Operational Application of 0-3 km Bulk Shear Vectors in Assessing Quasi Linear Convective System Mesovortex and Tornado Potential

Numerical model simulation results from the early 1990s showed the strength and longevity of quasi-linear convective systems (QLCSs) are highly dictated by horizontal vorticity rolls generated by both the system cold pool and ambient low-level shear. Portions of a QLCS that exhibit a general balance between the system cold pool and low-level environmental shear tend to persist longer, and have a greater probability for producing damaging straight-line winds. Additionally, observations have shown that the genesis and persistence of mesovortices within a QLCS tend to be favored where this balance exists, or where environmental shear is slightly dominant. Recent numerical simulation results have also shown that increasing magnitudes of low-level shear result in the increased potential for the genesis of mesovortices. While a large amount of research exists documenting the statistical significance of increasing shear magnitudes over varying layers, recent research has explored the importance of system normal magnitudes. Operational observations indicate that line normal 0-3 km bulk shear magnitudes greater than or equal to 15 m s-1 support an increased risk for QLCS mesovortices and attendant tornadoes.

One of the main goals of this study is to consolidate past research and present a methodology to mesoanalysts and radar operators on the utilization of 0-3 km bulk shear vectors. This methodology would then be used in conjunction with radar interrogation, ground-truth reports, objective analysis, etc. to highlight areas of potential QLCS tornadogenesis in real-time. Given that these vectors are relatively easy to derive and plot, they can be a very valuable tool not only in a real-time sense, but also in the forecast process. This study will specifically examine the effectiveness of system normal 0-3 km bulk shear vectors to diagnose areas within QLCSs where differing shear/cold pool regimes are favored. Additionally, 0-3 km bulk shear will be explored to determine if there is a correlation between mesovortex development and increasing amounts of system normal shear magnitude.

The first part of this presentation will serve as a review of differing shear/cold pool balance regimes. The second portion of this presentation will apply the use of 0-3 km shear vectors to three case studies. While it is fully acknowledged that many other atmospheric parameters should be closely scrutinized to assess the true potential for severe convection, the main focus of this presentation will be the utility of 0-3 km shear vectors. For the sake of orientation and familiarization, a brief overview of the synoptic and mesoscale setup will be presented for each case. Plots of the 0-3 km bulk shear vectors will be presented in conjunction with radar data and other atmospheric parameters. Key radar features critical to mesovortex genesis will also be briefly touched upon. Finally, we will show the lifecycle of at least one of the mesovortices utilizing a rotational velocity trace.

The first case that will be examined occurred during the overnight hours of 26-27 June 2011. This case featured a large QLCS that produced significant wind damage from southern Iowa into northeastern and east-central Missouri. This QLCS went on to produce multiple tornadoes across parts of northeastern Missouri as a portion of the line pivoted and became favorably oriented with the 0-3 km shear vectors. The second case that will be featured occurred across central Missouri during the late evening and early overnight hours of 18-19 June 2011. This case consisted of a relatively small bow echo that took on forward propagating characteristics. As this system accelerated eastward, three tornadoes were spawned on the north side of the bow apex roughly10-15 miles north and west of Springfield, Missouri. This system would then go on to spawn additional tornadoes farther east in Crawford County, Missouri. The third case depicts a forward propagating MCS that moved nearly due south across southern Illinois and eastern Missouri during the late afternoon and early evening of 19 July 2006. While this system was orientated nearly perpendicular to the 0-3 km shear vectors, magnitudes appeared to be too weak to support mesovortex development. This system did, however, produce a large swath of wind damage, with multiple reports of winds exceeding 35 m s-1.