Instability Maxima on the Cool Side of Lake-Breeze Fronts and Their Influence on Convection Initiation

Studies have noted that the cool side of airmass boundaries can, paradoxically, be characterized by higher near-surface equivalent potential temperature (θ_e) and associated CAPE, with the area of higher CAPE referred to as a Mesocale Airmass with High Theta-E (MAHTE). While the climatology of MAHTEs is not known, a growing number of case studies have been identified and demonstrate the potential for MAHTEs to increase the severity of storms that cross from the warm to cool side of the airmass boundary, into an environment with increased CAPE and vertical wind shear, along with a lower LCL. Notable cases include the supercell that produced the former world record hailstone in Aurora, NE in 2004, and the Dimmitt, TX supercell observed during the 1995 VORTEX campaign. To date, case studies have primarily focused on MAHTE development in thunderstorm outflow or on the cool side of synoptic-scale airmass boundaries.

This talk will present results from 36 LES that were run using the PSU/NCAR Cloud Model 1, with simulations that vary the base-state wind from calm to offshore (u = 0, -2.5 and -5 m s^-1), radiative forcing (15 June, 15 July, 15 August), and water surface temperature (25th, 50th, 75th, and 90th percentiles for Lake Michigan corresponding to the radiative forcing dates). In all simulations, the initial shore-parallel wind component was equal to 0 m s^-1. The base-state thermodynamic profile was constructed to allow for initiation of deep convection and was defined by a capping inversion at approximately 875 hPa (1240 m AGL) with a weakly stable layer below and a well-mixed layer above. The model domain was 200 × 20 × 18 km in the W-E, N-S and vertical dimensions, with a surface type of water in the western 50 km and land in the eastern 150 km. Inclusion of radiative parameterization, friction, and surface heat fluxes enabled development of realistic boundary layer structure and CI. Surprisingly, MAHTE developed in all simulations, presumably due to the presence of a pronounced EML in the base-state sounding, which favored entrainment of low θ_e air into the inland CBL due to more vigorous thermals and related vertical mixing. Trajectory analysis will be presented, to determine whether convection that initiates near the LBF contains parcels that originated within the MAHTE. To investigate processes related to MAHTE development, simulations with a substantially reduced EML vertical extent were run and show that this decreases both MAHTE magnitude and horizontal extent. Results will be presented in the context of mobile mesonet and transects and radiosonde data during lake-breeze MAHTE development, which further supports the important role that pronounced EML play in their development.