Sensitivity of Freezing Drizzle Development in Orographic Wintertime Clouds to Thermodynamic Stability and Airmass Source during SNOWIE

Observations of supercooled liquid water are nearly ubiquitous in wintertime orographic clouds throughout the intermountain west of the United States. The development of supercooled cloud water in such clouds is also well understood. However, observations of freezing drizzle (supercooled drops of size large enough to have appreciable fall velocities) are rarer and factors controlling their development and location are less-well documented. One of the overarching goals of the Seeded and Natural Orographic Wintertime clouds—the Idaho Experiment (SNOWIE) is to improve understanding of natural cloud structure and key dynamical and microphysical processes governing precipitation formation within mixed-phase, wintertime orographic clouds. Preliminary examination of cloud droplet statistics from the whole experiment found low droplet number concentrations (median N < 70 cm^-3), conditions closer to the expectation of maritime rather than continental airmass and associated with more rapid formation of freezing drizzle.

Here we examine all Intensive Observing Periods (IOPs) from SNOWIE and identify common characteristics for clouds in which freezing drizzle was observed. Freezing drizzle presence was determined from examination of detailed in situ and remote sensing measurements acquired in-cloud by the University of Wyoming King Air (UWKA) research aircraft. All of the IOPs for which freezing drizzle was observed at flight level were analyzed first by examining the drop size distributions and probe images. Cases were then categorized based on thermodynamic conditions and cloud structure using both aircraft in situ measurements and cloud radar profiles. We compare thermodynamic structure from soundings and air mass back trajectories for those days with observed freezing drizzle (and those days with no freezing drizzle) to local and regional climatology to elucidate how representative these cases are to broader conditions across the region and to investigate sensitivity of freezing drizzle development to these conditions.