Ice crystal number densities and size distributions from HOLODEC

Small ice crystals (≤100 μm extent) can dominate the optical properties of ice clouds such as high cirrus clouds and anvils of deep convective clouds. Yet there is uncertainty in measurements of small ice crystal number densities and size distributions as a result of variable sample volume due to depth of focus uncertainties in imaging instruments and non-spherical ice crystal habits in scattering instruments. 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.

The HOLODEC (Holographic Detector for Clouds) flew during the IDEAS 3 (Instrument Development and Education in Airborne Science Phase 3) project in August and September 2003 over northeastern Colorado. Holography, unlike imaging has no depth of focus, thus HOLODEC has a constant and 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 its sample volume. Presented are small and large ice crystal (10µm to 2000µm) number densities and size distributions from HOLODEC compared to that of the PMS 2DC, 2DP, and SPEC HVPS probes flown during IDEAS 3.