Vertical Temperature and Moisture Characteristics of Drizzle and Freezing Drizzle Environments

Freezing drizzle causes impacts and hazards to the public in a short amount of time and with relatively light precipitation amounts (e.g., a few hundredths of an inch). These light ice-glazing events can be difficult to forecast, and are typically characterized by supercooled liquid drops that grow in a saturated layer in the absence of ice nucleation. One operational forecast challenge is predicting if these saturated, or nearly-saturated, layers will manifest the collision-coalescence process to grow cloud droplets large enough to precipitate.

From 1973 to 1995, 83 events were identified consisting of three or more hours of drizzle (62 events) or freezing drizzle (21 events), inclusive of the upper-air sounding time (e.g., 0000 UTC or 1200 UTC) at St. Cloud, Minnesota. An additional subset of 52 freezing drizzle events from across the contiguous United States were derived from 422 freezing rain soundings used in Reeves et al. (2014).

All of the events were characterized by a surface-based moist layer consisting of high mean relative humidity (>85%), capped vertically by a temperature inversion, with pronounced drying above. Roughly 80% of the cases exhibited a surface-based moist layer deeper than 1 km. Further, 75% of the freezing drizzle events contained minimum temperatures warmer than -8C within that layer. Mean relative humidity values of 40-50% defined the lowest 2 km of the pronounced dry layer capping the surface-based moist layer.

A summary of the findings will be presented at the conference, which provide targets for identifying drizzle-promoting environments and improving freezing drizzle forecasts. Improved knowledge of these environments ultimately increases lead time and messaging to National Weather Service stakeholders.