Influence of small (sub-60micron) crystals on ice spectra
There have been many in-situ observations of small ice crystals in very large concentrations, using forward scattering cloud droplet probes. Ice size spectra with a seperate small mode for these tiny, barely falling crystals have been parameterised for input into GCMs and remote-sensing retrievals (Ivanova et al 2001, Donovan 2003). However, these measurements are uncertain because of shattering of larger ice crystals on the probe inlet, with the detector then counting the many tiny fragments. In an attempt to determine if large quantities of small crystals are genuinely typical of cirrus or an artefact, we have forward modelled their effect on the measured lidar Doppler velocity, using Ivanova et al's parameterisation. Comparison with 17 months of continuous lidar observations strongly suggests that these small crystals are an artefact in most cases, with measured Doppler velocities significantly faster than predicted by the size spectra with small mode added. Forward modelling of the size spectra with the small mode removed leads to a much better comparison with the lidar observations. We conclude that the small mode should not be included in NWP/climate model parameterisations if cirrus is to be simulated realistically. See pubs below.