Characteristics of Temperature Inversions in Wintry Mix Precipitation Events

Storms that produce a wintry mix of precipitation—a combination of snow, rain, sleet, graupel, and/or freezing rain—are responsible for much of the winter weather in the coastal northeast United States. These storms can be high impact events, with significant public safety and economic implications. However, forecasting precipitation type in these storms is challenging even shortly before the event. This is problematic for officials and the public, as different types of winter precipitation demand very different responses. The exact precipitation type in a wintry mix is heavily influenced by the presence of at least one temperature inversion layer of above-freezing temperatures embedded within the otherwise subfreezing atmosphere. This inversion is referred to as a “warm nose.” Precipitation type changes as it falls through this layer and melts. For example, if precipitation melts and refreezes, it falls to the ground as sleet, but if it remains melted and supercooled until contact with the ground, it becomes freezing rain. The characteristics of the temperature profile in combination with a detailed picture of the surface precipitation type have not formerly been available. We investigate wintry mix storms affecting Upton, NY from January 2002 to March 2016, choosing this location to take advantage of our vertically-pointing Micro Rain Radar (MRR) and Multi-Angle Snowflake Camera (MASC) deployed nearby at Stony Brook University. We use data from soundings in the Integrated Global Radiosonde Archive and from aircraft in the Aircraft Communications Addressing and Reporting System dataset to obtain vertical temperature and humidity profiles within the storms, and reference them against surface conditions and precipitation type data from the 5-minute Automated Surface Observation System, MRR data since 2007, and MASC snowflake pictures since 2015. We consider warm nose thickness, temperature, maximum temperature, height, and humidity, and address which variables are most influential on surface precipitation type. We also examine the effect of other contemporaneous atmospheric variables, such as the wind profile, turbulence, and near-surface humidity.