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Vincent Noel and Hélène Chepfer

Abstract

The goal of this paper is to retrieve information about ice particle orientation in cirrus clouds. This is achieved by comparing simulations of sunlight reflection on a cirrus cloud with measurements of polarized radiances from the spaceborne instrument Polarization and Directionality of the Earth's Reflectance (POLDER-1) on Advanced Earth Observing Satellite-1 (ADEOS-1). Results show that horizontal orientation of crystals can be spotted by the presence of a local maximum of polarized radiance in the direction of specular reflection. The angular width of the local maximum is shown to contain information on the particle maximum deviation angle, while the maximum intensity can provide information on particle shape and relative concentrations of ice crystals, horizontally and randomly oriented. The study of 31 ice cloud cases show that in 80% of them, the deviation angle is less than 3°. Also, the relative concentration of horizontally oriented crystals is less than 21%, depending on the angular distribution used for crystal deviation.

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Vincent Noel, Helene Chepfer, Martial Haeffelin, and Yohann Morille

Abstract

This paper presents a study of ice crystal shapes in midlatitude ice clouds inferred from a technique based on the comparison of ray-tracing simulations with lidar depolarization ratio measured at 532 nm. This technique is applied to three years of lidar depolarization ratio observations from the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA) observatory in Palaiseau, France, amounting to 322 different days of ice cloud observations. Particles in clouds are classified in three major groups: plates, columns, and irregular shapes with aspect ratios close to unity. Retrieved shapes are correlated with radiosounding observations from a close-by meteorological station: temperature, relative humidity, wind speed, and direction.

Results show a strong dependence of the relative concentration of different crystal shapes to most atmospheric variables, such as the temperature, with a clear successive dominance by platelike (temperature above −20°C), irregular (temperatures between −60° and −40°C), and columnlike shapes (temperatures below −60°C). Particle shapes are almost exclusively columnlike below −75°C. This is in sharp contrast with previous results of the same classification applied to tropical clouds, and highlights the high geographic variability of the ice clouds distribution of microphysical properties. Results also suggest that ice clouds created by jet streams (identified by high wind speeds) are strongly dominated by columnlike shapes, while front-created ice clouds (identified by lower wind speeds) show a much more variable mix of shapes, with the dominant shapes depending on other factors. Results also suggest different microphysical properties according to the average direction source of air masses and winds. Following these results, a possible parameterization of ice crystal shapes in midlatitude ice clouds as a function of temperature is provided.

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