Uncertainties in modeled and measured clear‐sky surface shortwave irradiances

A comparison of five independent measurements of the clear-sky downward shortwave irradiance at the surface shows that they scatter within a 5% range depending on their calibration constants. When the measurements are corrected using data from two cavity radiometers, three of the five independent measurements agree within 3 W m−2 over three clear-sky days, which is well within the estimated error limit of ±1.5%. A comparison of these three sets of irradiance measurements with the computed irradiance by a δ2-stream model reveals that the model overestimates the irradiance by 5%. Detailed investigation of the approximations and uncertainties associated with the computations (including the measurement error in the water vapor and ozone amounts, neglecting the state of polarization and trace gas absorption, the 2-stream approximation, the neglect of the spectral dependence of the surface albedo, and the uncertainties associated with aerosols) demonstrates that the discrepancy is not due to these approximations. Further analysis of the modeled and measured irradiance shows that the discrepancy is almost entirely due to the difference between modeled and measured diffuse field irradiances. An analysis of narrow-band diffuse to total irradiance ratios shows that this discrepancy is the largest near 400 nm and decreases with wavelength. These results rely on the absolute calibrations of two cavity radiometers, two shaded pyranometers, and one unshaded pyranometer, as well as ratios of irradiances measured by a multifilter rotating shadow-band radiometer. Therefore, in order for instrumental error to account for the diffuse field discrepancy, three independent measurements of the diffuse field irradiance must be biased low by at least 40%. For an aerosol to account for this discrepancy, it must be highly absorbing with a single-scattering albedo as low as 0.3. The unlikelihood of instrumental errors of 40% and aerosol single-scattering albedos of 0.3 suggests a third possibility: the neglect of some gaseous absorption process at visible wavelengths.

[1]  Anders Ångström,et al.  On the Atmospheric Transmission of Sun Radiation and on Dust in the Air , 1929 .

[2]  A. K. Pierce,et al.  Analysis of Limb Darkening Observations , 1961 .

[3]  M. Thekaekara.  Proposed standard values of the solar constant and the solar spectrum , 1970 .

[4]  James A. Weinman,et al.  Radiative Properties of Carbonaceous Aerosols , 1971 .

[5]  W. Pollock,et al.  Tropospheric aerosol: The relative contribution of marine and continental components , 1973 .

[6]  K. Liou.  Analytic Two-Stream and Four-Stream Solutions for Radiative Transfer , 1974 .

[7]  J. Hansen,et al.  Light scattering in planetary atmospheres , 1974 .

[8]  E. M. Patterson,et al.  Commonalities in measured size distributions for aerosols having a soil-derived component , 1977 .

[9]  K. T. Whitby.  THE PHYSICAL CHARACTERISTICS OF SULFUR AEROSOLS , 1978 .

[10]  Michael D. King,et al.  Aerosol size distributions obtained by inversion of spectral optical depth measurements , 1978 .

[11]  M. Box,et al.  Atmospheric scattering corrections to solar radiometry. , 1979, Applied optics.

[12]  Rudolf B. Husar,et al.  Spatial and temporal pattern of eastern U.S. haziness: A summary , 1981 .

[13]  A. T. Young.  On the Rayleigh-Scattering Optical Depth of the Atmosphere , 1981 .

[14]  Jack G. Calvert,et al.  Primary quantum yields of NO2 photodissociation , 1987 .

[15]  A Goldman,et al.  The HITRAN database: 1986 edition. , 1987, Applied optics.

[16]  F. X. Kneizys,et al.  Users Guide to LOWTRAN 7 , 1988 .

[17]  T. Ackerman,et al.  A simple formulation of the delta-four-stream approximation for radiative transfer parameterizations , 1988 .

[18]  C. McKay,et al.  Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres , 1989 .

[19]  P. Teillet,et al.  Rayleigh optical depth comparisons from various sources. , 1990, Applied optics.

[20]  B. Bonnel,et al.  Intercomparing shortwave radiation codes for climate studies , 1991 .

[21]  Eric P. Shettle,et al.  Atmospheric Aerosols: Global Climatology and Radiative Characteristics , 1991 .

[22]  Q. Fu,et al.  On the correlated k-distribution method for radiative transfer in nonhomogeneous atmospheres , 1992 .

[23]  P. Hobbs,et al.  Optical extinction properties of smoke from the Kuwait oil fires , 1992 .

[24]  G. Kattawar,et al.  Effect of volume-scattering function on the errors induced when polarization is neglected in radiance calculations in an atmosphere-ocean system. , 1993, Applied optics.

[25]  D. M. Moss,et al.  Spectral reflectance measurements in the genus Sphagnum , 1993 .

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

[27]  Andrew A. Lacis,et al.  Errors induced by the neglect of polarization in radiance calculations for rayleigh-scattering atmospheres , 1994 .

[28]  Darrel L. Williams,et al.  High-spectral resolution field and laboratory optical reflectance measurements of red spruce and eastern hemlock needles and branches , 1994 .

[29]  M. Iacono,et al.  Line‐by‐line calculation of atmospheric fluxes and cooling rates: 2. Application to carbon dioxide, ozone, methane, nitrous oxide and the halocarbons , 1995 .

[30]  Martin Wild,et al.  Validation of general circulation model radiative fluxes using surface observations , 1995 .

[31]  Barbara E. Carlson,et al.  Nonsphericity of dust‐like tropospheric aerosols: Implications for aerosol remote sensing and climate modeling , 1995 .

[32]  Proceedings of the Annual Conference on Atmospheric Transmission Models (17th) Held at Hanscom AFB, MA on 8-9 Jun 94. , 1995 .

[33]  Thomas P. Charlock,et al.  The CERES/ARM/GEWEX Experiment (CAGEX) for the Retrieval of Radiative Fluxes with Satellite Data , 1996 .

[34]  D. W. Nelson,et al.  Optimal Measurement of Surface Shortwave Irradiance Using Current Instrumentation , 1997 .