Sensitivity of Optical Array Probe (OAP) Derived Parameters with Respect to Particle Orientation

Information about the size distribution, orientation and the axial ratio of ice particles is important for precipitation retrievals by polarized radar. However, uncertainty in the natural particle orientation and axial ratios remains due to the difficulty in obtaining in situ observations of these parameters. This difficulty arises because of possible re-orientation of particles by airflow around aircraft instrumentation. Due to this possible re-orientation, observations of ice particles become a function of the viewing angle of the sampling instrumentation. For example, planar and columnar type ice crystals have been previously shown to fall with their broad face horizontal. However, previous studies have shown that columnar ice crystals have a preferred canting angle within OAP measurements. The two-dimensional stereo (2D-S) optical array probe (OAP) manufactured by SPEC, Inc. offers the capability for comparison between two orthogonally oriented sample volumes (vertical and horizontal). Analysis of 2D-S data allows for the determination of whether previously unknown errors in particle image aspect ratio, size distribution and other derived parameters exist due to the viewing angle of OAPs.             

To further understand the effect of particle orientation on OAP measurements, microphysical data collected with the University of North Dakota Citation II research aircraft during the Integrated Precipitation and Hydrology Experiment (IPHEx) and Olympic Mountain Experiment (OLYMPEx) are analyzed. Calculations of canting angle frequency within 2D-S data indicate a preferred canting angle within the vertically aligned sample volume of the instrument, but not within the horizontally aligned sample volume. Additionally, 2D-S measurements of aspect ratios indicate a preferred vertical orientation of planar crystals within the sample volume of the instrument. Analysis of the effects of this orientation suggests that planar crystals can be under-represented and under sized within OAP measurements.