Ground-Based Cirrus Observation

I. Observation System and Results of Frontal Cirrostratus Clouds on June 22 and 30, 1989

Abstract

A ground-based observation system of high-level ice clouds has been developed at the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) for simultaneous measurements of cloud microphysical, and radiative properties. The observation system has been designed by integrating the MRI hydrometeor-video-sondes (HYVIS), a lidar, and various radiometers to obtain simultaneous data both of the cloud microphysical properties and solar and atmospheric radiation measured at the surface. In this paper, the ground-based observation system is described and the cloud physical structures and radiative properties have been analyzed for thick cirrostratus clouds associated with the stationary Bai-u fronts, on June 22 and 30, 1989.
The size distribution of ice-crystals measured by HYVIS was approximated by a power-law function with an exponent of 3.2 averaged over the entire cloud layers. Evident temperature dependence was not found in the size distribution functions. The broadband solar transmittances derived from the solar irradiance measurements were related to the coincident visible optical thicknesses derived from the sunphotometer measurement, and compared with theoretically simulated transmittances. The comparison revealed that the single scattering asymmetry factor for ice-crystals should be smaller than that of spherical counterparts. The downward effective emissivity was estimated from the 10.5μm radiances measured by the radiation-thermometer. Further, spectral distribution of the downward effective emissivity in the wavenumber region from 800 to 1200cm^-1 was also estimated from the spectral zenith radiance measurement by the FTIR radiometer. The observed cirrostratus clouds were optically thick with visible optical thicknesses larger than 1.0, and effective emissivities greater than 0.4.