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A. Ono

Abstract

The growth mode of ice crystals in natural clouds has been studied by examination of replicas of more than 10,000 crystals sampled in about 120 clouds over the temperature range −2 to −32C.

The relationship between the basic ice crystal form deduced from the shape of recently formed small ice crystals and the temperature at which they have grown is found to agree well with the results of laboratory experiments.

Within each category of basic crystal form, there is some characteristic and systematic variation in the ratio of crystal axes which depends strongly upon the growth temperature. The growth of an ice crystal along either the a or c axes almost ceases after it reaches a certain length which depends upon the growth temperature. For columnar ice crystals, the ratio of the length of the c axis to the a axis at a given column length has a maximum value at about −5C. For plane ice crystals, the ratio of diameter to thickness for a given diameter has a maximum value at about −15C.

Some consideration is given to the growth rate of ice crystals based upon the observed ratio of the crystal axes and the bulk density. It is shown that the two pronounced peaks in the growth rate obtained by Hallett, a major peak at about −15C and a minor one at about −5C, can be explained in terms of the characteristic axial ratio of the crystals.

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A. Ono

Abstract

Individual ice crystals have been collected and replicated. The effect of the falling motion is found to enhance the growth along the “c” axis of columns and the dendritic growth of plane ice crystals. It is found that the onset of riming of ice crystals is controlled by crystal size; for columnar crystals the most sensitive parameter is the length along the “a” axis, while for plane crystals it is the diameter. Freely-falling ice crystals are oriented so as to present the maximum resistance to motion.

Some consideration is also given to the collection efficiency of columnar ice crystals for cloud droplets.

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S. C. Mossop and A. Ono

Abstract

Measurements of ice crystal concentration in five clouds in northern New South Wales are reported. These confirm earlier studies in which it was found that glaciated altostratus clouds contain approximately the same concentration of ice crystals and ice nuclei. On the other hand, cumulus and stratocumulus, generally sampled at temperatures >−10C, were found to contain about 103 times as many ice crystals as expected on the basis of ice nucleus measurements.

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A. Ono, F. Sakuma, K. Arai, Y. Yamaguchi, H. Fujisada, P. N. Slater, K. J. Thome, F. D. Palluconi, and H. H. Kieffer

Abstract

Preflight and in-flight radiometric calibration plans are described for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) that is a multispectral optical imager of high spatial resolution. It is designed for the remote sensing from orbit of land surfaces and clouds, and is expected to be launched in 1998 on NASA's EOS AM-I spacecraft. ASTER acquires images in three separate spectral regions, the visible and near-infrared (VNIR), the shortwave infrared (SWIR), and the thermal infrared (TIR) with three imaging radiometer subsystems. The absolute radiometric accuracy is required to be better than 4% for VNIR and SWIR radiance measurements and 1 to 3 K, depending on the temperature regions from 200 to 370 K, for TIR temperature measurements.

A reference beam is introduced at the entrance pupil of each imaging radiometer to provide the in-flight calibration. Thus, the ASTER instrument includes internal onboard calibration units that comprise incandescent lamps for the VNIR and SWIR and a blackbody radiator for the TIR as reference sources. The calibration reliability of the VNIR and SWIR is enhanced by a dual system of onboard calibration units as well as by high-stability halogen lamps. A ground calibration system of spectral radiances traceable to fixed-point blackbodies is used for the preflight VNIR and SWIR calibration.

Because of the possibility of nonuniform contamination effects on the partial-aperture onboard calibration, it is desirable to check their results with respect to other methods. Reflectance- and radiance-based vicarious methods have been developed for this purpose. These, and methods involving in-flight cross-calibration with other sensors are also described.

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