An Idealized Parameter Study of Instability Maxima on the Cool Side of Airmass Boundaries

The cool side of an airmass boundary can, paradoxically, consist of greater surface-based CAPE than the warm side of the boundary. This condition arises when the cool-side airmass is sufficiently moist relative to the warm-side airmass, with this phenomenon referred to as a Mesoscale Airmass with High Theta-E (MAHTE). This terminology is used since MAHTEs can also be identified using equivalent potential temperature and have a typical cross-frontal dimension of ~15 km. MAHTEs have the potential to enhance the severity of thunderstorms through their interaction with this region of increased CAPE, lower LFC, and greater low-level vertical wind shear. Recent climatological analysis determined that MAHTEs frequently occur in both coastal and continental regions, motivating an idealized parameter study of MAHTE development and characteristics near the coast and in areas with a uniform land surface type. While this climatological analysis was performed for the United States, characteristics of regions where MAHTEs are most common there suggests that MAHTEs likely occur frequently within Europe as well, particularly in southern coastal regions and in the lee of mountains.

Analysis of the coastal simulations indicates that the primary mechanism for MAHTE development is a relative decrease in moisture within the warm-side airmass through entrainment of dry air from above the boundary layer. MAHTE development is also supported through increased radiative forcing, which enhances the vigor of thermals, and increased water surface temperature, which increases moisture content in the cool-side airmass. This talk will expand on these earlier results through discussion of a series of CM1 large-eddy simulations whose domains are set in a continental location, with a broader parameter space including the initial temperature difference across the airmass boundary, radiative forcing, surface fluxes, moisture content above the boundary layer, and land surface type. Ultimately, the process-based understanding achieved through this study will improve our ability to forecast MAHTE development and characteristics over a broad parameter space.