Jacob Fugal
Department of Physics
Michigan Technological University
Small ice crystals (<100 &mu m extent) often dominate the optical properties of ice clouds such as
high cirrus clouds and anvils of deep convective clouds because of their over whelming populations
compared to larger ice crystals. Yet there is great uncertainty in measuring number densities and size
distributions of small ice crystals using standard aircraft instrumentation because of depth of focus
uncertainties and the difficulty in calibrating scattering instruments for non-spherical particles. Ice
crystals also shatter on probe tips and the shards are swept into the sample volume causing
undercounting of large ice crystals and overcounting of small ice crystals. There are some recently
developed instruments to address these problems. HOLODEC (Holographic Detector for Clouds) is one
such instrument.
The HOLODEC instrument flew on NCAR' s C130 during the IDEAS 3 project in August and
September 2003 over northeastern Colorado. Holography, unlike imaging, has no depth of focus, thus
HOLODEC has a well-defined sample volume. Holography also records information in all three spatial
dimensions. HOLODEC can measure the 3-D position, size and 2-D projected shape of ice crystals. It
can therefore detect which ice crystals are shards and which are natural because shards will appear as
strongly localized high concentrations of ice crystals in 3-D. Presented are small ice crystal (15 &mu m to
150 &mu m in extent) number densities and size distributions from HOLODEC compared to that of the
PMS 2DC, and FSSP probes flown during IDEAS 3. Also presented are limitations and advantages of
using holography for other particle measurements such as cloud particle clustering and cloud particle
collisions.
Tuesday November 14 at 10:00am in FL2-1022.
|
