Numerical Simulations of Supercell Tornadogenesis: The Structure and Evolution of Vortex Lines within Low-Level Mesocyclones and Tornadoes

Mashiko et al. (2009, MWR) simulated a supercell tornadogenesis within the outer rainbands of a landfalling tropical cyclone using a cloud resolving model with a horizontal grid spacing of 50 m. The environment around the supercell was characterized by strong low-level veering sheer and a highly humid air throughout the troposphere. Backward trajectories and vorticity budget analysis revealed that the secondary rear-flank downdraft (RFD) surge played a key role in tornadogenesis by transporting the large streamwise vorticity associated with the environmental low-level veering shear. Moreover, the secondary RFD outflow surge enhanced the horizontal convergence, contributing to the rapid amplification of the vertically tilted streamwise vorticity. This result is probably inconsistent with recent observational studies of low-level mesocyclone evolution in the midwestern United States (e.g., Markowski et al. (2008, MWR)).

In this study, the evolution of vortex lines within low-level mesocyclones and tornadoes is examined in order to confirm the above mentioned conclusions by comparing it to the recent observational studies. As a result, a low-level mesocyclone contains a lot of vortex lines originating from environmental streamwise voriticy. After the development of the low-level mesocyclone, vortex lines having environmental streamwise vorticity wrap around the low-level mesocyclone, and a secondary RFD surge depresses the vortex lines downward in a nearly barotropical manner, which leads to tornadogenesis.

Meanwhile, as to F3 Supercell tornado, which hit Tsukuba City in Japan on 6 May 2012, vortex lines emanating from the low-level mesocyclone form the arches straddling the hook-shaped hydrometers distribution. This result indicates that baroclinic vorticity generation within RFD region is effective for a low-level mesocyclone formation as in the recent observational studies of suprecells in midwestern United States.

Thus, mechanisms of supercell tornadogenesis vary depending on the environmental conditions.