Atmospheric InstrumentationM. D. Eastin Measurement of Radiation

Atmospheric Instrumentation M. D. Eastin

Measurement of Radiation


Outline


• Review of Atmospheric Radiation

• Review of Solar Geometry

• Radiometers• Thermopile Radiometer• Pyranometer• Pyrheliometer• Albedometer• Pyrgeometer• Pyrradiometer• Pyrometer

• Exposure Errors


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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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




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




•The net radiation (Rn) observed at the surface consists of all combined incoming and outgoing longwave and shortwave radiation


UDUDBN LLSSSR

Direct beamShortwave

(SB)

Top of Atmosphereirradiance (STOA)

UpwardsReflectedShortwave

(SU)

DiffuseShortwave

(SD)

DownwellingLongwave

(LD)

UpwellingLongwave

(LU)



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



Global Mean Energy Flows (W m-2)




• 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



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


Review of Solar Geometry

365

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24

360 oo tth

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)


Review of Solar Geometry

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0TOA hhSS sincoscossinsin86400

00


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


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


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


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


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)


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


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


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)


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)


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


Radiometers

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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


Summary


• Review of Atmospheric Radiation

• Review of Solar Geometry

• Radiometers• Thermopile Radiometer• Pyranometer• Pyroheliometer• Albedometer• Pyrradiometer• Pyrogeometer• Pyrometer

• Exposure Errors


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.

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Atmospheric InstrumentationM. D. Eastin Measurement of Radiation