Using in situ estimates of ice water content, volume extinction coefficient, and the total solar optical depth obtained during the tropical ACTIVE campaign to test an ensemble model of cirrus ice crystals
Authors: Baran Anthony J. , Connolly Paul J. , Heymsfield A. J. , Bansemer A. 
Published: January 2011

An ensemble model of cirrus ice crystals combined with a parametrized particle size distribution function (PSD) is used to predict the ice water content (IWC), column‐integrated IWC (ice water path, IWP), volume extinction coeﬃcient, and the total solar optical depth, for ﬁve tropical cirrus cases. The PSD is estimated from the IWC and in‐cloud temperature, and comparisons are presented between the ensemble model predictions and in situ estimates of these microphysical and macrophysical quantities. The in situ estimates were obtained during the Aerosol and Chemical Transport In tropical conVection (ACTIVE) campaign between November–December 2005 and January–February 2006, based at Darwin, Australia. The microphysical instrumentation deployed on the Airborne Australia Egrett research aircraft were the SPEC Cloud Particle Imaging (CPI) probe, Cloud and Aerosol Spectrometer (CAS) and Cloud Imaging Probe (CIP). The CPI was used to measure ice crystal size from about 5 to 1800 μm, ice crystal number concentration, and to estimate ice crystal shape, IWC, IWP, volume extinction coeﬃcient and the total solar optical depth. The CIP instrument was also used to measure ice crystal size from about 25 to 1550 μm, ice crystal number concentration and to estimate IWC. For all ﬂights the limited CPI shape recognition algorithm recorded that about 80% or greater of the ice crystal populations were composed of small irregular or ‘quasi‐spherical’ ice crystals. The CPI‐ and CIP‐estimated IWC distributions are compared against each other and it is shown that the distributions are not signiﬁcantly diﬀerent at the 95% level of conﬁdence. The CPI‐estimated averaged IWC ranged between approximately 5.3 and 98.2 mg m−3. The CPI‐estimated IWP and total solar optical depth ranged between ∼1.0 ± 0.5 and 35.0 ± 17 g m−2 and between 0.1 ± 0.05 and 1.46 ± 0.73, respectively.

To predict the IWC and IWP, an ensemble model eﬀective density‐size relationship is derived, and it is shown that the uncertainty in the model predictions are generally within the uncertainty of the CPI estimates for all cases considered. It is also demonstrated that, when the CPI‐estimated total solar optical depth is greater than unity, the ensemble model combined with the PSD scheme predicts an uncertainty in the volume extinction coeﬃcient and total solar optical depth that is within the CPI experimental range of uncertainty. However, for total solar optical depths much less than unity, the ensemble model combined with the PSD scheme does not generally predict an uncertainty in the volume extinction coeﬃcient and total solar optical depth that is within the lower range of the CPI uncertainty; the physical reason for this is further explored.

The paper demonstrates that there is predictive value in combining an ensemble model of ice crystals with a universal PSD scheme to predict the microphysical and macrophysical properties of importance to radiative transfer through tropical cirrus. Moreover, in the case of very low IWC tropical cirrus, further characterization of the PSD is required using a number of in situ instruments. Copyright © Royal Meteorological Society and Crown Copyright, 2011