An Observational Study of High Theta-E Airmasses

Typically, the “cool” side of a thermal boundary is more stable to vertical motions compared to the “warm” side; however, under certain conditions it is possible for the cool airmass directly along the thermal boundary to obtain a higher equivalent potential temperature and higher surface based convective available potential energy (SBCAPE). This phenomenon is referred to as a Mesoscale Airmass with High Theta-E (MAHTE). Prior observations indicate that MATHEs are only on the order of 5-10 km in width, but they can have a significant impact on storms that interact with them. Supercells that interacted with observed MAHTEs produced several tornadoes during the VORTEX-95 field campaign, and a seven inch record breaking hailstone during the BAMEX field campaign. Analysis also indicates the Cullman supercell, which produced an EF4 tornado on 27 April 2011, may have ingested MAHTE air near the time of tornadogenesis. Due to the localized increase in conditional instability, understanding the processes responsible for MAHTE formation and evolution is important for forecasters to properly assess severe weather potential.

The authors have conducted an observational study to characterize and examine the processes responsible for the formation and evolution of MAHTEs. Surface observations of a MAHTE have been collected via a mobile mesonet, and additional events will be targeted using both a mobile mesonet and an unmanned aircraft system (UAS). It is hypothesized that differential vertical mixing on either side of the boundary is important for MAHTE formation. As deep vertical mixing is suppressed on the cool side of the boundary, moisture is able to remain higher as insolation warms the environment, creating a localized maximum in theta-e.