Surface-based Radiation Observations ( primarily broadband w/ a climate bias)

Surface-based Radiation Observations (primarily broadband w/ a climate bias)

Outline• Observable Radiation Quantities (review)• General and Specific Applications

• Applied Radiometry – Incoming (downwelling) Solar Irradiance– Downwelling Thermal Infrared– Reflected and Surface-emitted Upwelling Irrad.– Remote Sensing of the Atmosphere at Solar Wavelengths– Recent Advances

•Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009

Ellsworth G. DuttonNOAA, Earth System Research Laboratory

Boulder, [email protected]

Observable Radiation Quantities(energy per unit time per unit area)

• Emitted

• Absorbed

• Reflected

• Transmitted

E = A + R + T, 1.0 = a + r + t

Two Types of Radiation Emission Sources

• Full-spectrum - Black and grey bodies (opaque mass/objects) • Molecular emission lines (semi-transparent gases/mediums)

RT

A

E


SI Units W m-2 (Joules sec-1 m-2)

Kirchoff’s LawMass emits the

same as it absorbs

1. Opaque bodies, such as a hot, dense gas or solids produce a continuous spectrum – Known as a blackbody if absorption at all wavelengths is 100%

2. A hot transparent gas produces an emission line spectrum – a series of bright spectral lines against a dark background.

3. A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum – a series of dark spectral lines among the colors of the continuous spectrum.

physics.unl.edu/~klee/ast204/lectures/Ast204_lecture14_01.ppt •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009

Opaque Solid

Wein’s Law

I (r

elat

ive

unit

s λ-1

)

I -- Radiation (W m-2 nm-1)c -- speed of lighth -- Planck constantK -- Boltzman constantT -- Temperatureλ -- Wavelength

Planck’s Law for Full-spectrum Emission

Stephan-BoltzmanLaw (ideal BB)

∫ I dλ = σ T4

(realistic, grey)

∫ I dλ = εσ T4

ε = 0→ <1.0


Some Terminology

• Radiance – Radiant power per unit area per steradian (3-D conical solid angle)

W m-2 ster-1 (remote sensing, mapping)• Irradiance – Radiant power per unit area W m-2 (fluxes)

Either can be specified spectrally as per unit wavelength or integrated over some spectral interval



Illustration only

ModTran 5.0

Note: 0.3 μm to 3.0 μm(includes downward scattering)


ModTran 5.0

Note: 3.5 μm to >50 μm

Examples of Applications for Radiation Observations

• Satellite observed constraints on inversions• Model (rad transfer, Wx, Climate) comparisons• Surface and boundary layer energy budgets, dynamics and

surface flux partitioning• Vertical profiles for flux divergence (heating rates)• Radiation climatology• IR trends related to Global Warming• Solar Dimming & Brightening variations• Estimating cloud and aerosol effects• International cooperative observational programs


Trenberth et al 2009


ModelsValidation

Valid

atio

n

More accurate radiative transfer in

weather, and climate simulations

Assimilation

BSRN

…

Surface Radiation Budget Observations and Research An Integrated approach

Improvement

Retrieval

Direct Observations

RT

Climate-quality surface observations


WMO

IOC

Regions

Oceanic Tropics Desert Polar Coastal Rain forest Agricultural Prairie

EG Dutton, 4Oct2007

Features• Site scientists• 18+ countries• Stand. Specs.

• Long-term• Central archive• Ref. Std. Devlp.

• GRP review• GCOS

Archiving Provisional

Goal:To acquire the highest possible quality, climati-cally-diverse, surface-based radiation measurements for climate research

Data Applications

• GCM comparisons• Satellite validation• Regional climatologies• Global radiation budget• Radiation model testing

Measurements• Direct & diffuse solar*• Downward infrared *• Upwelling rad.• PAR & UV• Aerosol optical depth• Surface meteorology*• Upper air met.* all sites

Monthly Averages for 4 Years

0

100

200

300

400

500

1995 1996 1997 1998 1999Year

W m

-2

Daily Averages for 6 Months

0

100

200

300

400

500

1997 1997.2 1997.4

Year

W m

-2

Radiation Budget Components, time averaging

LW

LW SW SW

Erie TowerNOAA/CMDL

STAR

1-Minute Averages for 1 day

0

200

400

600

800

1000

75.4 75.9 76.4

Day of Year (GMT, 1997)

W m

-2

Yearly Averages for 14 Years

0

100

200

300

400

1985 1990 1995 2000Year

W m

-2


ISCCP Sat. constrained model

BS

RN

NASA / CERES / SARB -T Charlock / D. Rutan

NASA / WCRP / ISCCP - Zhang et al

NASA /CERES /SOFA –

D. Kratz

NASA / GEWEX / SRB - P. StackhouseBSRN

Comparison of Satellite vs. Ground-based Surface Irradiance Products

LW bias <2.SW bias <2.



E.G. DuttonPan-GEWEX Mtg10 Oct 2006Frascati, Italy•Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009

s

BSRN251

BSRN – International Baseline Surface Radiation Network

Compared to Observations

Avg. (337)BSRN (344)

Circa 2005

GCM models(global means) M. Wild 2001& 2005

Over Last 6 Years, Climate Models Approach BSRN Downwelling IR Results

BSRN (344 W m-2)

Model Avg. (329)

Circa 1999

GCM models (global means)


Ohmura 2009, JGR

Longest Surface Solar records, Europe


Early Brightening Dimming Brightening

Long Term Variability

IPCC AR4 GCM Means---- GHG forcing only---- GHG + direct aerosol foricing---- GHG + direct and Indirect aerosol forcing

GHG forcing


~2.5 W m-2/dec

BSRN Observed

Observations Confirming Surface IR Global Warming Increases w/ Feedbacks

∫∫∫∫∫∫∫ I(t,z,x,y,θ,φ,λ) dtdθdφdλdzdxdy = 7.38 mV

“It can be a lot easier to measure something than it is to know what it is that you measured!”

Particularly true in radiometry.

0.3 0.6 1.0 1.7 3.0

10

5

2.5

7.5

Wavelength (m)

Alt

itu

de

(km

)

0.3 0.6 1.0 1.7 3.0

10

5

2.5

7.5

Wavelength (m)

Alt

itu

de

(km

)


Generic Radiometer and View-Scene Components

Detector

Spectral selection(filter)

View limiter

Intervening medium(possible sources and sinks) Source

Signal processor

Radiometer

(x), T(x)T,

May or may not have all these components

Detector types Spectral selection View limitersPhoto multiplier Detector sensitivity Adjustable aperturePhotoelectric cell Interference filter Fixed apertureThermopile Absorption filter Flat platePyroelectric Prism/Grating OpticsCavity Shutter

Desirable Characteristics•Stable w/ time•Defined spectral response•Defined geometry•Linear output•Fast response•Low cost


Common Surface-based Broadband Radiometers for Surface Radiation Budget Measurements

• Pyrheliometer – Direct solar beam (on a perpendicular surface)• Pyranometer – Total (diffuse+ direct, Global) solar on a horizontal surface• Shaded pyranometer – Diffuse solar (horizontal surface)• Pyrgeometer – Thermal IR “ “

Previously widely used by surface energy budget community* • Net radiometer – single element • Subtraction to get IR or solar from total spectrum *generally sub-standard accuracy by today’s standards - lacking component resolution

and calibration reference standards.

Notes:Total solar = Normal Direct ∙ cos(θ) + Diffuse, θ = solar zenith angle

Surface Rad. Bud. (net) = Total solar + sky LW - reflected solar - surface emitted LW


Surface Radiation BudgetComponent Quantities (A-E)

EARTH

SPACE

A

B C D

ESOLAR THERMAL INFRAREDObs. Site

A – Diffuse solar B -- Direct solar, normalC – Reflected solar D – Downward IRE – Upward IRTotal Downward Solar = A + cos (Θ) *B

Θ


Direct Solar Irradiance

• Easiest component to model• High sensitivity to attenuators • Dominant component of total

solar when sky is clear

Measurement• Relatively easy and accurate to

calibrate by absolute cavity• Alignment and tracking

required• 0.28 μm to ~3.5 μm

5.7 º

Clock motor on equatorial (right ascension) axis

Declination axis(manual on this tracker)

Eppley Labs pyrheliometer (NIP) & tracker•Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009

RefTemp.

- +

Temp.

Comp.

Solar Typical Pyrheliometer

(with spectral filter → sun photometer)

Clear glass window

(filter and Wx protect)

Thermopile

View limiting aperture

Baffles

Connector

(to voltmeter)

Im = cc ·VV = f(ΔT,T)ΔT = ε · nIi

Im -- Measured direct solar Irradiancecc -- Assigned linear calibration constantV -- Output voltageT -- TemperatureΔT -- Thermopile T – Ref Tε -- Proportionality “constant”nIi -- Net incident irradiance at detector = Id + Is – Ir + LWi – LWo

Id = Direct solar irradianceIs = Scattered solarIr = Reflected solarLWi = Incoming thermal LW (IR)LWo = Outgoing thermal LW (IR)•Instrument design requirement: Id proportional to V•General User application: Find cc and assume that Im = Id

•Reality for the most accurate meas.: Id = f(Im)

Electronics


Error sources in Pyrheliometer Meas.

Instrument Characteristics• Calibration stability• Linearity• Window spectral transmission• Sensitivity temperature dependency• …User Characteristics• Calibration reference & transfer• Alignment & obstructions• Window cleanliness • Data collection system• …

Kipp& Zonen CH-1

Alignment aid (diopter)


Diffuse Solar Irradiance

• Information on downward scattering, high sensitivity to scatters

• Difficult to model, other than Rayleigh• Sometimes is 100% of total solar

component• Sensitive to surface albedo

Measurement issues• No universal calibration reference• Calibration: transfer from calibrated

pyranometer under diffuse conditions, or as total pyranometer

• Direct solar blockage required, tracking disk highly preferred


Error Sources in Diffuse Meas.

• Same as direct component plus:

• Unknown absolute reference

• Dome thermal offsets (black thermopile detectors)

• Shade geometry

• Cosine response of flat plate

• Subtle obstructions


Total (Global) Solar Irradiance

• Primary desired quantity in radiation budget• Sensitivity to forward scatters is reduced • Difficult to model –absorption and scat. phase• Sensitive to surface albedo• Many other applications – agriculture, renewable energy…

Measurement issues• No absolute calibration reference• Very serious cosine (non-linearity) response issues• Calibration: transfer of WRR by shade-cal, component sum

cal in clear sky, or laboratory reference source• Lots of instrument choices, some quite bad


RefTemp.

- +

Temp.

Comp.

LW (IR) exchangeSolar

Typical Pyranometer

Clear glass domes

Direct

Diffuse

IT = cc · V


SW = cc ∙Voltage

Error Sources in Total Solar (measured by a single pyranometer)

• Same as direct and diffuse, minus tracking

• Cosine error aggravated by large direct component

• Leveling• Proliferation of cheap

sensors



Thermal IR Meas.

• Typically measured over ~3.5 μm to ~50 μm• Difficult to model (clouds and cloud bases, H2O)• Generally hemispherically diffuse • Downwelling is total “greenhouse” effect• Detector elements emitting at measured wavelengths• Blackbody references possible• Trends predicted• Routine measurements maturing

Eppley PIR•Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009

RefTemp.

- +

Temp.

Comp.

LW exchange

IncidentLW

Typical Pyrgeometer

Solar

Silicon dome (silver)

IR↓ = cc · V + σTc

4

+ kσ(Tc4-Td4)

.Tdome

Connector


Tcase

Error Sources in IR Meas

• Contamination from instrument elements

• Absolute calibration

• No universal reference other than BB

• Instrument noise

• Source definition (solar source included?)

• Spectral definition

• Data system, obstructions, etc


Upwelling irradiances


• The other “half” of the problem

• Same instrumentation, inverted

• Completely diffuse fields

• Better constrained (near the surface)

• Representativeness & interpretation issues

• Instrument deployment problems (shadows, height, service)

END(sort of)


Remote Sensing of the atmosphere at Solar Wavelenghts

• Spectral Aerosol optical depth– Total column attenuation– Relative size distribution– Scattering angle (phase function or asymmetry factor– Absorption

• Ozone• Water Vapor• NO2• Exotic trace species



τλ= τλH2O + τλO3 + τλaerosol +…

AOD = Aerosol Optical Depth

= τλa

Iλ/I0λ = exp(-τλmr)

Vλ/V0λ = exp(-τλmr)

Beer’s Law

Iλ = cc ∙ Vλ

Sprectral Optical Depth Remote Sensing Basics


Silicon cell response (RHS)

Recent Advances In Direct Solar Meas.

• Sustained definition of World Radiation Reference scale, since 1976

• Better solar tracking, circa early 1990s (computerized/interactive)

• Introduction of calcium fluoride (CaF4) windows, high trans. to 10 μm

• All-weather cavity radiometers

• New understanding of thermal off-sets and non-linearities in operational pyrheliometers


Recent Advances in Solar Diffuse Meas.

• Better tracking of shade disk

• Comparisons to the Rayleigh limit

• Corrections and reductions of thermal offset error

• Revival of black and white detectors

• Consensus intercomparisons

• Recommend Reference Stnd. -- Michalsky et al 2007


Recent Advances in Total Solar Meas.

• Refined “shade/unshade” calibration• Improved cosine response• Reduced dome thermal offset

– Ventilation “conditioning”

– Dome thermal contact to base

– Correction algorithms


Recent Advances in IR Meas.

• Proper dome temperature correction

• Pyrgeometer tracking shading• Absolute scanning reference

radiometer• Isothermal domes• “Flatter” domes• Ventilation of domes• Interim International Reference

Standard established (World Radiation Center, Davos, Switzerland)


Example BSRN SITES

Barrow, AlaskaBoulder, Colorado

Kwajalein, M.I.

E.G.Dutton NOAA/CMDL

Kwajalein

Barrow

Boulder - BAO

http://cmdl1.cmdl.noaa.gov:8000/~star/pictures/100-0100.HTML

The Three Ways to Transport Thermal Energy

• Convention/Advection

• Conduction

• Electromagnetic Radiation


General Applications for Measured E/M Radiation in Atmos Sci.

• Account for amount of heat energy transferred from a source to a surface or volume of interest

• Determine temperature of emitting sources/gases• Determine quantity and type of emitting gases or

absorbing gases between source and receiver • Determine quantity and/or some characteristics of

intervening suspended particulates (scattering and absorption)

• Mapping of emission, reflection and/or absorption

Remote Sensing


Selected references – Related to Broadband Surface Radiation Measurements - Ells Dutton CSU/AT650 23Sept2007(Also look under Publications at http://bsrn.ethz.ch)

Solar, direct and diffuse• Hengstberger, F., (ed.) 1989: Absolute Radiometry – Electrically Calibrated Thermal Detectors for Optical Radiation Academic Press, Boston, 266p.• Frohlich, C., 1991: History of solar radiometry and the World Radiometric Reference. Metrologia 3, 111-15.• Romero, J., et al., 1991: First comparison of the solar and SI radiometric scale. Metrologia, 28, 125-128.• Romero, J. et al., 1995: Improved comparison of the World Radiometric Reference and the SI radiometric scale Metrologia, 32, 523.• Michalsky, J., et al., 1999: Optimal measurements of surface shortwave irradiance using current instrumentation . J. Tech., 16, 55-69• Bush, B.C. and F.P.J. Valero, 1999: Comparison of ARESE clear sky surface radiation measurements. J. Quant. Spectrosc. Radiat. Transfer, 61, 249-264.• Bush, B.C., et al., 2000: Characterization of thermal effects in pyranometers: A data correction algorithm for improved measurement of surface insolation. J. Tech. 17, 165-

175.• Halthore, R. et al., 1997: Comparison of model estimated and measured direct-normal soar irradiance. JGR 102, 29,991-20,002.• Halthore, R., et al., 1998: Models overestimate diffuse clear-sky surface irradiance: a case for excess atmospheric absorption. GRL 25, 3591-3594.• Kato, S., et al., 1997: Uncertainties in modeled and measured clear-sky surface shortwave irradiances. JGR 102, 25,882-25,898.• Kato, S. et al., 1999: a comparison of modeled and measured surface shortwave irradiance for a molecular atmosphere. J Quant. Spectrosc. Radiat. Transfer 61, 493-502• Dutton, E. G., et al., 2001: Measurement of broadband diffuse solar irradiance using current commercial instrumentation with a correction for thermal offset errors. J. Tech.

18, 297-314.• Haeffelin, M., et al., 2001: Determination of the Thermal Offset of the Eppley Precision Spectral Pyranometer , Applied Optics-OT, Volume 40, Issue 4, 472-484.• Cess, R. D., et al, 2000: Consistency tests applied to the measurement of total, direct, and diffuse shortwave radiation at the surface. JGR 105, 24,881-24,887.• Ohmura, A., et al., 1998: Baseline Surface Radiation Network (BSRN/WCRP): New precision radiometry for climate research. BAMS, 79, 2115-2136.• Philipona R, 2002:Underestimation of solar global and diffuse radiation measured at Earth's surface J. Geophys. Res. VOL. 107, NO. D22, 4654, doi:10.1029/2002JD002396,

2002• Zamora, R.J., et al., 2002: Comparing MM5 radiative fluxes with observations gathered during the 1995 and 1999 Nashville Southern Oxidants Studies, J Geophys. Res. 108,

D2,4050,doi:10.1029/202JD002122.• Dutton, E.G., A. Farhadi, R.S. Stone, C. Long, and D. W. Nelson: 2004: Long-term variations in the occurrence and effective solar transmission of clouds determined from

surface irradiance observations. J. Geophys. Res., Vol. 109, No. D3, D0320410.1029/2003JD003568.• Michalsky, J.J., et al., 2003: Results from the first ARM diffuse horizontal shortwave irradiance comparison. J. Geophys. Res. 108, D3, 4108, doi:10.1029/2002JD002825.• Augustine, J. A., et al.,, An update on SURFRAD—The GCOS surface radiation budget network for the continental United States, J. Atmos. Ocean. Tech., 22, 1460-1472,

2005.• Michalsky, J. J., et al., Toward the development of a diffuse horizontal shortwave irradiance working standard, J. Geophys. Res., 110, D06107, doi:10.1029/2004JD005265,

2005.• Michalsky, J. J., et al,, Shortwave radiative closure studies for clear skies during the Atmospheric Radiation Measurement 2003 Aerosol Intensive Observation Period, J.

Geophys. Res., 111, D14S90, doi:10.1029/2005JD006341, 2006.• Pinker R.T., B. Zhang, and E. G. Dutton, 2005: Do Satellites Detect Trends in Surface Solar Radiation? Science, Vol 308, Issue 5723, 850-854 , 6 May 2005• Wild, M., et al.,. (2005). From Dimming to Brightening: Decadal Changes in Solar Radiation at Earth's Surface. Science, 308: 847-850.• Zamora, R.J., et al., 2005: The accuracy of solar irradiance calculations used in mesoscale numerical weather prediction. Mon. Wea. Rev., 133, 783-792. • Reda, I., et al 2005: Using a blackbody to calculate net-longwave responsivity of shortwave solar pyranometers to correct for their thermal offset error during outdoor

calibration using the summation method, J. Oceanic and Atmos. Tech. 22, 1531–1540. • Dutton E.G., et al., 2006: Decadal Variations in Surface Solar Irradiance as Observed in a Globally Remote Network . 2006: J. Geophys. Res., 111, D19101,

doi:10.1029/2005JD006901.• J. J. Michalsky, et al., 2007: A proposed working standard for the measurement of diffuse horizontal shortwave irradiance. J GEOPHYS RES 112, D16112,

doi:10.1029/2007JD008651, 2007Longwave (thermal IR)• Albrecht, B., et al., 1974: Pyrgeometer measurements from aircraft. Rev Sci. Instrum., 45, 33-38. • Albrecht, B., and S. Cox, 1977: Procedures for improving pyrgeometer performance. JAM, 16, 188-197.• Dutton, E., 1993: An extended comparison between LOWTRAN7-computed and observed broadband thermal irradiance: Global extreme and intermediated surface conditions.

J. Tech. 10, 326-336.• Philipona, R., et al., 1995: Characterization of pyrgeometers and the accuracy of atmospheric long-wave radiation measurements. App. Optics 34, 1598-160.• Philipona, R. et al., 1998: The Baseline Surface Radiation Network pyrgeometer round robin calibration experiment. J. Tech. 15, 687-696.• Fairall, C.W., et al., 1998: A new look at the calibration and use of Eppley precision infrared radiometers. Part I Theory and applications. J. Tech. 15, 1229-1242.• Philipona, R. et al., 2001 Atmospheric longwave irradiance uncertainty: Pyrgeometer compared to an Absolute sky-scanning radiometer, AERI and radiative transfer model

calculations. J. Geophys. Res. 106, 28,129-28,141.• Marty, C., R. etal., 2002: Longwave irradiance uncertainty under arctic atmospheres: Comparisons between measured and modeled downward longwave fluxes. J. Geophys.

Res., VOL. 108, NO. D12, 4358, doi:10.1029/2002JD002937, 2003• Philipona, R, et al.: Radiative forcing - measured at Earth's surface - corroborate the increasing greenhouse effect GEOPHYS RES LET, 31 (3): Art. No. L03202 FEB 6 2004•Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009

Documents

Surface-based Radiation Observations ( primarily broadband w/ a climate bias)