Freezing Drizzle Formation Over The Oregon Cascades during IMPROVE II

A number of recent field programs have focused on the meteorological conditions leading to aircraft icing (WISP, CFDE, NASA Glenn). These programs have primarily focused on aircraft icing conditions over relatively flat terrain (Front Range of Colorado, New Foundland, and Cleveland) in association with cold fronts, warm fronts, and other features associated with mid-latitude cyclones. One of the key results coming out of recent studies is the significantly higher frequency of freezing drizzle than previous thought (1 – 2% of all icing conditions versus 0.1% previously). Freezing drizzle can be particularly hazardous to aircraft due to the impaction of the larger droplets beyond current deicing boots, leading to a significant reduction in aircraft performance that cannot be alleviated by the activation of ice protection devices such as pneumatic boots. These field programs have also shown that freezing drizzle is mostly formed via a collision-coalescence process rather then melting ice falling into below freezing air. This study examines the formation of freezing drizzle over complex terrain, a topic which is important operationally due to the high frequency of PIREPs of icing over complex terrain, but to date infrequently studied. In particular, data over the Oregon Cascades from the recent IMPROVE II program will analyzed and modeled to understand the processes producing freezing drizzle during winter storms impacting the Pacific Northwest.