Nexus of ambient flow and squall line via turbulence in the March 2018 meso-scale convective system over Southeast China

Abstract

On March 4, 2018, an extremely strong squall line occurs suddenly over Southeast China, incurring devastatingly strong surface winds. In this study, we first present a high-resolution model simulation, then apply the recently developed theory of canonical transfer, which bears a form reminiscent of the Poisson bracket in Hamiltonian mechanics, to investigate the underlying dynamical processes. Using a recently developed functional analysis apparatus, multiscale window transform, the fields are reconstructed onto three scale windows, namely, ambient flow window, squall line-scale window, and turbulence window, which for convenience are denoted as windows 0, 1, 2, respectively. It is found that, among all the canonical transfers of kinetic energy (KE), only that between windows 0 and 2, and that between windows 1 and 2, are significant, and the two correspond remarkably well in space, except for opposite signs. Specifically, at the leading edge, KE is first transferred from the ambient flow over the spectrum all the way to the turbulence scale processes, and then goes to window 1 to form the squall line. At the trailing edge, this process is reversed. While the downscale cascading at the leading edge and upscale transfer at the trailing edge are expected in the classical shock hypothesis of squall lines, the existence of the secondary upscale and downscale transfers, respectively, at the leading and trailing edge, are not seen before. Most importantly, these secondary canonical transfers make the turbulence-scale window function like a hub, bridging the ambient flow and the squall line. To our knowledge, such an energetic scenario, i.e., the nexus of two adjacent scales in a spectrum via a “remote mediator” is not seen before in the literature. This study testifies to the importance of turbulent flows in the maintenance of squall lines, and may yield a clue to a better simulation.