Some Microphysical Properties of an Ice Cloud from Lidar Observation of Horizontally Oriented Crystals

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Abstract
Lidar observations of a winter ice cloud in the zenith gave a very high reflection but a very small depolarization. When the lidar was tilted more than 0.5° away from the zenith, the reflection amplitude fell to 3% of it's zenith value, but the depolarization increased. The above properties proved unambiguously that reflection was occurring from the specular surfaces of horizontal crystals. These properties were used to estimate some cloud microphysical properties. At a selected time, the estimates gave a mean “diameter” of 74 μm for the horizontal faces, a crystal number density of 0.78 ℓ−1, and a maximum departure of the crystal axis from the horizontal of 0.5°. The fraction of the total crystal cross section which was specularly reflecting was estimated as unity. Abstract Lidar observations of a winter ice cloud in the zenith gave a very high reflection but a very small depolarization. When the lidar was tilted more than 0.5° away from the zenith, the reflection amplitude fell to 3% of it's zenith value, but the depolarization increased. The above properties proved unambiguously that reflection was occurring from the specular surfaces of horizontal crystals. These properties were used to estimate some cloud microphysical properties. At a selected time, the estimates gave a mean “diameter” of 74 μm for the horizontal faces, a crystal number density of 0.78 ℓ−1, and a maximum departure of the crystal axis from the horizontal of 0.5°. The fraction of the total crystal cross section which was specularly reflecting was estimated as unity.

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