Environment characteristics associated with tornado events near closed cold core 500 mb lows

As a follow-up to previous work by Davies and Guyer (2004), this study will focus on thermodynamic and kinematic characteristics in the environments of tornado events associated with closed cold core 500 mb lows. The prior study found that such tornadoes tend to occur within roughly 300 km of a closed 500 mb low, often in the presence of marginal boundary layer moisture (surface dewpoints as low as 48-55° F), and relatively weak instability (e.g., CAPE of 1000 J/kg or less). These factors can render some tornado events near mid-latitude closed 500 mb lows difficult to forecast, motivating a closer examination of specific environment characteristics associated with these events.

For this study, a comparison was made of 39 tornadic cases to 21 non-tornadic severe cases near closed 500 mb lows using RUC proximity analysis soundings. It was found that tornadic cases tended to have more low level (0-3 km) CAPE, larger 0-1 km SRH, and greater 0-1 km VGP (vorticity generation parameter) values. In comparison to more “typical” supercell tornado environments, the cold core tornadic cases tended to have nearly double the amount of 0-3 km CAPE, often the result of a “fatter” positive CAPE area located relatively low in the vertical profile (e.g., closer to the ground). This suggests that low level parcel accelerations and resultant stretching may be important for tornado development in events associated with closed 500 mb lows.

Additionally, it was found that CAPE calculations using a shallower mixed-layer lifted parcel (e.g., the lowest 50 mb), or even surface-based parcels, appear better suited for assessing thermodynamic characteristics in tornado events associated with closed 500 mb lows. Given the apparent relatively shallow moisture layer indicated by the RUC soundings in many such events, the lowest 100 mb often may be too deep a mixed layer to properly represent buoyancy and associated environment characteristics in such settings.

References:

Davies, J.M., and J.L. Guyer, 2004: A preliminary climatology of tornado events with closed cold core 500 mb lows in the central and eastern United States. Preprints, 22nd Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. Soc. (CD ROM).