Mesovortices Formed within Bow Echoes: Their Genesis and Sensitivity to the Environment and System Cold Pool

Recent observational and modeling studies have elucidated the important role that shallow, small-scale vortices formed on the gust front of bow echoes play in the creation of damaging surface winds. Specifically, these “mesovortices” have been shown to spawn tornadoes and are also capable of producing long, narrow swaths of straight-line wind damage that has been previously attributed to a descending rear-inflow jet.

Our knowledge of how mesovortices form stems from idealized modeling results that suggest convective-scale downdrafts tilt baroclinic horizontal vorticity creating a vortex pair. The anticyclonic vortex dissipates while stretching of planetary vorticity helps to amplify the cyclonic member. The generality of this mechanism in not known.

Further, a spectrum of mesovortex strengths is often observed within bow echoes. Some are damaging while others are not. From a forecasting perspective, it is important to discriminate between the stronger damaging vortices from the weaker non damaging circulations. The factors that are important for creating stronger vortices are not well understood.

In this study, simulation results of the 10 June 2003 Saint Louis bow echo that occurred during the Bow Echo and MCV Experiment (BAMEX) are presented. This bow echo produced a number of tornadoes and an 80 km straight-line wind damage swath. All damage was produced by mesovortices. The objective of this study is to understand how environmental conditions and the system cold pool may affect mesovortex strength and longevity and their genesis mechanism(s).

The simulations were performed with the WRF-ARW model. The model was initialized with an 18 UTC sounding launched at Springfield, MO that appeared to well represent the environment on this day. The sounding was characterized by moderate low-level shear where the wind vector difference between the surface and 2.5 km was about 15 ms-1. The convective available potential energy for a surface-based parcel was about 2560 Jkg-1.

A series of sensitivity simulations were performed at 750 m horizontal resolution to examine the impact of shallow and deep environmental shear, coriolis forcing, and cold pool strength on mesovortex evolution. In the control run, the sounding described above was used along with the Lin microphysics scheme. The coriolis parameter was set to 1x10-4 s-1. As will be shown at the conference, the control run produced a number of strong, long-lived mesovortices, similar to the observed 10 June 2003 event. It will also be shown that mesovortex evolution is quite sensitive to the environmental shear profile, coriolis forcing, and cold pool strength. In particular, mesovortex longevity and strength is quite sensitive to the 0-2.5 km environmental shear with stronger, longer-lived vortices forming in higher shear environments. Interestingly, all deeper layer (0-5 km) shear runs produced shorter-lived, weaker vortices relative to the control suggesting that low-level shear is more important for mesovortex strength and longevity. Consistent with previous modeling results, mesovortex evolution is also very sensitive to coriolis forcing with stronger, longer-lived vortices formed as f is increased. Finally, it will be shown that weakening the cold pool relative to the control run, produces weaker and fewer mesovortices within the simulated bow echo. The physical processes responsible for the aforementioned sensitivities will be presented at the conference.

To examine the mesovortex genesis mechanism, analyses of a higher resolution nested simulation will be presented. This simulation is similar to the aforementioned control run except that the outer domain was run at 1 km horizontal resolution with an inner 333 m domain centered on the leading edge of the bow echo. Analyses of the higher resolution domain data illustrating the genesis mechanism of the mesovortices is ongoing and will be presented at the conference.