Manifestation of elevated convection within wintertime extratropical cyclones during impacts

Past work has established 5 natural sources of instability within extratropical cyclones including: boundary-layer, mid-layer, surface-based, elevated potential instability, and cloud top instability. This work focuses on instability above frontal zones (both elevated potential instability and cloud top instability). Elevated potential instability develops from dry air descending above a warm/occluded front in the convective region of the comma head and results in elevated convection associated with riming, enhanced snowfall, and sometimes thundersnow. Cloud top instability develops from longwave radiational cooling in the stratiform region of the comma head and leads to generating cells. Although past case studies of wintertime extratropical cyclones analyzed in depth the thermodynamic environment supporting elevated instability and resulting vertical motions, their conclusions are unique to the selected cases. It has not been determined if results from case studies can be generalized for multiple cyclones. Past studies of elevated convection within many wintertime extratropical cyclones also lack an in-depth analysis of the vertical structure of elevated instability and vertical motions. The motivation for this work is to provide the first in-depth investigation on the characteristics of elevated instability and resulting vertical motions within various wintertime extratropical cyclone types and strengths. This work uses radar data within the comma head of 18 cyclones during the Investigation of Microphysics and Precipitation for Atlantic Coast Threatening Snowstorms (IMPACTS) field campaign and RAP model analysis equivalent potential temperature (e) that was transformed into low-relative coordinates to statistically composite and analyze data from multiple cyclones based on its location relative to the center of the low. Based on cyclogenesis location and its development in the ERA5 reanalysis, the 18 cyclones were categorized into established cyclone types: Miller type A and B cyclones, gulf coast cyclones, great plains cyclones, and Alberta clippers. Using RAP e, characteristics of elevated instability such as its spatial distribution, frequency, strength, number of layers, and depth were investigated in various cyclone types and strengths. Vertical Doppler radial velocity (Vr) from IMPACTS were used to determine how vertical motions varied by cyclone type, strength, and elevated layer stability. This work answers the following questions: 1. How common is elevated instability and where does it develop in cyclones? Does the occurrence of elevated instability vary based on cyclone type and strength? 2. What are the characteristics of elevated instability (number of layers, depth of layers, and strength of elevated instability)? How do these characteristics vary based on cyclone type and strength? 3. How do Vr distributions vary by cyclone type? How do Vr distributions vary between stable layers and layers with different strengths of elevated instability? We found that elevated instability occurred in 71% of 1 km columns along flight legs typically between 270-530 km from the center of the low and in the northern region of the comma head. 95% of elevated unstable layers had de/dz > -2.9 K km-1 and the strongest instability occurred in Miller type B cyclones, ~200 km from the center of the low, and west of the low with the base of layers 2-4 km AGL. Vr distributions were broader and shifted toward positive upward values in stronger cyclones and unstable layers due to the presence of elevated convection and turbulence. As elevated instability strength increased, greater occurrences of -Vr were present associated with riming in elevated convection.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Atmospheric Sciences

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Heimes, Kaylee Elizabeth

Contributors dc:contributor

• Rauber, Robert
• Nesbitt, Steve
• Di Girolamo, Larry

• Extratropical Cyclones
• Winter Weather
• Snow
• Elevated Instability

Statement dc:rights

• Copyright 2024 Kaylee Heimes

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/124283

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F124283