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Haruma Ishida and Shoji Asano

Abstract

A new calculation scheme is proposed for the explicitly discretized solution of the three-dimensional (3D) radiation transfer equation (RTE) for inhomogeneous atmospheres. To separate the independent variables involved in the 3D RTE approach, the spherical harmonic series expansion was used to discretize the terms, depending on the direction of the radiance, and the finite-volume method was applied to discretize the terms, depending on the spatial coordinates. A bidirectional upwind difference scheme, which is a specialized scheme for the discretization of the partial differential terms in the spherical harmonic-transformed RTE, was developed to make the equation determinate. The 3D RTE can be formulated as a simultaneous linear equation, which is expressed in the form of a vector–matrix equation with a sparse matrix. The successive overrelaxation method was applied to solve this equation. Radiative transfer calculations of the solar radiation in two-dimensional cloud models have shown that this method can properly simulate the radiation field in inhomogeneous clouds. A comparison of the results obtained using this method with those using the Monte Carlo method shows reasonable agreement for the upward flux, the total downward flux, and the intensities of radiance.

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Masataka Shiobara and Shoji Asano

Abstract

A method is proposed to estimate the optical thickness of cirrus clouds from ground-based sun photometry. Transfer calculations of solar radiation in ice clouds were made by the Monte Carlo method. A scattering phase function presented by Takano and Liou was employed for ice clouds. Simulations of sun photometry, which include strong forward scattering into the instrument's field of view, give a simple relationship between the true and apparent optical thicknesses. The correction method was applied to Sun photometer measurements for cirrostratus clouds observed at Tsukuba, Japan. The relationship between the visible optical thickness and the broadband solar flux transmittance obtained from observations agreed well with that theoretically expected for cloud optical thickness up to about 10.

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Giichi Yamamoto, Masayuki Tanaka, and Shoji Asano

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The problem of diffuse reflection, transmission and emission of infrared radiation by water clouds is investigated in the wavelength region from 5–50 μ. The drop-size distribution of clouds is assumed to be that of altostratus measured by Diem. The phase function and other optical properties of the clouds are estimated from the value of the refractive index of water proposed by Pontier and Dechambenoy. Radiative processes due to both cloud droplets and water vapor in the cloud are taken into account, and a method of averaging the solution over a spectral interval including a number of absorption lines is developed.

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Shoji Asano, Masataka Shiobara, and Akihiro Uchiyama

Abstract

A new method is proposed to retrieve various cloud physical parameters of water clouds from the solar-flux reflectances at four wavelengths measured by using the airborne Multi-channel Cloud Pyranometer (MCP) system. The MCP system was designed to measure the spectral reflectances at nine wavelengths in the visible and near-infrared region. The estimation procedure assumes a locally plane-parallel and vertically homogeneous water-cloud layer with monomodal particle size distributions of a fixed width. The cloud optical thickness τ500 and the effective particle radius re, of the water-cloud layer can be simultaneously retrieved from the MCP reflectances measured at the visible channel of λ=500 nm and at the near-infrared channel of λ=1650 nm. Under the assumption that cloud pressure height is known, the cloud liquid water content (LWC) can then be retrieved from the MCP reflectance at the oxygen absorption-band channel cantered at λ=760 nm. Finally, the in-cloud water vapor amount u H2O can be estimated from the MCP reflectance at the water vapor absorption-band channel centered at λ=938 nm. Using these directly retrieved parameters, we can estimate byproduct parameters such as integrated liquid water path, cloud particle concentration, and geometrical thickness of the homogeneous cloud layer. A reliably applicable range of the present method was estimated to be 1≤&tau500<100, 4 µm ≤re<25 µm, LWC≤1 g m−3, and u H2O≤10 g m−3.

The present retrieval method was applied to the MCP spectral reflectance data obtained through aircraft observations for wintertime stratocumulus clouds over the ocean south of Tokyo, made as part of the Western North Pacific Cloud-Radiation Experiment/Meteorological Research Institute program in Decembers of 1989 and 1990. Reasonable values of the cloud physical parameters were successfully retrieved for the stratocumulus clouds. However, a comparison with the simultaneous in situ cloud measurement showed that the estimated effective particle radii and liquid water content were lager than the in situ measured values.

Statistical microphysical features of the marine stratocumulus clouds have been investigated by taking various correlations among the contemporaneous cloud physical parameters estimated from the MCP spectral reflectances. It was found that, for thin water clouds with τ500<∼20, the effective particle radius was positively correlated with cloud optical thickness, but for thicker clouds with τ500<∼20, there was a negative correlation between them. For both cases, however, liquid water content was positively correlated with cloud optical thickness. It is suggested that, for thin clouds in the dissipating stage, evaporation might be dominant process of cloud particle formation; on the other hand, coalescence might be dominant in thick clouds in the mature stage with precipitation.

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Shoji Asano, Masataka Shiobara, Yuji Nakanishi, and Yukiharu Miyake

Abstract

The design and performance of a spectral radiometer system are described for airborne measurements of solar flux reflectance by clouds. The system consists of a pair of identical multichannel pyranometers: one installed on the top and the other on the bottom of an aircraft fuselage to measure the downward and upward solar irradiances, respectively. This measurement scheme has an advantage in that reflectances derived from ratios between the upward and downward irradiances can avoid the need for absolute radiometric calibrations. The multichannel cloud pyranometer (MCP) system measures near-monochromatic solar irradiances at nine discrete wavelengths between 420 and 1650 nm by using interference filters with very narrow bandwidths. Included among these wavelengths are 760 and 938 nm in the oxygen and water vapor absorption bands, respectively. Solar radiation passing through the filters is instantly detected by a silicon photodiode for wavelength λ<1 µm and by a germanium photodiode for λ>1 µm. Good performance of the MCP system was confirmed through laboratory calibrations and airborne tests. The MCP system is suitable for remote sensing application to retrieve cloud physical parameters of water clouds from airborne spectral reflectance measurements.

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Masataka Shiobara, James D. Spinhirne, Akihiro Uchiyama, and Shoji Asano

Abstract

Optical depths in the visible to infrared spectral region were obtained from solar extinction measurements with two sun photometers during the First ISCCP Regional Experiment Phase II Cirrus Intensive Field Observation in Kansas.

A method is described to correct sun photometry for gaseous absorption and is extended to estimate the water vapor amount. The approach uses a prior computation of gaseous absorption for the narrowband-pass sun photometry, parameterized with the slant-path absorber amount. These produce correction coefficients for gaseous absorption, as determined by LOWTRAN 7 models. Near-infrared channels were calibrated by modified Langley plots taking account of gaseous absorption.

After the correction and calibration, the aerosol optical depths at the wavelength of 0.4–4 µm were obtained for clear sky conditions. The aerosol optical depth at the wavelength λ = 0.5 µm was 0.1–0.2 during the campaign. The cloud optical depth at λ = 0.5 µm was obtained for cirrus events on 26 November and 5 December 1991 correction of multiple scattering effects involved in sun photometry. The column amount of water vapor was estimated from the 0.94-µm-channel measurement and compared with results from radiosonde measurements. The comparison has shown a good agreement within a 10% difference during the campaign when the equivalent water vapor amount ranges from 0.3 to 1.2 g cm−2.

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Shoji Asano, Akihiro Uchiyama, Akihiro Yamazaki, and Katsuyuki Kuchiki

Abstract

As an international collaborative research activity within the Japanese Cloud-Climate Study (JACCS) program, the authors participated in the second Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE II) using the Meteorological Research Institute (MRI) radiometers. This paper describes results of ARESE II, as well as specifications and calibration of the MRI radiometers. The solar radiation budget for 2 days of typical clear sky (27 February and 20 March 2000) and overcast sky (3 and 21 March 2000) has been analyzed using spatially collocated, total-band solar irradiances measured by the MRI pyranometers (Kipp & Zonen CM21). These were installed on a Twin Otter aircraft, and deployed at the ARM Southern Great Planes Central Facility site. On average, the clear-sky and overcast-sky air columns between the surface and the Twin Otter flight level of 7 km absorbed about 13% ± 2% and 20% ± 3%, respectively, of the total-band solar radiation incident on the column top. The measured solar radiation budgets agree well with those computed for models of clear and cloudy atmospheres. The present results indicate no evidence of anomalous solar absorption for either the clear- or cloudy-sky cases. It is suggested that about half of the observed absorption enhancement of 7% for the overcast-sky cases could be caused by the presence of larger water vapor, compared with the clear-sky cases, and that the other half could be caused by increased absorption within and above the rather low cloud layers.

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Tomoaki Nishizawa, Shoji Asano, Akihiro Uchiyama, and Akihiro Yamazaki

Abstract

The surface direct radiative forcing and optical properties of aerosols have been analyzed from a ground-based solar radiation measurement, which was made under clear-sky conditions in Tsukuba, Japan, over two years from April 1997 to March 1999. The global and diffuse irradiances in the total and near-infrared (NIR) solar spectral regions were simultaneously measured by using two sets of the total-band and NIR-band pyranometers, respectively. The visible (VIS)-band irradiances were estimated by taking differences between the total-band and NIR-band irradiances. Spectral aerosol optical thicknesses (AOTs) in the air column were also measured, using a sun photometer. By combining the spectral AOTs and the surface diffuse irradiances, a retrieval algorithm for simultaneously estimating the effective aerosol size distribution and imaginary index of refraction (m i) was developed. Seasonal variations of the broadband surface radiative forcings and retrieved optical properties of the columnar aerosols have been studied. A close correlation was found among these parameters with similar features of seasonal variations. In winter the columnar aerosols exhibit the minimum surface radiative forcing and a minimum AOT, but the maximum m i value of 0.04. The opposite is true in summer, when the minimum m i value of 0.02 was estimated. The surface radiative forcing in the VIS band was estimated to be almost 4 times larger than in the NIR band. The total-band aerosol forcing efficiency is defined as the change in the surface radiative forcing in the total band due to a unit increase of AOT at 500 nm. This has its largest magnitude of −219 W m−2 in winter and its smallest magnitude of −150 W m−2 in summer. The results suggest that the correlated seasonal variations between the aerosol radiative forcing and the optical properties may result from seasonal changes in the dominant aerosol components.

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