Testing the influence of small crystals on ice size spectra using Doppler lidar observations

Analysis of the vertical velocity of ice crystals observed with a 1.5 mu m Doppler lidar from a continuous sample of stratiform ice clouds over 17 months show that the distribution of Doppler velocity varies strongly with temperature, with mean velocities of 0.2ms(-1) at -40 degrees C, increasing to 0.6ms(-1) at -10 degrees C due to particle growth and broadening of the size spectrum. We examine the likely influence of crystals smaller than 60 mu m by forward modelling their effect on the area-weighted fall speed, and comparing the results to the lidar observations. The comparison strongly suggests that the concentration of small crystals in most clouds is much lower than measured in-situ by some cloud droplet probes. We argue that the discrepancy is likely due to shattering of large crystals on the probe inlet, and that numerous small particles should not be included in numerical weather and climate model parameterizations. Citation: Westbrook, C. D., and A. J. Illingworth (2009), Testing the influence of small crystals on ice size spectra using Doppler lidar observations, Geophys. Res. Lett., 36, L12810, doi: 10.1029/2009GL038186.