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Sherwood, SC (2005). Detection of faceted crystals in deep convective clouds via the antisolar peak. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 110(D14), D14205.

[1] It is shown that an antisolar peak can clearly be detected in backscattered near-infrared reflectances from deep convective cloud tops. This peak should be caused only by regularly faceted ice crystals, primarily columns and plates, rather than rimed or irregular particles. Using 13 years of AVHRR ( advanced very high resolution radiometer) data, the average peak stands 22% above the background for sampled clouds having brightness temperatures below 220 K; the implied mean contribution of pristine crystals to total ice surface area, based on a published model, is 13 +/- 5%. Local estimates of this percentage agree roughly with available field data. The distribution of peak strength is positively skewed, suggesting that a few deep cumuli may be rich in regular particles while a majority contain even less than the 13% mean. The antisolar peak varies geographically, with largest mean values over the central Atlantic and Pacific ITCZs, and smallest values over the Caribbean/ American region during summer. Curiously, there is no significant land- ocean contrast in the antisolar peak, even though MODIS ( Moderate Resolution Imaging Spectroradiometer) data indicate a strong land- ocean contrast in cumulus glaciation temperature, so it does not appear that supercooled water amounts at lower levels regulate the habit distribution observed at cloud top in any straightforward way. Variations in the antisolar peak may be significant for backscatter lidar retrievals of cloud extinction.



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