A Nexrad-Based Dual-Polarization in-Flight Icing Algorithm Compared to Research Flight Data

Several precursor flights for this winter's 'Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment' (SNOWIE) field campaign in the Payette Mountains of Idaho were conducted in February of 2014.  The overall goals of the project are to understanding the natural dynamical and microphysical processes by which precipitation forms and evolves within orographic winter storms.  Researchers have a high concentration of ground-based instrumention in the area around the Idaho Power Company's seeding operations in the Payette Mountains to collect data.  The University of Wyoming's King Air research aircraft carried a full suite of microphysical instrumentation, which was able to sample an atmospheric river event with supercooled liquid and mixed-phase conditions aloft.

In this work, the prototype 'Radar Icing Algorithm' (RadIA) is compared to in-situ microphysical data from the research aircraft. RadIA uses polarized moment fields from operational S-band weather radars plus a numerical weather prediction model temperature profile as inputs. The height of the freezing level is determined from the moment data and then adjustments are made to the model temperature profile. Non-meteorological targets and radar return at non-freezing heights in the volume are masked out. Fuzzy logic membership functions are then defined to determine the presence of freezing drizzle, homogeneous small drop supercooled liquid, mixed phase conditions and the existence of plate-shaped crystal species on a pixel-by-pixel basis. The resulting interest values of each of these four calculations are combined through rule-based thresholding to identify areas of the radar volume that RadIA has a high, medium or low interest in the presence of these various forms of in-flight icing. Correlation statistics are presented and discussed related to hydrometeor phase at discrete points along the aircraft flight track for one Pre-SNOWIE flight case compared to RadIA's icing classification.