Vertical Temperature and Moisture Characteristics of
Drizzle and Freezing Drizzle Environments (189)
Dan Baumgardt, NOAA/National
Weather Service, La Crosse, WI
Andrew Just, NOAA/National
Weather Service
Todd Rieck, NOAA/National
Weather Service
Abstract:
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.
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.