Testing an ensemble model of cirrus ice crystals using midlatitude in situ estimates of ice water content, volume extinction coefficient and the total solar optical depth

Abstract

In this paper an ensemble model of cirrus ice crystals is tested against midlatitude in situ estimates of ice water content, volume extinction coefficient and the total solar optical depth. During the Winter of 2005 and Spring 2006 the FAAM (Facility for Airborne Atmospheric Measurements) BAE-146 G-LUXE aircraft flew three flights as part of the CAESAR (Cirrus and Anvils: European Satellite and Airborne Radiation measurements project) campaign of flying in cirrus around the UK. The suite of microphysical instrumentation onboard the aircraft included the PMS 2D-C probe and the Stratton Park Engineering Company (SPEC) cloud particle imager (CPI). The campaign characterized cirrus properties such as ice water content, volume extinction coefficient, ice crystal geometric shape and ice crystal effective dimension. Cirrus cloud temperatures ranged approximately between 215 and 240 K. From the CPI instrument 60–80% of the ice crystal habits were estimated to be either indeterminate or ‘irregular’ (though such irregular crystals could be composed of pristine components) of some form with hexagonal columns and hexagonal plates accounting for generally much less than 3% of the ice crystal population. The CPI estimated integrated ice water content ranged between 5±2 and 45±22 gm −2, whilst the CPI estimate of the total solar optical depth was found to lie between 0.2±0.1 and 1.0±0.5. The CPI estimate of the mean ice crystal effective dimension was found to range between about 59±9 and 90±75 μm. The particle size distribution (PSD) function was estimated using a PSD scheme that requires as input the in situ estimated IWC and measured in-cloud temperature. The CPI estimates of the bulk and microphysical properties of the midlatitude cirrus are used to test whether an ensemble model of cirrus ice crystals together with a PSD scheme can predict CPI in situ estimates to within the experimental uncertainty. This paper demonstrates that the ensemble model coupled with a PSD scheme can predict the ice water content and the integrated ice water content to generally well within the experimental uncertainty if a varying density with respect to size is assumed. The ensemble model together with a PSD scheme is also shown to predict the CPI estimated volume extinction coefficient and the derived total solar optical depth to generally well within the experimental uncertainty. The paper demonstrates that an ensemble model of cirrus combined with a PSD scheme can predict the radiative properties of cirrus without the need to invoke the concept of an ice crystal effective dimension.