Strengths and Limitations of NSSL's Experimental Warn-on-Forecast System for Ensembles in Forecasting Cyclic Mesocyclogenesis

Cyclic mesocyclogenesis is the process by which a supercell produces multiple mesocyclones with similar lifecycles. Cyclic supercells have the potential to produce several tornadoes that could cause widespread damage, so an improved ability to forecast them would be beneficial to public safety. However, cyclic mesocyclogenesis is highly sensitive to both environmental and computational parameters, so whether storm-scale models can resolve and predict cycling has yet to be determined. Therefore, this study analyzes forecasts generated by NSSL’s Experimental Warn-on-Forecast System for Ensembles (NEWS-e) of four cyclic supercells occurring 9 May, 16 May, and 18 May 2017. NEWS-e was created as part of the Warn-on-Forecast (WoF) initiative, and is a convection-allowing model with 18 forecast members. The goal of WoF is to produce accurate, probabilistic, short-range (0-3 hr) forecasts for severe convective events.

The purpose of this research is to test the capability of a short-term, storm-scale model in resolving the cycling process, whether this process is physically representative of the current understanding for cyclic supercells, and if it can be used to forecast these storms. In order to provide a source of verification for the model output, a database of observations was created from manually analyzed WSR-88D radar reflectivity and radial velocities for each supercell. Using the real-time NEWS-e forecasts as a control, sensitivity experiments will be conducted that include changing the NEWS-e grid spacing from 3 km to 1 km, examining the effects of each forecast member’s different PBL and radiation schemes, and how changes in the environment across the different storms can affect the cycling process.

Radar analyses of cyclic supercells exhibiting both nonoccluding and occluding forms of cyclic mesocyclogenesis is observed in radar data collected within a single supercell. Additionally, preliminary analyses of NEWS-e forecasts with a 3-km grid spacing show evidence of cyclic mesocyclogenesis-like processes, which was previously thought to be highly improbable from research that suggested the coarsest grid resolution to resolve cycling was 1 km. This research supports the long-term goal to produce an advanced version of NEWS-e that would run at a finer horizontal resolution, such as 1 km, to better represent and forecast severe convection events, like cyclic supercells.