Direct aerosol forcing: Calculation from observables and sensitivities to inputs

[1] Understanding sources of uncertainty in aerosol direct radiative forcing (DRF), the difference in a given radiative flux component with and without aerosol, is essential to quantifying changes in Earth’s radiation budget. We examine the uncertainty in DRF owing to measurement uncertainty in the quantities on which it depends: aerosol optical depth, single scattering albedo, asymmetry parameter, solar geometry, and surface albedo. Direct radiative forcing at the top of the atmosphere and at the surface is calculated at three locations representing distinct aerosol types and radiative environments. Sensitivities, the changes in DRF in response to unit changes in individual aerosol or surface properties, are also calculated for these conditions. The uncertainty in DRF associated with a given property is computed as the product of the sensitivity and typical measurement uncertainty in the respective property. Sensitivity and uncertainty values permit estimation of total uncertainty in calculated DRF and identification of properties that most limit accuracy in estimating forcing. Absolute total uncertainties in modeled local diurnally averaged forcing range from 0.2 to 3.1 W m � 2 for the ranges of properties examined here. Relative total uncertainties range from � 20 to 80% with larger values at higher latitudes, where fluxes are low. The largest contributor to total uncertainty in DRF is single scattering albedo; however, decreasing measurement uncertainties for any property would increase accuracy in DRF. Comparison of two radiative transfer models suggests the contribution of modeling error is small compared to the total uncertainty although comparable to uncertainty arising from some individual properties.

[1]  W. S. Hartley,et al.  Case Studies of the Vertical Structure of the Direct Shortwave Aerosol Radiative Forcing During TARFOX , 2000 .

[2]  J. Penner,et al.  Aerosol direct radiative effects over the northwest Atlantic, northwest Pacific, and North Indian Oceans: estimates based on in-situ chemical and optical measurements and chemical transport modeling , 2005 .

[3]  P. Chylek,et al.  Effect of absorbing aerosols on global radiation budget , 1995 .

[4]  R. Betts,et al.  Changes in Atmospheric Constituents and in Radiative Forcing. Chapter 2 , 2007 .

[5]  K. Stamnes,et al.  Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media. , 1988, Applied optics.

[6]  C. Gueymard.  The sun's total and spectral irradiance for solar energy applications and solar radiation models , 2004 .

[7]  R. Charlson,et al.  Sulphate aerosol and climate , 1990, Nature.

[8]  Catherine Gautier,et al.  SBDART: A Research and Teaching Software Tool for Plane-Parallel Radiative Transfer in the Earth's Atmosphere. , 1998 .

[9]  S. Kinne,et al.  Aerosol climate effects: Local radiative forcing and column closure experiments , 1997 .

[10]  E. Clothiaux,et al.  Uncertainties in modeled and measured clear‐sky surface shortwave irradiances , 1997 .

[11]  O. Boucher,et al.  Global estimate of aerosol direct radiative forcing from satellite measurements , 2005, Nature.

[12]  A. Jefferson,et al.  Spatial variability of submicrometer aerosol radiative properties over the Indian Ocean during INDOEX , 2002 .

[13]  J. Michalsky,et al.  Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements. , 1994, Applied optics.

[14]  David Crisp,et al.  Intercomparison of shortwave radiative transfer codes and measurements , 2005 .

[15]  S. Schwartz,et al.  Aerosol Optical Depth over Oceans: High Space- and Time-Resolution Retrieval and Error Budget from Satellite Radiometry , 1997 .

[16]  Michael D. King,et al.  A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements , 2000 .

[17]  Alexander Smirnov,et al.  How well do State-of-the-Art Techniques Measuring the Vertical Profile of Tropospheric Aerosol Extinction Compare? , 2006 .

[18]  M. Wendisch,et al.  Measurement-based aerosol forcing calculations: The influence of model complexity , 2001 .

[19]  T. Eck,et al.  Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements , 2000 .

[20]  D. Murcray.  Optical Properties of the Atmosphere , 1968 .

[21]  Robert J. Charlson,et al.  Perturbation of the northern hemisphere radiative balance by backscattering from anthropogenic sulfate aerosols , 1991 .

[22]  Evgueni I. Kassianov,et al.  Retrieval of aerosol microphysical properties using surface MultiFilter Rotating Shadowband Radiometer (MFRSR) data: Modeling and observations , 2005 .

[23]  Yoram J. Kaufman,et al.  An “A-Train” Strategy for Quantifying Direct Climate Forcing by Anthropogenic Aerosols , 2005 .

[24]  B. Forgan,et al.  Aerosol Measurement in the Australian Outback: Intercomparison of Sun Photometers , 2003 .

[25]  P. Russell,et al.  Estimation of aerosol direct radiative effects over the mid‐latitude North Atlantic from satellite and in situ measurements , 1999 .

[26]  J. Coakley,et al.  Climate Forcing by Anthropogenic Aerosols , 1992, Science.

[27]  D. Chu,et al.  Multi‐grid‐cell validation of satellite aerosol property retrievals in INTEX/ITCT/ICARTT 2004 , 2007 .

[28]  L. J. Cox.  Optical Properties of the Atmosphere , 1979 .

[29]  Shepard A. Clough,et al.  Atmospheric radiative transfer modeling: a summary of the AER codes , 2005 .

[30]  Vincent R. Gray.  Climate Change 2007: The Physical Science Basis Summary for Policymakers , 2007 .

[31]  Reto Knutti,et al.  Climate Forcing by Aerosols--a Hazy Picture , 2003, Science.

[32]  J. Haywood,et al.  The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget , 1995 .

[33]  J. Penner,et al.  Quantifying and minimizing uncertainty of climate forcing by anthropogenic aerosols , 1994 .

[34]  Warren J. Wiscombe,et al.  The backscattered fraction in two-stream approximations. , 1976 .

[35]  J. D. Wheeler,et al.  Aerosol backscatter fraction and single scattering albedo: Measured values and uncertainties at a coastal station in the Pacific Northwest , 1999 .

[36]  M. Chin,et al.  A review of measurement-based assessments of the aerosol direct radiative effect and forcing , 2005 .

[37]  A. Kirkevåg,et al.  Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols , 1998 .

[38]  Stephen E. Schwartz,et al.  Comparison of model-estimated and measured diffuse downward irradiance at surface in cloud-free skies , 2000 .

[39]  Christine A. O'Neill,et al.  Effects of Aerosol from Biomass Burning on the Global Radiation Budget , 1992, Science.

[40]  Stephen E. Schwartz,et al.  Uncertainty Requirements in Radiative Forcing of Climate Change , 2004, Journal of the Air & Waste Management Association.

[41]  Michael Schulz,et al.  Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations , 2006 .

[42]  A. Smirnov,et al.  AERONET-a federated instrument network and data archive for aerosol Characterization , 1998 .

[43]  P. Pilewskie,et al.  Airborne measurements of spectral direct aerosol radiative forcing in the Intercontinental chemical Transport Experiment/Intercontinental Transport and Chemical Transformation of anthropogenic pollution, 2004 , 2006 .

[44]  Gail P. Anderson,et al.  Shortwave radiative closure studies for clear skies during the Atmospheric Radiation Measurement 2003 Aerosol Intensive Observation Period , 2006 .

[45]  David J. Delene,et al.  Variability of Aerosol Optical Properties at Four North American Surface Monitoring Sites , 2002 .

[46]  C. Gautier,et al.  The Effect of Non-Lambertian Surface Reflectance on Aerosol Radiative Forcing , 2005 .