Numerical Simulation Of A Squall Line Process In Southern China And Its Multi-scale Dynamics Stud

Squall lines are a type of devastating mesoscale convective systems(MCSs);squall line study is thence of great significance for disaster prevention.However,due to the involvement of different small-scale or higher-frequency processes,this system is very complicated and mostly yet to be explored.On March 4,2018,an extremely strong squall line occurs suddenly over Southeast China,incurring devastatingly strong surface winds.In this study we investigate this extreme case,using a recently developed functional analysis tool(Liang,2016),namely,multiscale window transform(MWT),and the MWT-based multiscale energy and vorticity analysis(MS-EVA)and canonical transfer theory.We first present a high-resolution model simulation,with outputs including temperature,air pressure,and wind vector(threedimensional).These fields are then 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.Also found in this study is that,for the convective cloud area in the mid-upper level of the squall line,the available potential energy(APE)of squall line is mainly converted into KE via buoyancy conversion,and then transferred to the lower level through vertical pressure work,enhancing the wind near the front of the cold pool.Near the surface cold pool,buoyancy conversion is positively and negatively offset between updrafts and downdrafts;that is,buoyancy does positive work on updrafts,with KE converted to APE,while negative work is done on downdrafts,with APE converted to KE.This energy cycle facilitates the exchange of KE and APE,so that the energy of the cold pool reaches a dynamic equilibrium and continues to exist.