In late April, Jessica King, presented a summary of much of her CSTAR research simulating severe and non-severe high shear, low CAPE convective events. The presentation focused on the examination of the rapid destabilization that occurs in the few hours leading up to severe convection in the simulated events and the mechanisms responsible for the rapid changes in CAPE. Links to the PowerPoint slides and the recorded webinar are available at the bottom of the post. Some notes from the presentation are shared below.
Low instability severe thunderstorms are heavily concentrated in the Southeast especially in the Ohio, Tennessee, and Mississippi Valleys. These events tend to occur in the cool season, often in the overnight and early morning hours, and frequently with a convective mode of mini-supercells or QLCSs. The occurrence of severe convection in low instability environments can often be explained by the presence of synoptic scale forcing and mid-latitude cyclones. The research goal was to determine mechanisms by which environmental conditioning occurs in severe and non-severe high shear, low CAPE thunderstorm events.
Jessica conducted real-data simulations of high shear, low CAPE events when there was at least a “slight” risk for severe storms and the SfcOA (SPC mesoanalysis) CAPE ≤ 1000 J kg-1 and 0-3 km shear ≥ 18 m s-1. She developed a 3-hour time series analysis for separate points in a 7×7 grid for the three hours prior to severe convection. The 0-1km wind shear showed some discrimination between severe and non-severe events with values higher in severe events, especially at night. The wind shear tended to remain relatively steady over time while the CAPE increased over time.
Calculating Contributions to CAPE
Advection of high theta-e air often leads to an increase in CAPE. A plot of 3-hour change in SBCAPE up to the event time shows a much larger increase in SBCAPE prior to the event for severe events with a smaller increase for non-events. An increase in CAPE can be realized by 1) increasing surface temperature 2) increasing surface moisture or 3) decreasing temperature aloft. The mechanisms for destabilization varied significantly among all environments with significant destabilization occurring in the 3 hours prior to severe events. The change in surface moisture was a positive contribution to increase in CAPE for all cases. Warming near the surface was important in destabilizing all of the severe events as well, and cooling near the surface may be detrimental in the non-severe events. The increase in surface temperature was noted for both daytime and nocturnal cases.
Synoptic Forcing for Ascent
Forcing for ascent can be driven by processes such as warm advection, lifting by a front or boundary, and cyclonic vorticity advection aloft. These processes can lead to the release of potential instability. The release of potential instability through layer lifting occurred prominently in 4 of the simulated severe events. In these 4 cases, the 3km vertical velocity increases significantly and the 0-3km lapse rate became less negative in the hour before the severe event as the synoptic forcing approaches.
The SHERBS1 and to a lesser extent the SHERBS3 were discriminators between the severe and non-severe events. The skill of the SHERBS1 was likely a result of the 0-1km shear serving as a good differentiator. Work is underway to modify the SHERB parameters to perhaps include a term to account for the release of potential instability.
Future Work and Looking Ahead
• Investigate larger scale observations and climatologies to identify recurring synoptic-to-mesoscale patterns in cool season, low instability events in the Southeast.
• Examine the vertical distribution of CAPE.
• The recognition of patterns in high-resolution model data and observations that indicate a low CAPE environment has the potential to evolve into a severe-weather producing environment.
• The availability of datasets and tools to examine data in the highest temporal resolution possible is critical. In many events, some of the higher resolution data doesn’t show a supportive environment 3 hours in advance of the event but a rapid change in low-level moisture or temperature can occur just ahead of the line that provides the needed buoyancy. Sub-hourly time scales or other perspective may provide an opportunity to anticipate these events.
• There may be an R2O pathway with the near-storm situational awareness environment tool that is under development which provides a means for detailed monitoring of the environment.
• There was also an interest in requesting the SPC produce some 1 or 2 hour change fields for surface temperature and moisture.