Effects of Ice Crystal Habit on the Radiative Properties and Forcing of Cirrus Clouds

Profiles of in situ measurements of ice crystal size distribution of subtropical cirrus were used to calculate terrestrial (spectral and broadband) irradiances above and below the clouds and their respective radiative forcing. Spheres and non-spherical ice crystal habits (columns, hollows, plates, bullets, and aggregates) were assumed in the calculations. The simulation results were compared to irradiance measurements from pyrgeometers. The microphysical and radiation data were collected by three aircraft during CRYSTAL-FACE. Two cirrus cases (optical thickness of about 1 and 7) from two mission dates (26 and 23 July 2002) were investigated in detail.

The ice crystal shape effects on terrestrial irradiances are most prominent for the optically thin cirrus (up to 70%) observed on 25 July 2002. Spectrally, the largest shape effects are identified in the atmospheric window between about 8-12 microns wavelength. For the low cirrus of moderate optical thickness (23 July 2002) the terrestrial irradiances are less sensitive to crystal habit. For both cirrus clouds it is shown that the terrestrial radiation is completely insensitive to ice crystal shape within the major gas absorption bands. Furthermore it is concluded, that the terrestrial radiative forcing at the top of the atmosphere contains significant crystal shape dependence for the optically thin cirrus cloud only.