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  • Author or Editor: Masataka Shiobara x
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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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