Sensitivities of cirrus cloud radiative forcing as well as solar albedo and
infrared emittances to ice crystal size spectrum and ice crystal shape wer
e examined using a coupled cloud-radiation model. The single- and hi-modal
crystal size distribution were considered and simulated based on field meas
urements. Optical parameters of ice crystals shaped as hexagonal columns an
d random polycrystals (being frequently found in cirrus clouds) were calcul
ated with a ray-tracing method. Both solar and infrared cirrus radiative fo
rcing are influenced by the pattern of crystal size spectra. The net radiat
ive 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 cir
rus 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 se
cond maximum peak. (C) 1999 Elsevier Science B.V. All rights reserved.