Several recent formal and informal studies have been conducted to verify the Integrated Icing Diagnostic Algorithm (IIDA) using observations of icing from pilot reports (PIREPs) and research aircraft. Results have shown that IIDA performance can vary for different meteorological situations and in different parts of the U.S. In particular, IIDA sometimes has problems identifying or underestimates the potential for icing in the Pacific Northwest when vigorous synoptic-scale storms affect that area. These events and others where IIDA does not perform well appear to have common characteristics, suggesting that an analysis of the IIDA input datasets used and the synoptic-scale meteorology for these cases may provide insight and potential solutions.

The first aspect of this analysis involved matching available PIREPs with IIDA icing potential values and creation of a time-height cross-section for the days in question. The values used in these plots are the maximum IIDA values at the four grid points surrounding a city (Portland or Seattle), and range from 0.0 (no icing) to 1.0 (icing very likely). RUC model-based relative humidity and temperature are used in conjunction with observations from satellite, radar, and surface stations to assess where clouds are present and to estimate their phase. Together these plots help to provide an initial hypothesis of why IIDA did not perform well for these events. The synoptic-scale meteorology of these separate circumstances is examined to identify the weather patterns that existed during these events. Some basic tools that will be used include constant pressure charts from required levels, soundings from the closest upper-air station, surface charts, satellite and radar data, and surface observations. These analyses will provide the developers of IIDA with some reasonable ways to adjust the algorithm with respect to frequently occurring synoptic events in the Pacific Northwest.

Author Contact Information: Michael Chapman, NCAR, PO Box 3000, Boulder CO 80307. Phone: (303) 497-8371; Fax: (303) 497-8401; e-mail: mchapman@ucar.edu