Surface-based Radiation Observations (primarily broadband w/ a climate bias) Ellsworth G. Dutton NOAA, Earth System Research Laboratory Boulder, Colo [email protected] Outline • Observable Radiation Quantities (review) • General.
Download ReportTranscript Surface-based Radiation Observations (primarily broadband w/ a climate bias) Ellsworth G. Dutton NOAA, Earth System Research Laboratory Boulder, Colo [email protected] Outline • Observable Radiation Quantities (review) • General.
Surface-based Radiation Observations (primarily broadband w/ a climate bias) Ellsworth G. Dutton NOAA, Earth System Research Laboratory Boulder, Colo [email protected] 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 Observable Radiation Quantities (energy per unit time per unit area) • • • • Emitted Absorbed Reflected Transmitted E = A + R + T, R E T A SI Units W m-2 (Joules sec-1 m-2) 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) •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Opaque Solid Kirchoff’s Law Mass emits the same as it absorbs 1. 2. 3. 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% A hot transparent gas produces an emission line spectrum – a series of bright spectral lines against a dark background. 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 Planck’s Law for Full-spectrum Emission I -- Radiation (W m-2 nm-1) c -- speed of light h -- Planck constant K -- Boltzman constant T -- Temperature λ -- Wavelength •Ells Dutton NOAA/ESRL I (relative units λ-1) Stephan-Boltzman Law (ideal BB) ∫ I dλ = σ T4 (realistic, grey) ∫ I dλ = εσ T4 ε = 0→ <1.0 Wein’s Law NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Note: 0.3 μm to 3.0 μm (includes downward scattering) Illustration only ModTran 5.0 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Note: 3.5 μm to >50 μm ModTran 5.0 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Trenberth et al 2009 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Surface Radiation Budget Observations and Research An Integrated approach … RT Models Climate-quality surface BSRN observations More accurate radiative transfer in weather, and climate simulations Direct Observations •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Features • Site scientists • 18+ countries • Stand. Specs. • Long-term IOC • Central archive • Ref. Std. Devlp. • GRP review • GCOS WMO Goal: To acquire the highest possible quality, climatically-diverse, surface-based radiation measurements for climate research Measurements • Direct & diffuse solar* • Downward infrared * • Upwelling rad. • PAR & UV • Aerosol optical depth • Surface meteorology* • Upper air met. * all sites Archiving Provisional Regions Oceanic Tropics Desert Polar Coastal Rain forest Agricultural Prairie EG Dutton, 4Oct2007 Data Applications • GCM comparisons • Satellite validation • Regional climatologies • Global radiation budget • Radiation model testing Radiation Budget Components, time averaging 1-Minute Averages for 1 day Daily Averages for 6 Months 800 400 600 300 Wm W m -2 500 -2 1000 400 200 200 100 0 75.4 75.9 76.4 Day of Year (GMT, 1997) Monthly Averages for 4 Years 500 LW 1997 200 Yearly Averages for 14 Years 300 200 100 100 0 1995 1997.4 400 W m-2 300 1997.2 Year LW SW SW 400 W m-2 0 1996 1997 Year •Ells Dutton NOAA/ESRL 1998 1999 Erie Tower NOAA/CMDL STAR 0 1985 1990 1995 2000 Year NCAR/ASP 1 June 2009 Comparison of Satellite vs. Ground-based Surface Irradiance Products NASA / CERES / SOFA – D. Kratz BSRN BSRN NASA / GEWEX / SRB - P. Stackhouse LW bias <2. SW bias <2. ISCCP Sat. constrained model NASA / CERES / SARB T Charlock / D. Rutan NASA / WCRP / ISCCP - Zhang et al •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Compared to Observations BSRN 251 s BSRN – International Baseline Surface Radiation Network E.G. Dutton •Ells Dutton NOAA/ESRL Pan-GEWEX Mtg 10 Oct 2006 Frascati, Italy NCAR/ASP 1 June 2009 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) BSRN (344) Avg. (337) Circa 2005 GCM models (global means) •Ells Dutton NOAA/ESRL M. Wild 2001& 2005 NCAR/ASP 1 June 2009 Long Term Variability Longest Surface Solar records, Europe Ohmura 2009, JGR Early Brightening •Ells Dutton NOAA/ESRL Dimming Brightening NCAR/ASP 1 June 2009 Observations Confirming Surface IR Global Warming Increases w/ Feedbacks IPCC AR4 GCM Means ---- GHG forcing only ---- GHG + direct aerosol foricing ---- GHG + direct and Indirect aerosol forcing BSRN Observed ~2.5 W m-2/dec GHG forcing •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 “It can be a lot easier to measure something than it is to know what it is that you measured!” Particularly true in radiometry. ∫∫∫∫∫∫∫ I(t,z,x,y,θ,φ,λ) dtdθdφdλdzdxdy = 7.38 mV 10 Altitude (km) 7.5 5 2.5 0.3 •Ells Dutton NOAA/ESRL 0.6 1.0 1.7 3.0 Wavelength (m) NCAR/ASP 1 June 2009 Generic Radiometer and View-Scene Components Radiometer (x), T(x) Signal processor View limiter Intervening medium (possible sources and sinks) Desirable Characteristics T, Source Detector Spectral selection (filter) May or may not have all these components •Stable w/ time •Defined spectral response •Defined geometry •Linear output •Fast response •Low cost Detector types Spectral selection View limiters Photo multiplier Photoelectric cell Thermopile Pyroelectric Cavity Detector sensitivity Interference filter Absorption filter Prism/Grating Adjustable aperture Fixed aperture Flat plate Optics Shutter •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Surface Radiation Budget Component Quantities (A-E) SPACE Θ B SOLAR A EARTH •Ells Dutton NOAA/ESRL C D E THERMAL INFRARED Obs. Site A – Diffuse solar B -- Direct solar, normal C – Reflected solar D – Downward IR E – Upward IR Total Downward Solar = A + cos (Θ) *B NCAR/ASP 1 June 2009 Direct Solar Irradiance • Easiest component to model • High sensitivity to attenuators • Dominant component of total solar when sky is clear 5.7 º Measurement • Relatively easy and accurate to calibrate by absolute cavity • Alignment and tracking required • 0.28 μm to ~3.5 μm Declination axis (manual on this tracker) •Ells Dutton NOAA/ESRL Clock motor on equatorial (right ascension) axis Eppley Labs pyrheliometer (NIP) & tracker NCAR/ASP 1 June 2009 Typical Pyrheliometer Solar Im = cc ·V (with spectral filter → sun photometer) V = f(ΔT,T) ΔT = ε · nIi View limiting aperture Im -- Measured direct solar Irradiance cc -- Assigned linear calibration constant V -- Output voltage T -- Temperature ΔT -- Thermopile T – Ref T ε -- Proportionality “constant” nIi -- Net incident irradiance at detector = Id + Is – Ir + LWi – LWo Id = Direct solar irradiance Is = Scattered solar Ir = Reflected solar LWi = Incoming thermal LW (IR) LWo = Outgoing thermal LW (IR) Clear glass window (filter and Wx protect) Baffles Thermopile Electronics Temp. Comp. Ref Temp. Connector (to voltmeter) •Ells Dutton NOAA/ESRL + •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) NCAR/ASP 1 June 2009 Error sources in Pyrheliometer Meas. Alignment aid (diopter) Instrument Characteristics • Calibration stability • Linearity • Window spectral transmission • Sensitivity temperature dependency • … User Characteristics • Calibration reference & transfer • Alignment & obstructions • Window cleanliness • Data collection system • … •Ells Dutton NOAA/ESRL Kipp& Zonen CH-1 NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Typical Pyranometer Direct IT = cc · V Solar LW (IR) exchange Diffuse SW = cc ∙Voltage Clear glass domes - •Ells Dutton NOAA/ESRL Temp. Comp. Ref Temp. + NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL Eppley PIR NCAR/ASP 1 June 2009 Typical Pyrgeometer IR↓ = cc · V + σTc4 + kσ(Tc4-Td4) Silicon dome (silver) Tdome Incident LW Solar . LW exchange Tcase - Temp. Comp. Ref Temp. + Connector •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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) •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 END (sort of) •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Sprectral Optical Depth Remote Sensing Basics AOD = Aerosol Optical Depth = τλa Beer’s Law Iλ/I0λ = exp(-τλmr) Iλ = cc ∙ Vλ Vλ/V0λ = exp(-τλmr) τλ= τλH2O + τλO3 + τλaerosol +… •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Silicon cell response (RHS) •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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) •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 Barrow Example BSRN SITES Barrow, Alaska Boulder, Colorado Kwajalein, M.I. Boulder - BAO Kwajalein The Three Ways to Transport Thermal Energy • Convention/Advection • Conduction • Electromagnetic Radiation •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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 •Ells Dutton NOAA/ESRL NCAR/ASP 1 June 2009 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, 2007 Longwave (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