Date of Award

January 2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Atmospheric Sciences

First Advisor

Michael R. Poellot

Abstract

Information on the size distribution, orientation and the axial ratio of ice particles is important for the improvement of 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 sampling instrumentation. Due to this possible re-orientation, observations of ice particles become a function of the viewing angle of the sampling instrumentation. The two-dimensional stereo (2D-S) optical array probe (OAP) manufactured by SPEC, Inc. offers the capability for comparison between two orthogonal sample volumes (vertical and horizontal) and the determination of whether previously unknown errors in particle image aspect ratio, size distribution and other derived parameters arise due to the viewing angle of imaging instruments.

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. Planar and columnar type ice crystals have been previously shown to fall with their broad face horizontal. However, 2D-S measurements of aspect ratios indicate a preferred vertical orientation of these particles within the sample volume of the instrument. Analysis of the effects of this orientation suggest that planar crystals are under-represented, and under sized, within OAP measurements.

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