A Test of Recent Bulk Optical Models to Represent the Radiative Properties of Cirrus Cloud Across the Mid- and Far-Infrared.

Abstract:

Measurements of mid- to far-infrared nadir radiances obtained from the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX) are used to assess the performance of a selection of currently available ice cloud bulk optical (single-scattering) property models. Through use of a minimisation approach, we find that the simulations can reproduce the observed spectra in the mid-infrared to within measurement uncertainty but are unable to simultaneously match the observations over the far-infrared frequency range. When both mid and far-infrared observations are used to minimise residuals, first order estimates of the flux differences between the best performing simulations and observations indicate a strong compensation effect between the mid- and far-infrared such that the absolute broadband difference is less than 0.7 W m^-2. However, simply matching the spectra using the mid-infrared observations in isolation leads to substantially larger discrepancies, with absolute differences reaching approximately 1.8 W m^-2. These results demonstrate the potential of far infrared observations to better constrain retrievals of ice cloud properties and their radiative impact, and provide guidance for the development of more representative ice cloud optical models.