Effect of crystal size spectrum and crystal shape on stratiform cirrus radiative forcing

Sensitivities of cirrus cloud radiative forcing as well as solar albedo and infrared emittances to ice crystal size spectrum and ice crystal shape were examined using a coupled cloud-radiation model. The single- and hi-modal crystal size distribution were considered and simulated based on field measurements. Optical parameters of ice crystals shaped as hexagonal columns and random polycrystals (being frequently found in cirrus clouds) were calculated with a ray-tracing method. Both solar and infrared cirrus radiative forcing are influenced by the pattern of crystal size spectra. The net radiative forcing is lower for bi-modal than for single-modal spectra. The solar radiative forcing of cirrus cloud is reduced by nonspherical ice crystals, due to larger albedo effects of nonspherical crystals compared to those of equivalent spherical crystals. Moreover, this reduction in solar radiative forcing by random polycrystals is even larger than that by hexagonal column crystals. The cloud radiative forcing, solar albedo and infrared emittance are changed significantly as the mean crystal size: approaches the smaller size end. Furthermore, net cloud radiative forcing is positive in most cirrus cases. Exceptions are cirrus clouds with a large number (> 10(7) m(-3)) of small (mean maximum dimension < 30 mu m) ice crystals and cirrus clouds with bi-modal crystal size distribution and large particle size for the second maximum peak. (C) 1999 Elsevier Science B.V. All rights reserved.