An Observational and Modeling Study of Mesoscale Air Masses with High Theta-E

Abstract

Abstract Typically, the cool side of an airmass boundary is stable to vertical motions due to its associated negative buoyancy. However, under certain conditions, the air on the cool side of the boundary can undergo a transition wherein it assumes an equivalent potential temperature and surface-based convective available potential energy that are higher than those of the air mass on the warm side of the boundary. The resultant air mass is herein referred to as a mesoscale air mass with high theta-e (MAHTE). Results are presented from an observational and mesoscale modeling study designed to examine MAHTE characteristics and the processes responsible for MAHTE formation and evolution. Observational analysis focuses on near-surface observations of an MAHTE in northwestern Kansas on 20 June 2016 collected with a Combined Mesonet and Tracker. The highest equivalent potential temperature is found to be 15–20 K higher than what was observed in the warm sector and located 2–5 km on the cool side of the boundary. This case was also modeled using WRF-ARW to examine the processes involved in MAHTE formation that could not be inferred through observations alone. Model analysis indicates that differential vertical advection of equivalent potential temperature across the boundary is important for simulated MAHTE formation. Specifically, deeper vertical mixing/advection in the warm sector reduces moisture (equivalent potential temperature), while vertical motion/mixing is suppressed on the cool side of the boundary, thereby allowing largely unmitigated insolation-driven increases in equivalent potential temperature. Model analysis also suggests that surface moisture fluxes were unimportant in simulated MAHTE formation.