MECHANISMS FOR THE GENERATION OF MESOSCALE VORTICES WITHIN QUASI-LINEAR CONVECTIVE SYSTEMS

Previous idealized simulations of convective systems have demonstrated that the development of mesoscale vortices within quasi-linear convec tive systems may be a natural consequence of the finite extent of the convective line, its horizontal vorticity is tilted into the vertical at the line ends. However, the source of this horizontal vorticity has not yet been clearly established, either being associated with the am bient shear or else generated within the system. In this paper, result s are presented from a series of idealized simulations that demonstrat e that the source, strength, and scale of these vortices depends on th e strength of the ambient vertical wind shear, the strength of the sys tem-generated cold pool, the scale of the convective line segments, as well as the phase within the life cycle of the convective system. In particular, for systems that develop in an environment with weak-to-mo derate shear, a line-end vortex pair is generated primarily via the ti lting of horizontal vorticity generated within the system-scale cold p ool, as the associated vortex lines are lifted within the laterally fi nite front-to-rear ascending current. Similar mechanisms also operate in environments with stronger or deeper shear, but subsystem-scale vor tices can also originate via the tilting of the ambient horizontal vor ticity within supercell updraft-downdraft couplets. In all cases, conv ergence at midlevels enhances Coriolis rotation over the longer term, leading to the preferred development of a cyclonic vortex, as is frequ ently observed in asymmetric convective systems.