Advancing Understanding of Mesoscale Airmasses with High Theta-E through Airmass Boundary Classification

The cool side of an airmass boundary is typically stable and less favorable for deep convection. However, under certain conditions, a narrow region immediately on the cool side of a boundary can obtain higher equivalent potential temperature (θe) than the warm side of the boundary, resulting in a mesoscale airmass with high theta-e (MAHTE). MAHTEs are characteristic of higher convective available potential energy, lower lifting condensation level, and often higher 0–6 km bulk wind differential, making them potentially more favorable for severe convection and tornadoes. Previous work has identified plausible mechanisms of MAHTE formation and developed a climatology of MAHTEs across the contiguous United States which quantified their characteristics, seasonality, and distribution. However, the type of boundary along which a MAHTE typically forms has not been formally analyzed. Both synoptic boundaries and outflow boundaries have been identified in association with MAHTEs in case studies. Additionally, the climatology revealed an abundance of MAHTEs in coastal regions with onshore wind, suggesting a sea/lake breeze. Thus, this work seeks to identify which type of parent airmass boundary is most commonly associated with MAHTEs. The aim of this research is to examine a subset of the climatology to classify the parent boundaries as synoptic, outflow, or sea/lake breeze through the analysis of archived radar reflectivity, frontal analysis, and ASOS wind data. Results reveal that new outflow boundaries are the most common boundary type associated with the MAHTEs examined and that there is seasonal and diurnal variation in boundary type frequency. Additionally, some MAHTE characteristics are mostly consistent regardless of boundary type, such as the increases in theta-e or CAPE, while others vary depending on boundary type. These results help to further the understanding of MAHTE formation in different conditions or situations.