On the Genesis and Strength of Mesovortices Formed within Bow Echoes

It has been shown that “mesovortices” formed on the leading edge of bow echoes are capable of producing long swaths of straight-line wind damage and tornadoes. The most significant wind damage within a bow echo may be produced by mesovortices rather than a descending rear-inflow jet. Furthermore, a spectrum of mesovortex strengths have been observed within a given bow echo event such that some are damaging while others are not. To date, the genesis mechanism(s) and factors that govern mesovortex strength are not well understood.

In this study, the mechanisms that generate mesovortices within bow echoes and govern their strength are investigated by simulating an observed damaging bow echo event that occurred on 10 June 2003 over the greater Saint Louis, MO area. This event produced 11 mesovortices of which five produced tornadoes. Two mesovortices produced a 70 km long straight-line wind damage swath. Six of the vortices produced little or no damage.

The simulations were performed with the Advanced Research WRF (ARW) model. A nested-grid configuration was employed such that outer 400 x 400 km domain with 1 km horizontal resolution contains an inner 120 x 120km domain with 333 m horizontal grid resolution. The inner grid is centered on the cold pool and leading edge of the simulated bow echo. The coarse domain is initialized with a nearby sounding launched at 18 UTC at Springfield, MO. The inner domain is initialized by interpolating the base state from the outer domain.

In the control run, a well-defined bow echo forms and evolves in a similar manner as the observed system. Well-defined mesovortices of varying strengths are produced on the bow echo gust front and are well-sampled by the inner domain. A number of sensitivity experiments will be presented where the influence of cold pool strength, low-level shear, and coriolis forcing on mesovortex evolution will be presented. A discussion of the mesovortex genesis mechanism will also be presented and discussed in the context of previous observational and modeling results.