Effects of ice crystal habit on thermal infrared radiative properties and forcing of cirrus
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The impact of assumed ice crystal morphology on thermal infrared (IR) radiative properties of subtropical cirrus is quantified. In particular, the crystal-shape-dependent profiles of downwelling and upwelling thermal IR (broadband and spectral) irradiances and the radiative forcing of cirrus (at the top and bottom of the atmosphere) are investigated. For this purpose, airborne measurements of ice crystal size distribution (in terms of ice crystal maximum dimension) from the CRYSTAL-FACE campaign and a recently published library of thermal IR optical properties of nonspherical ice crystal habits are implemented into radiative transfer simulations. Two cirrus cases are studied in detail: (1) a high (cold) cirrus with small optical thickness (1 at 10.8 m wavelength) and (2) a lower (warmer) cirrus of relatively large optical thickness (7). The relative effects of ice crystal shape on thermal IR irradiance are substantial for the high, optically thin cirrus (up to 70%). Spectrally, the largest effects of ice crystal shape are identified in the atmospheric window spanning from 8 to 12 m wavelengths, especially for the upwelling irradiance above the cirrus. For the low cirrus of large optical thickness the thermal IR irradiance is only slightly sensitive to ice crystal habit (less than 15-20%). Within the major gas absorption bands the thermal IR radiation is essentially insensitive to ice crystal shape. Furthermore, it is concluded that the thermal IR radiative forcing at the top of the atmosphere contains significant ice crystal shape dependence for the high cirrus case. Copyright 2007 by the American Geophysical Union.
