An Observational and Mesoscale Modeling Study of Mesoscale Airmasses with High Theta-e (MAHTEs)

Although the cool side of a thermal boundary should be more stable to vertical motions than the warm side, under certain conditions it is possible for the leading edge on the cool side to undergo a transition where it obtains a higher theta-e and surface based convective available potential energy (SBCAPE) than the warm side. This phenomenon will hereafter be referred to as a mesoscale air mass with high theta-e, or MAHTE. In some instances, MAHTE formation has been resolved by the HRRR and NAM-4K operational forecast models, however prior MAHTE observations suggest that this will not always occur. Two MAHTEs observed during the VORTEX-95 and BAMEX field campaigns were on the order of 5-10 km in width; a feature this small will not be accurately resolved by a numerical model with grid spacings of 3-4 km. Due to the locally enhanced conditional instability, understanding the processes responsible for MAHTE formation and evolution are important for forecasters to properly assess the probability of severe storm formation.

The authors have undertaken an observational and mesoscale modeling study aimed at addressing the processes responsible for MAHTE formation and evolution. Surface observations of a MAHTE have been collected via a mobile mesonet and additional events may be targeted with both a mobile mesonet and unmanned aircraft system. These data will complement real-data simulations using WRF-ARW. Numerical experiments will be conducted within a parameter space that includes insolation and surface fluxes. 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. It is also hypothesized that increased surface fluxes of moisture and temperature on the cool side of the thermal boundary maintains or increases moisture required for MAHTE formation.