  Measurements of the vertical velocity of ice crystals using a 1.5micron
Doppler lidar are described. The statistics from a continuous sample of
stratiform ice clouds over 17 months are analysed: the distribution of velocity
varies strongly with temperature, with average Doppler velocities of 0.2m/s at
-40C increasing to 0.6m/s at -10C presumably due to particle growth and
broadening of the size spectrum. We examine the likely influence of small
crystals less than 60 microns 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 these small crystals
in most ice clouds is much lower than measured in-situ by cloud droplet probes.
The discrepancy is attributed to shattering of large crystals on the probe
inlet, and we argue that these numerous small particles should not be included
in numerical weather and climate model parametrizations.
