (b) from contaminants that could alter the spectral characteristics of the two thermopiles Often mounted on a flat horizontal surface facing either upward or downward The properties of the dome also define the range of wavelengths permitted to reach the sensors GlassShortwave observations Transparent to λ = 0.2 – 2.8 μm SiliconLongwave Observations Transparent to λ = 3.0 – 50.0 μm Radiometers Black Thermopile White Thermopile Dome R TOT

Atmospheric InstrumentationM. D. Eastin Generic Thermopile Radiometer – Basic Concept The total irradiance (R TOT ) can be measured via incoming and outgoing energy balance considerations between (1) the black thermopile and (2) the white thermopile IncomingOutgoing Black Thermopile: White Thermopile: Combining to the two equations and making a number of algebraic approximations (see your text) results in the following linear response function where:C= unique instrument constant (found in calibration) Radiometers Black Thermopile Temperature T b White Thermopile Temperature T w Dome Temperature T d R TOT

Atmospheric InstrumentationM. D. Eastin Generic Thermopile Radiometer – Typical Specifications There are three “quality levels” for basic radiometer instruments Selection of instrument quality depends on available budget and observational goals CharacteristicStandardFirst ClassSecond Class Accuracy(W m -2 ) ±1 ±2±10 Resolution (W m -2 ) Response Time (s)< 25< 60 < 240 Generic Thermopile Radiometer – Type of Radiation Measured The type of solar or terrestrial radiation measured is a function of the following 1. Dome Material Glass (all S types) Silicon (all L types) 2. Mounting Orientation Horizontal – upwards (S G and L D ) Horizontal – downwards (S U and L U ) Tracking (S B and S D ) 3. ShieldingShielded (S D ) None (all others) Radiometers

Atmospheric InstrumentationM. D. Eastin Pyranometer (S G ): Measures global solar irradiance by mounting an upward-facing glass-domed radiometer on a flat horizontal surface away from any obstructions Even in clear skies, the measured global hemispheric irradiance in less than that determined from top of the atmosphere (TOA) calculations due to absorption by atmospheric gases In partly cloudy or cloudy conditions, considerable variability occurs along with significant reductions in measure irradiance Radiometers Daily Irradiance – Boulder (CO)

Atmospheric InstrumentationM. D. Eastin Pyrheliometer (S B ): Measures the direct beam solar irradiance at a normal incidence by using a global hemispheric pyranometer attached to a sophisticated solar tracking mount that moves in both azimuth and elevation as the sun crosses the sky Radiometers

Atmospheric InstrumentationM. D. Eastin Diffuse Solar Irradiance (S D ): Option #1Use a pyrheliometer (S B ), and a pyranometer (S G ) and then calculate the diffuse irradiance where: h = solar zenith angle Option #2Use a shielded pyranometer (S D ) that blocks the direct beam using either an occulting disk (mounted to a solar tracker) or a shade-ring (that blocks direct solar radiation throughout the day in all months) Radiometers

Atmospheric InstrumentationM. D. Eastin Albedometer (S U ): Measures total reflected solar irradiance by mounting a downward-facing glass-domed radiometer on a flat horizontal surface When paired with a pyranometer (S G ), the local albedo ( α ) can be easily calculated Radiometers Albedometer (S U ) Pyranometer (S G ) Pyrogeometer (L U ) Pyrogeometer (L D )

Albedometer (S U ) Pyranometer (S G ) Pyrogeometer (L U ) Pyrogeometer (L D ) Atmospheric InstrumentationM. D. Eastin Pyrgeometer (L U and L D ): Measures total terrestrial (longwave) irradiance by mounting both an upward-facing and a downward-facing silicon-domed radiometer on a flat horizontal surface At nighttime, differences between L D and L U determine the local net radiation (R N ) In daytime, measurements of S G, S D, and S U are also needed Radiometers LULU LDLD

Albedometer (S U ) Pyranometer (S G ) Pyrogeometer (L U ) Pyrogeometer (L D ) Pyrradiometer (R N ): Measures net surface radiation at a local site through the combination of four radiometers 1. Pyranometer (S G ) 2. Albedometer (S U ) 3. Pyrgeometer (L D ) 4. Pyrgeometer (L U ) These observations can be easily combined to compute the net radiation via Most observation sites install these instruments since they provide the full compliment of required radiation measurements to compute a full surface energy balance (radiation, moisture, and heat fluxes) Atmospheric InstrumentationM. D. Eastin Radiometers

Pyrometer (T S ) – Basic Concept and Specifications Estimates the surface temperature of an “object” by restricting the instrument’s field of view and confining the radiation to a narrow window within the infrared (longwave) spectrum Temperature is estimated using the Stefan-Boltzmann relationship with (or without) emissivity ( ε ) corrections Thus, the measured surface temperature represents a weighted average temperature of all objects within the field of view that are emitting radiation in that wavelength window Used by research aircraft to measure flight-level air temperature and the underlying ground / sea surface temperature Accuracy ±2.0 °C Resolution 0.1 °C Response Time< 1-10 s Atmospheric InstrumentationM. D. Eastin Radiometers

Atmospheric InstrumentationM. D. Eastin Errors Unique to Radiometers: (1) Instruments must be kept level (2) Instruments should always be kept clean → dust, rain, dew, and bird droppings can adversely affect window transparency → daily cleaning → fan aspirators should maintain regular flow of air over radiometer domes to keep them free of dust, dew, and rain (3) No condensation inside the instrument (4) Site must exhibit no shadows for all annual sun angles (5) Site must exhibit no reflections for all annual sun angles Exposure Errors

Atmospheric InstrumentationM. D. Eastin Summary Measurement of Radiation Review of Atmospheric Radiation Review of Solar Geometry Radiometers Thermopile Radiometer Pyranometer Pyroheliometer Albedometer Pyrradiometer Pyrogeometer Pyrometer Exposure Errors

Atmospheric InstrumentationM. D. Eastin References Aceves-Navarro, L. A., K. G. Hubbard, and J. Schmidt, 1988: Group calibration of silicon cell pyranometers or use in an automated network. Journal of Atmospheric and Oceanic Technology, 5, Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp. Brock, F. V., K. C. Crawford, R. L. Elliot, G. W. Cuperus, S. J. Stadler, H. L. Johnston, M.D. Eilts, 1993: The Oklahoma Mesonet - A technical overview. Journal of Atmospheric and Oceanic Technology, 12, Delany, A. C., and S. R. Semmer, 1998: An integrated surface radiation measurement system. Journal of Atmospheric and Oceanic Technology, 15, Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp. Halldin, S., and A. Lindroth, 1992: Errors in net radiometry: Comparison and evaluation. Journal of Atmospheric and Oceanic Technology, 9,