A Three-Dimensional Numerical Model of an Isolated Thunderstorm. Part II: Dynamics of Updraft Splitting and Mesovortex Couplet Evolution

This study analyzes the dynamics of an isolated convective storm embedded within marked ambient vertical wind shear dominated by low-level veering, as simulated by a three-dimensional anelastic numerical modeling experiment. Two particular aspects of the model results, namely, cell splitting and the development of a cyclonic-anticyclonic mesovortex couplet in the updraft, are of main interest. The analysis includes parcel trajectory calculations, decomposition of the pressure perturbation, and examination of the tilting and concentration (stretching) terms as contributors to vorticity generation. It is found that: 1) The updraft is fed by potentially warm low-level air from downshear. This is also the case for the lower part of the main downdraft, which is located upshear of the updraft. Potentially cold middle-level air originating from upshear of the cloud feeds the upper part of the mean downdraft, as well as a secondary downdraft that splits the updraft by propagating inward from the downshear cloud edge. 2) Twin mesolow centers aloft at the cloud flanks aid the updraft splitting process in four ways, the first two from an Eulerian viewpoint and the fast two from a Lagrangian viewpoint: (i) a divergent horizontal pressure gradient force field within the cloud is induced between the flanks; (ii) a low-level upward pressure gradient force is concentrated beneath the flanks; (iii) parcels feeding the secondary downdraft are steered quasi-cyclostrophically around the flanking low centers and into the front of the cloud; and (iv) these parcels encounter a downward pressure gradient force. 3) Updraft parcels are accelerated upward first by the pressure gradient force and then by thermal buoyancy, also receiving a weak upward impetus from the pressure buoyancy. Inward penetration of the secondary downdraft is aided by both a downward pressure gradient force and by precipitation drag. 4) All three perturbation pressure components (dynamic, hydrostatic and drag-induced) have comparable maximum magnitudes, and each component contributes importantly to the surface mesohigh, but the double configuration of the mesolow is fully dynamic in origin. 5) An evaluation of the vorticity equation shows that tilting supports the outer portions of the vortices, while concentration supports their inner portions and has maximum magnitudes roughly half as great as for the tilting. Upward advection enables the couplet to penetrate into the updraft outflow.