Tropical Cirrus Parameterization for Bimodal Ice Size Spectra

Ivanova, D.C.(a), Mitchell, D.L.(a), and McFarquhar, G.M.(b), Desert Research Institute, Reno, NV (a) University of Illinois at Urbana-Champaign, IL (b)
Thirteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting

The role of small ice crystals in cirrus and the use of the FSSP (Forward Scattering Spectrometer Probe) to characterize the small crystal mode continues to be uncertain, although recent evidence indicates that the FSSP may, given some criteria, measure adequately the small ice crystal mode for D<50 m.Therefore, it makes sense to include an estimate of the small crystal mode rather than ignore it as in earlier studies. Evidence presented here suggests the FSSP does a reasonable job at measuring the small SD mode. This study focuses on two particular tropical cirrus cloud parameterizations: McFarquhar and Heymsfield (1997) and Mitchell et al. (2000). FSSP data from CEPEX (Central Equatorial Pacific Experiment) is contrasted with size spectra predicted by these two parameterizations. McFarquhar and Heymsfield used VIPS (Video Ice Particle Sampler) data to determine the small crystal mode. Mitchell et al. extrapolated the first two usable bins of the 2D-C probe to estimate ice crystal concentrations for D < 100 m, while the exponential slopes of the larger particles in the size distribution were estimated well as a function of temperature. Then using observed MODIS solar radiances for the CEPEX case study, the small mode was adjusted slightly to yield the observed radiances. We are presenting the difference between these two parameterizations and FSSP data. At the time of this writing, the McFarquhar & Heymsfield scheme appears to underestimate the concentrations of the smallest crystals relative to the FSSP, while the Mitchell et al. scheme appears to overestimate them somewhat. However, both schemes appear relatively consistent with the FSSP concentrations. In addition, a parameterization for bimodal size spectra in tropical cirrus, based on 4,028 SDs, taken from 3 flights during CEPEX is described. This parameterization is derived, following the methodology described in our mid- latitude study (Ivanova et al., 2001) and provides three gamma SD parameters for the small ice crystal mode. This parameterization is compared with the observed size spectra, as well as the parameterizations mentioned above.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).