Manifestation of Elevated Convection within Wintertime Extratropical Cyclones during IMPACTS. Part I: Analysis of Elevated Potential Instability

Abstract Elevated potential instability (EPI) often occurs in the comma head of wintertime extratropical cyclones as air within the storm’s dry slot moves above a warm or occluded frontal zone. Lifting of EPI layers may result in elevated convection, enhanced snowfall, and thundersnow. High-Resolution Rapid Refresh (HRRR) initialization values of equivalent potential temperature θ e are used to analyze EPI characteristics along tracks of the NASA Earth Resources-2 (ER-2) aircraft within the comma head of 14 cyclones sampled during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign. EPI was found in 53% of HRRR 1-km-wide vertical columns along ER-2 flight legs, typically 210–410 km from the low pressure center within the northern comma head region, but 0–400 km from the low in the western region of strong cyclones [sea level pressure gradients > 4 hPa (100 km) −1 ]. The highest and lowest frequencies of EPI were in Miller type B (MB) and Great Plains cyclones, respectively. In 67% of 1-km-long columns exhibiting EPI, EPI occurred within a single layer. The median base of the potentially unstable layers (Δ θ e /Δ z < 0) was 4.2 km, the median depth was 0.59 km, and the median equilibrium level was 0.94 km above the base. MBs had more, shallower EPI layers compared to fewer, deeper layers in Miller type A and Gulf Coast cyclones. The median value of Δ θ e /Δ z within potentially unstable layers was −0.6 K km −1 , the 95th percentile was −2.8 K km −1 , and the greatest values of EPI occurred in MBs and cyclones with sea level pressure gradients < 2 hPa (100 km) −1 . Significance Statement Snowfall associated with wintertime extratropical cyclones can cause large disruptions to everyday life, impacting transportation, schools, businesses, and power. Snowfall arises from different circulations in the atmosphere ranging from broad-scale ascent of air to small-scale buoyant circulations associated with elevated potential instability (EPI) above frontal zones. This paper examines the distribution, frequency, and intensity of EPI within storms sampled during the 3-yr NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign and the relationship of the instability to cyclone type and intensity. We address two questions: 1) How common is EPI and where does it develop within the comma head region of cyclones? Does the occurrence of EPI vary based on cyclone type and strength? And 2) what are the characteristics of EPI (number of layers, depth of layers, and strength of EPI)? How do these characteristics vary based on cyclone type and strength? Detailed, quantitative answers to these questions are provided.