Early evolution of vertical vorticity in a numerically-simulated idealized convective line

The generation and organization of mesoscale convective vortices (MCVs) is a recurring theme in middle latitude and tropical meteorology during the warm season. In this work an idealized simulation of a finite-length convective line is investigated in the absence of ambient vertical vorticity. Previous simulations have focused primarily on the tilting of either ambient or storm-generated horizontal vorticity to account for the formation of MCVs. This study confirms recent work suggesting that tilting of both ambient and storm-generated horizontal vorticity generates vertical vorticity at early times in the simulation. At later times, however, a Lagrangian budget analysis of the vertical vorticity equation shows that vorticity convergence becomes a comparable and at times dominant mechanism for the production and long-term organization of vertical vorticity.

Considering the convective line in the context of hydraulic jump theory allows one to apply a recently derived Bernoulli theorem to obtain a new and exact expression for the potential vorticity (PV) flux along the convective system. Rapid deceleration in the horizontal winds at the gust front boundary results in a reduction in the Bernoulli function, thereby producing a horizontal PV flux which is related to gradients in potential temperature and the Bernoulli function. Due to a larger Bernoulli function gradient at the leading convective line, a net northward transport of PV occurs along the convective line, thus favoring the development of mesoscale positive and negative PV (and hence vertical vorticity) anomalies at the northern and southern line ends, respectively.

This work suggests a new and useful way of thinking about the organization of vertical vorticity in mesoscale convective systems.