Max Ungar - March 11

Convective Meteorology (Mesoscale Dynamics)

Using Radiosonde Observations to Assess The ‘Three Ingredients’ Method to Forecast QLCS Mesovortices

Max Ungar

Friday, March 11

03:00 PM

Quasi-Linear Convective System (QLCS) mesovortices have a propensity to produce impactful severe weather outcomes, including tornadoes and enhanced straight-line wind damage. Despite advances in understanding the origins of these meso-gamma scale vortices over the past two decades, a relative lack of operational tools remain for anticipating their genesis. The “Three Ingredients” (3-I) Method, developed by Schaumann and Przybylinski (2012), identifies favored QLCS mesovortexgenesis conditions for operational benefit. The 3-I Method assesses both internal system characteristics along with the pre-convective environment to aid in early identification of mesovortex potential. The 3-I Method are: 1) a portion of a QLCS in which the system cold pool and ambient low-level shear are said to be nearly “balanced” or “slightly shear dominant”, 2) where 0-3 km line-normal bulk wind shear magnitudes are greater than or equal to 15 ms-1/30 knots, and 3) where a rear-inflow jet or enhanced outflow causes a surge or bow along the convective line. The first ingredient refers to the theory that attributes a strongly forced updraft to a “balance” between cold-pool-generated horizontal vorticity and the horizontal vorticity (of opposite sign) in the ambient environment. Additionally, these ingredients relate mesovortexgenesis to the tilting of horizontal vorticity by either updrafts or convectively induced downdrafts. Numerical simulations have revealed these processes to be favored in the presence of moderate to strong line-normal wind shear in the lowest few kilometers.

Environmental data from radiosonde observations in proximity to 50 observed QLCSs, including 9 from past field programs (PECAN, VORTEX-SE, MESO18-19), were compared against QLCS characteristics revealed by nearby WSR-88D observations. Analysis of internal characteristics included assessing the shear/cold pool balance, rear-inflow surges, and observed mesovortices. Initial results show that nearly half (46%) of mesovortexgenesis events were observed in regimes of sub-critical line-normal 0-3 km wind shear. This agrees with recent studies that found mesovortexgenesis processes can be quite varied and can occur for line-normal shear under the 15 ms-1/30 knot threshold. Additionally, Updraft Downdraft Convergence Zone (UDCZ)-to-reflectivity distance, related to shear/cold pool balance, appears to offer more skill in discriminating mesovortexgenesis potential than line-normal 0-3 km wind shear. This study also offers potential objective criteria for defining favorable shear/cold pool balance “regimes” for mesovortexgenesis. Further analysis will explore the distribution of shear over shallower layers, along with line-parallel components, to attempt to explain frequent mesovortexgenesis for sub-critical 0-3 km line-normal wind shear.