A GCM parameterization for bimodal size spectra and ice mass removal rates in mid-latitude cirrus clouds

This study describes a parameterization for bimodal size spectra in mid-latitude cirrus, based on 996 size distributions (SD) taken from 17 flights in non-convective cirrus during ARM and FIRE intensive observation periods (IOPs). Based on recent work and results given here, the Forward Scattering Spectrometer Probe (FSSP) appears to be a viable instrument for approximating the concentration and sizes of small ice crystals for the cirrus studied here. This, in combination with new methodologies presented, allows the small particle mode of the SID (D less than or similar to 100 mum) to be characterized. The large mode was characterized from laser imaging 2DC probe measurements. Recent work indicates that the thermal single scattering properties of ice clouds are not adequately determined by effective diameter (D-eff) and ice water content (IWC) alone, and that information on SID dispersion (e.g. degree of bimodality) is also required. This parameterization satisfies this need, providing three gamma SD parameters for each mode of the bimodal distribution. Three of these six parameters are approximated as constant, while the other three are expressed in terms of cloud IWC and/or temperature (7). These simple inputs make the parameterization convenient for large scale models, such as Global Climate Models (GCMs). In addition, this parameterization provides a physically rigorous means for parameterizing the mass sedimentation rates from ice clouds in a microphysical sense. Accurate knowledge of mass removal rates is critical for predicting cirrus IWCs and radiative properties. Finally, the data in this study was used to parameterize D-eff as a function of T for use in schemes parameterizing bulk short-wave single scattering properties. Uncertainties in estimating D-eff drop considerably if most mid-latitude cirrus crystals are irregular, as recent studies suggest. (C) 2001 Elsevier Science B.V All rights reserved.