Development and Evolution of a High Theta-E Outflow Airmass

Thunderstorm outflow airmasses consist of diabatically-cooled air, which is generally more stable than that of the ambient environment. Following dissipation or departure of the storm that produced the outflow airmass, the temperature within the outflow moderates. Under certain conditions, the leading edge of the outflow airmass evolves such that the cool side of the outflow boundary consists of higher equivalent potential temperature, and therefore greater CAPE, assuming horizontal gradients in mid- and upper-level temperature and dew point above the boundary are negligible. The cooler, but relatively unstable air, is referred to as a MAHTE (Mesoscale Airmass with High Theta-e). Thunderstorms that ingest air from within MAHTE have been observed to intensify due to the higher CAPE, lower LCL, and increased vertical wind shear; observed cases of MAHTE-storm interaction include the record-setting Aurora, NE hailstorm in 2003, and the 2 June 1995 Texas Panhandle tornado outbreak during VORTEX. While there is much to be learned about the climatology of MAHTE, the growing list of documented cases demonstrates the need for further study of this phenomenon given its potential implications on the severity of thunderstorms that ingest MAHTE air.

Hypotheses for the development of MAHTE include a relative decrease in dew point within the ambient environment over time due to entrainment of dry air from above the CBL, and an enhancement in surface moisture fluxes near the leading edge of the outflow airmass. However, these hypotheses are difficult to test due to the small horizontal scale of MAHTE relative to the spacing of ASOS and other fixed observational platforms. High spatial and temporal observations of MAHTE development and evolution were obtained on 15 July 2019 using the Central Michigan University mobile mesonet. A series of north-south transects were performed from ~1730-0000 UTC ahead of developing storms, through fresh outflow where both temperature and dew point were lower than in the ambient environment, following transition of the outflow into a MAHTE, and during MAHTE evolution into the evening. At its most pronounced state, theta-e was 10K higher than in the ambient environment, due to a ~4°C higher increase in dew point and a ~2°C decrease in temperature when crossing the gust front. A gradual decrease in dew point in the ambient environment from 21 to 18°C to over the course of the afternoon supports the entrainment hypothesis for MAHTE development. These and other observations of the evolution of thermodynamic and kinematic properties within the outflow and ambient environment will be discussed in the context of ASOS, radar, satellite data, and other operational data.