Role of small ice crystals in radiative properties of cirrus: a case study

Aircraft observations of cirrus cloud were made near Coffeyville, Kansas, during November 1991 as part of the FIRE II project. Cloud ice particle spectra measurements were made using both a PMS 2DC probe and an ice particle replicator. Particles larger than 200 μm were column rosettes. The replicator shows the presence of large numbers of ice crystals smaller than 66 μm (two PMS size bins) that are not recorded by the PMS 2DC probe. Calculations based on the replicator data of the geometrical blocked area and absorption cross section of the cloud per unit volume show that small particles can contribute significantly to and sometimes dominate both the solar extinction and the infrared emission. Intercomparison is made of the ice particle size, area, and mass distributions determined by these different instruments. Power law relationships for area occluded by a crystal as a function of crystal maximum dimension were computed from the PMS 2DC data. The wavelength-dependent infrared absorption cross section per volume was computed using a simple model based on anomalous diffraction and area and mass dimensional relationships for the ice crystals.

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