Masataka Shiobara

Calibration of a sunphotometer by simultaneous measurements of direct-solar and circumsolar radiations

A new method is proposed for the calibration of the sunphotometer. Well-known difficulties of the usual Langley-plot method when applied to unsteady turbidity conditions can be avoided by monitoring the circumsolar radiation. To realize this idea, an alternate of the Langley-plot method is developed, in which the logarithm of the sunphotometer reading is plotted against the ratio of intensity of singly scattered circumsolar radiation to that of direct solar radiation instead of the optical air mass in the usual Langley-plot method. Results of numerical simulations and field tests with a newly developed instrument show that the rms error of the calibration constant could be reduced to 1/5–1/10 of the usual method for wavelengths larger than 500 nm.

Spectral Measurements of Direct Solar Radiation and the Sky Brightness Distribution at Syowa Station, Antarctica

Spectral measurements of the solar radiation were performed at Syowa Station (69°00′ S: 39°35′E), Antarctica, from June 1984 to January 1985 as a research project of the 25 th Japanese Antarctic Research Expedition. The spectral direct-solar radiation was measured by a sunphotometer which was equipped with eight interference filters of 330, 369, 500, 675, 776, 862, 938, and 1048 nm. The aerosol optical thickness at the 500 nm wavelength was about 0.08 as an average of the observed values during January to April 1984, but it decreased to about 0.04 during September 1984 to January 1985. These values of the aerosol optical thickness were much larger than those reported previously by several authors, and were attributed to El Chichon aerosols. The inversion of these data showed that a mono-modal volume spectrum of aerosols with the mode radius of about 0.4 μm was predominant throughout the year, and that the loading of giant particles was relatively enhanced in spring and autumn. The surface albedo around Syowa Station was estimated from measured values of the brightness and degree of polarization of skylight.

Optical Properties of the Turbid Atmosphere in the Yellow Sand Event Over Japan

The optical properties of yellow sand particles were examined by comprehensive measurements carried out during the period of April 21–May 12, 1982 in Nagasaki, Japan. In situ measurements of the phase functions of aerosols were performed for both parallel and perpendicularly polarized components with a portable polar nephelometer, as well as spectral extinction measurements with a sunphotometer, and measurements of the aureole intensity with an aureolemeter. Large depolarization ratio values of about 0.3 and a power-law size distribution with an exponent around 3.0 were characteristically found in the yellow sand event, indicating an abundance of large, irregular sand particles. On the other hand, a powerlaw size distribution with an exponent of around 4.0, i. e., a typical Junge distribution, was well developed in the atmosphere for normal conditions over Japan. The size spectrum and complex index of refraction of aerosols retrieved from the light scattering measurements were found to be useful in parameterizing the phase functions of yellow sand particles, although an unrealistically large value was retrieved for the imaginary index of refraction.

Atmospheric Turbidity Monitoring by use of a Scanning Radiometer

The atmospheric turbidity was monitored by a 5-channel scanning radiometer (aureolemeter) for the period of September 1981–May 1985 in the suburbs of Sendai, Japan, The aureolemeter was designed to be able to measure both direct-solar and circumsolar radiation simultaneously, and was calibrated by a newly developed algorithm, which is far more accurate than the usual Langley-plot method. The measured intensity of circumsolar radiation was reduced to the single scattering phase function after subtraction of multiply scattered radiation. The obtained values of the phase function at a scattering angle of 20° were linearly correlated with the aerosol optical thickness and, thus, provided good indices of the atmospheric turbidity. Temporal variations of the aerosol optical thickness and the volume spectrum were retrieved from the spectral extinction and aureole data. An enhancement of monomodal aerosols, with a mode radius of about 0.5 μm and due to the El Chichon eruption, was distinguished from typical bimodal spectra of background aerosols for the period from the autumn of 1982 to the spring of 1984.