How Mountains Alter Quasi-linear Convective System Dynamics and Tornado Potential

Though tornadoes commonly form in quasi-linear convective systems (QLCSs), predicting their development remains a challenge, due to their rapid formation, relatively short lifetimes as compared to supercell tornadoes, and the likelihood of formation along any segment of the line. Rapid changes in near-surface vertical vorticity associated with QLCS tornadogenesis can result from environmental heterogeneities, including those associated with mountainous landscapes. Mountains can induce variations in stability and vertical wind shear, for example, due to changes in elevation and the configuration of the valleys. Such heterogeneities can influence the evolution of QLCS structures, and thus vertical vorticity. Sloping mountain surfaces can also induce variations in cold pool characteristics and advancement which may further alter storm structural evolutions. Potential systematic linkages between QLCS vertical vorticity and mountains may offer hope toward improved QLCS tornado prediction.

This study leverages idealized numerical simulations to quantify the potential role of mountainous landscapes in the development and evolution of QLCS vertical vorticity. Two different base-state environments are tested, including a profile observed during a QLCS-tornadic event and an analytic profile with several different vertical wind shear configurations. For each base-state environment, we evaluate the sensitivity of QLCS vertical vorticity evolution to a suite of simplified mountain configurations, including a plateau with either a downward or upward sloping surface, each with or without a localized valley of different widths within the sloping surface. Preliminary results indicate that as QLCSs move over a mountain slope with a valley, line echo wave patterns form, which have been associated with QLCS tornado damage. Thus, terrain may be important for the development of QLCS tornadoes through the modification of the linear structure by the underlying irregular surface landscape. Localized valleys also favor CI and the development of intense convective cells, which may increase tornado potential within and near the valley.