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- Author or Editor: Tsutomu Takashima x
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Abstract
To investigate the feasibility of retrieving spectral aerosol optical thickness from polarization measurements, the degree of polarization of the sky radiation was measured at a 90° angle from the solar direction in the principal plane with a multichannel polarimeter from ships on the Pacific Ocean and on the Inland Sea of Japan in 1997. The direct solar radiation was also measured simultaneously. Both measurements were made at wavelengths (λ) of 443, 490, 565, 670, 765, and 865 nm.
A lookup table of the degree of polarization with three parameters (the solar zenith angle, the aerosol optical thickness at λ = 550 nm, and the Ångström coefficient) was created from radiative transfer calculations. The aerosol model over the ocean is assumed to be composed of the externally mixed “oceanic” and “water soluble” components, whose size distributions are expressed by the lognormal functions. The size distribution parameters and refractive indices are adopted from the International Radiation Commission reports with a minor modification.
The effective aerosol optical thickness at λ = 550 nm and the Ångström coefficient were simultaneously retrieved from polarization measurements at λ = 443 and 865 nm by referring to the lookup table. The mean and standard deviation of the difference between the aerosol optical thickness retrieved from the polarization measurements and those determined from the direct solar radiation measurements were 0.00 and 0.02. The retrieved Ångström coefficients agreed well with those derived from the direct solar radiation measurements when the optical thickness exceeds 0.1, whereas the difference was about 0.5 when the aerosol optical thickness was only 0.05.
Sensitivities of the retrieval algorithm to the size distribution parameters and the refractive indices of aerosol models are also examined.
Abstract
To investigate the feasibility of retrieving spectral aerosol optical thickness from polarization measurements, the degree of polarization of the sky radiation was measured at a 90° angle from the solar direction in the principal plane with a multichannel polarimeter from ships on the Pacific Ocean and on the Inland Sea of Japan in 1997. The direct solar radiation was also measured simultaneously. Both measurements were made at wavelengths (λ) of 443, 490, 565, 670, 765, and 865 nm.
A lookup table of the degree of polarization with three parameters (the solar zenith angle, the aerosol optical thickness at λ = 550 nm, and the Ångström coefficient) was created from radiative transfer calculations. The aerosol model over the ocean is assumed to be composed of the externally mixed “oceanic” and “water soluble” components, whose size distributions are expressed by the lognormal functions. The size distribution parameters and refractive indices are adopted from the International Radiation Commission reports with a minor modification.
The effective aerosol optical thickness at λ = 550 nm and the Ångström coefficient were simultaneously retrieved from polarization measurements at λ = 443 and 865 nm by referring to the lookup table. The mean and standard deviation of the difference between the aerosol optical thickness retrieved from the polarization measurements and those determined from the direct solar radiation measurements were 0.00 and 0.02. The retrieved Ångström coefficients agreed well with those derived from the direct solar radiation measurements when the optical thickness exceeds 0.1, whereas the difference was about 0.5 when the aerosol optical thickness was only 0.05.
Sensitivities of the retrieval algorithm to the size distribution parameters and the refractive indices of aerosol models are also examined.
Abstract
Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known.
Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends strongly on SZA and moderately on precipitable water and cloud type. The linear relationship provides a simple parameterization to estimate surface-absorbed flux from satellite-measured reflected flux at the TOA. Unlike other models, the present model makes explicit use of the SZA. Precipitable water is included as a secondary parameter. Surface-absorbed fluxes deduced from this simple parameterized model generally agree to within 10 W m−2 with the absorbed fluxes determined from detailed radiative transfer calculations, without including information on the presence or absence of cloud, cloud type, optical thickness, or surface type.
Abstract
Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known.
Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends strongly on SZA and moderately on precipitable water and cloud type. The linear relationship provides a simple parameterization to estimate surface-absorbed flux from satellite-measured reflected flux at the TOA. Unlike other models, the present model makes explicit use of the SZA. Precipitable water is included as a secondary parameter. Surface-absorbed fluxes deduced from this simple parameterized model generally agree to within 10 W m−2 with the absorbed fluxes determined from detailed radiative transfer calculations, without including information on the presence or absence of cloud, cloud type, optical thickness, or surface type.
