Numerical simulation of quasi-linear convective systems in heterogeneous mesoscale environments

This study examines the relationship between quasi-linear convective systems (QLCSs) such as squall lines and bow echoes and the heterogeneous environments within which they evolve. Idealized numerical simulations produce apparently severe QLCSs in the absence of environmental heterogeneity. Yet, observational studies suggest that significant surface boundaries and/or isolated thunderstorms cells in advance of QLCSs are precursors to severe weather. Satisfactory resolution of this scientific issue cannot be accomplished through analysis of incomplete observational data or idealized numerical simulations, which use horizontally homogeneous initial conditions. The primary objective of this study, therefore, is to perform real-data numerical simulations of observed QLCSs in order to: i) identify the various environmental features that modify the structure and evolution of simulated QLCSs, and quantify their characteristics; and ii) determine the role of such interactions in enhancing QLCS severity, and how it differs in homogeneous versus inhomogeneous scenarios.

The Advanced WRF (WRF-ARW) Version 2 has been used to perform real-data numerical simulations of the 24 October 2001 and 6 July 2003 bow echo events. In each simulation, on the subsystem-scale, a diversity of low-level mesovortices form along the main convective line's leading edge, consistent with single-Doppler radial velocity data for that event. For the simulated bow echo of the 6 July 2003, time series of the time-integrated contributions to vertical vorticity from tilting and stretching show that mesovortex strength can significantly increase through vertical vortex stretching, owing to a focused region of enhanced convergence at a QLCS-boundary point of intersection. In contrast, low-level mesovortices that form within the simulated squall-line bow echo of 24 October 2001 achieve strengths greater than three times mesocyclone-scale vorticity in the absence of significant surface boundaries and other mesoscale features. The suggestion is then made that significant environmental heterogeneity is a sufficient condition for the development of severe weather.