A Multi-Instrument Comparison of Small Ice Particle Size Distributions Measured from Aircraft

Despite recent advances in instrument and data processing technologies, the reliable measurement of small ice concentrations from airborne platforms remains a difficult problem. Here, we compare measurements of ice particle size distributions from a number of different instruments, including optical array probes, high-resolution imagers, a holographic imager, and forward scattering instruments. Comparisons are made in a variety of ice cloud conditions, including cirrus, convective anvils, stratiform precipitation, and sublimating precipitation.

The comparisons show a wide range of estimated particle concentrations and size distribution shapes among the instruments, even within the same cloud. Particles with a maximum dimension smaller than 200 microns have the largest uncertainties. It is shown that optical array probes are not able to reliably measure the absence of small ice due to their tendency to overcount small particles, which is then compounded by an uncertain and size-dependent sample volume in this size range. This behavior is also demonstrated in synthetic data sets where small particles are known to be absent. High-resolution imagers and forward scattering probes also tend to overestimate small ice concentrations due to inlets, shrouds, or other physical shapes that promote particle shattering, which is further influenced by external factors such as aircraft speed and attitude. The holographic imager, which does not have a size-dependent sample volume and can readily detect shattering events, generally shows lower small ice particle concentrations. While we consider the holographic measurements to be more reliable, the small sample volume and difficulty in acquiring and processing the data remain obstacles to routine measurement.

The overall goal of this work is to define the range and uncertainty in small ice concentrations that are reported from aircraft measurements. Finally, we present a new data processing method that may be used to provide more reliable estimates of small ice particle concentrations from optical array probes.