Atmospheric Instrumentation M. D. Eastin
Measurement of Radiation
Atmospheric Instrumentation M. 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 Instrumentation M. 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 originatesis a function of the body’s temperature → Planck’s Law
The integral of this energy (the area under each curve below) definesthe 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
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Atmospheric Instrumentation M. 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 originatesis a function of the body’s temperature → Planck’s Law
Since the black-body temperatures for the Sun (TBB = 5500-6000 K) and Earth (TBB = 210-310 K) are significantly different, atmospheric radiationis divided into two distinct broadband spectrums
Solar Radiation → Shortwave RadiationTerrestrial Radiation → Longwave Radiation
Review of Atmospheric Radiation
Atmospheric Instrumentation M. 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 Instrumentation M. D. Eastin
Definitions and Concepts:
•The net radiation (Rn) observed at the surface consists of all combined incoming and outgoing longwave and shortwave radiation
Review of Atmospheric Radiation
UDUDBN LLSSSR
Direct beamShortwave
(SB)
Top of Atmosphereirradiance (STOA)
UpwardsReflectedShortwave
(SU)
DiffuseShortwave
(SD)
DownwellingLongwave
(LD)
UpwellingLongwave
(LU)
Atmospheric Instrumentation M. 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 angleof 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 mediumReflectance: Fraction of irradiance that is reflected by a mediumTransmittance: Fraction of irradiance that is transmitted by a medium
SI units and W = wattsMeteorology: sr = steradian (solid angle unit)
m = meters
Instrument: Radiometer
SolidAngle
Review of Atmospheric Radiation
Atmospheric Instrumentation M. D. Eastin
Global Mean Energy Flows (W m-2)
Review of Atmospheric Radiation
Atmospheric Instrumentation M. D. Eastin
Definitions and Concepts:
• Up to 90% of the total top of the atmosphereshortwave irradiance (~1370 W m-2)
reaches the Earth’s surface
• Longwave emissions can reach 800 W m-2
• Surface radiometers should exhibit adynamic 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 rotationrelative to the orbital (elliptical) plane
• Declination angle (δ) describes this tilt as afunction 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 (t0) (when the Sun is directly overhead)
•Local solar time (measured on a sundial) differs from a standardized clock by up to 1 hour
Atmospheric Instrumentation M. D. Eastin
Review of Solar Geometry
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Local Day Length (Lday):
• The length of a given day defines the durationof solar heating at a given location
where: φ = latitude (degrees)δ = declination angle (degrees)
Local Irradiance (STOA):
•The daily amount of solar radiation received overa given location at the top of the atmosphere
where: S0 = total solar irradiance (W m-2) h0 = hour angle between noon and sunset (degrees)φ = latitude (degrees)δ = declination angle (degrees)
Atmospheric Instrumentation M. D. Eastin
Review of Solar Geometry
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Atmospheric Instrumentation M. D. Eastin
Summary List of Radiometer Types
Wavelength Instrument Purpose
Shortwave Pyranometer Measures global solar radiation (SG ≈ SB + SD) over a hemispheric field of view
Pyrheliometer Measures direct beam solar radiation (SB)
Albedometer Measures solar radiation received by andreflected from a surface (SU)
Longwave Pyrgeometer Measures terrestrial radiation of the upwardor downward hemisphere (LU or LD)
Pyrometer ** Estimates an object’s temperature throughmeasurement of the longwave radianceemitted by the object
Both Pyrradiometer Measures total radiative flux of solar andterrestrial radiation (RN)
Radiometers
Atmospheric Instrumentation M. D. Eastin
Generic Thermopile Radiometer – Basic Concept
•Measures hemispheric irradiance by detecting the temperature difference between (1) ablack thermopile (an array of thermistors) and (2) a white thermopile housed beneath a domethat protects the sensors from (a) thermal lossdue 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 facingeither upward or downward
•The properties of the dome also define the rangeof wavelengths permitted to reach the sensors
Glass Shortwave observationsTransparent to λ = 0.2 – 2.8 μm
Silicon Longwave Observations Transparent to λ = 3.0 – 50.0 μm
Radiometers
BlackThermopile
WhiteThermopile
Dome
RTOT
Atmospheric Instrumentation M. D. Eastin
Generic Thermopile Radiometer – Basic Concept
•The total irradiance (RTOT) can be measured via incoming and outgoing energy balanceconsiderations between (1) the black thermopile and (2) the white thermopile
Incoming Outgoing
Black Thermopile:
White Thermopile:
•Combining to the two equations and making a numberof algebraic approximations (see your text) resultsin the following linear response function
where: C = unique instrument constant(found in calibration)
Radiometers
44bdTOT TTR
44wd TT
BlackThermopile
TemperatureTb
WhiteThermopile
TemperatureTw
DomeTemperature
Td
RTOT
wbTOT TTCR
Atmospheric Instrumentation M. 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
Characteristic Standard First Class Second ClassAccuracy (W m-2) ±1 ±2 ±10Resolution (W m-2) 0.5 1.0 5.0Response 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 (SG and LD)Horizontal – downwards (SU and LU)Tracking (SB and SD)
3. Shielding Shielded (SD)None (all others)
Radiometers
Atmospheric Instrumentation M. D. Eastin
Pyranometer (SG):
•Measures global solar irradianceby mounting an upward-facingglass-domed radiometer on a flathorizontal surface away from anyobstructions
•Even in clear skies, the measuredglobal hemispheric irradiancein less than that determined fromtop of the atmosphere (TOA) calculations due to absorptionby atmospheric gases
•In partly cloudy or cloudyconditions, considerablevariability occurs alongwith significant reductionsin measure irradiance
Radiometers
Daily Irradiance – Boulder (CO)
Atmospheric Instrumentation M. D. Eastin
Pyrheliometer (SB):
•Measures the direct beam solar irradiance at anormal incidence by using a global hemisphericpyranometer attached to a sophisticated solartracking mount that moves in both azimuth andelevation as the sun crosses the sky
Radiometers
Atmospheric Instrumentation M. D. Eastin
Diffuse Solar Irradiance (SD):
Option #1 Use a pyrheliometer (SB), anda pyranometer (SG) and then calculate the diffuse irradiance
where: h = solar zenith angle
Option #2 Use a shielded pyranometer (SD)that blocks the direct beam usingeither an occulting disk (mountedto a solar tracker) or a shade-ring(that blocks direct solar radiationthroughout the day in all months)
Radiometers
DBG ShSS cos
Atmospheric Instrumentation M. D. Eastin
Albedometer (SU):
•Measures total reflected solar irradiance by mounting a downward-facing glass-domedradiometer on a flat horizontal surface
•When paired with a pyranometer (SG), thelocal albedo (α) can be easily calculated
Radiometers
Albedometer (SU)
Pyranometer (SG)
Pyrogeometer (LU)
Pyrogeometer (LD)
GU SS
Albedometer (SU)
Pyranometer (SG)
Pyrogeometer (LU)
Pyrogeometer (LD)
Atmospheric Instrumentation M. D. Eastin
Pyrgeometer (LU and LD):
•Measures total terrestrial (longwave) irradiance by mounting both an upward-facing and a
downward-facing silicon-domed radiometeron a flat horizontal surface
•At nighttime, differences between LD and LU determine the local net radiation (RN)
•In daytime, measurements of SG, SD, and SU
are also needed
Radiometers
LU
LD
Albedometer (SU)
Pyranometer (SG)
Pyrogeometer (LU)
Pyrogeometer (LD)
Pyrradiometer (RN):
•Measures net surface radiation at a local sitethrough the combination of four radiometers
1. Pyranometer (SG)2. Albedometer (SU)3. Pyrgeometer (LD)4. Pyrgeometer (LU)
•These observations can be easily combinedto compute the net radiation via
•Most observation sites install theseinstruments since they provide thefull compliment of required radiationmeasurements to compute a fullsurface energy balance (radiation, moisture, and heat fluxes)
Atmospheric Instrumentation M. D. Eastin
Radiometers
UDUGN LLSSR
Pyrometer (TS) – Basic Concept and Specifications
•Estimates the surface temperature of an “object” by restrictingthe instrument’s field of view and confining the radiation to anarrow window within the infrared (longwave) spectrum
•Temperature is estimated using the Stefan-Boltzmannrelationship 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 °CResolution 0.1 °C Response Time < 1-10 s
Atmospheric Instrumentation M. D. Eastin
Radiometers
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Atmospheric Instrumentation M. 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 Instrumentation M. 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 Instrumentation M. 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, 875-879.
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, 5-19.
Delany, A. C., and S. R. Semmer, 1998: An integrated surface radiation measurement system. Journal of Atmospheric and Oceanic Technology, 15, 46-53
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, 762-783.