Very high-resolution prediction of tornado-like vortices in a long-lived mesovortex

An observed mesoscale-convective system (MCS) together with an embedded line-end vortex (LEV) has recently been successfully simulated at 2-km horizontal resolution assimilating data from the operational WSR-88D radars and experimental X-band CASA radar network (Schenkman et al. 2010, Mon. Wea. Rev.). In a follow-up study, a 400 m grid spacing is used and it successfully simulated the development of an observed strong, long-lived (~ 1 hr) mesovortex when CASA radial velocity observations are assimilated. In the present study, we further refine the grid resolution by placing a nested 100-m grid within the 400-m grid in attempt to predict the tornadoes that were spawned by the long-lived mesovortex in this case. Tornadoes occurring in such settings are generally believed to be different from those occurring in supercell storms.

Simulations on the 100-m grid show that several smaller low-level vortices develop within the larger mesovortex circulation. These vortices have a lifetime of less than 5 min, and tend to form in a serial manner with the decay of one vortex coinciding with the genesis of the next. A positive feedback mechanism appears to occur as each subsequent vortex is stronger than the last. The strongest of the vortices obtains tornado-like vorticity values. A detailed description of the forecast evolution, along with a possible explanation of the positive feedback mechanism will be discussed. The feasibility of generating useful forecasts in realtime for tornadic vortices will also be discussed.