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Kazuhiko Masuda
,
H. G. Leighton
, and
Zhanqing Li

Abstract

An earlier parameterization that relates the outgoing solar flux at the top of the atmosphere to the flux absorbed at the surface is modified and extended to allow for variations in atmospheric properties that were not considered in the original parameterization. Changes to the parameterization have also been introduced as a result of better treatment of water vapor absorption in the detailed radiative transfer calculations. Corrections are introduced that account for the height of the surface (surface pressure), ozone amount, aerosol type and amount, and cloud height and cloud type, which is characterized by an effective cloud droplet radius. Finally, the results of applying the parameterization to Earth Radiation Budget Satellite measurements are compared with the measurements of the net solar flux measured from two instrumented towers.

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Kazuhiko Masuda
,
Masayuki Sasaki
,
Tsutomu Takashima
, and
Hiroshi Ishida

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.

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Zhanqing Li
,
H. G. Leighton
,
Kazuhiko Masuda
, and
Tsutomu Takashima

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.

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