Effects of Horizontal Grid Spacing and Inflow Environment on Cyclic Mesocyclogenesis in NSSL's Warn-on-Forecast System (WOFS)

Cyclic mesocyclogenesis is the process by which a supercell produces multiple mesocyclones with similar life cycles. These storms have the potential to produce several tornadoes that may cause widespread damage. Therefore, having the ability to forecast the potential for and cycling frequency of cyclic supercells may be beneficial to forecasters when issuing watches and warnings. However, idealized simulations of cyclic mesocyclogenesis have found cycling to be highly sensitive to both environmental and computational parameters. Thus, whether convective allowing models can resolve and predict cycling has yet to be determined.

The purpose of this study is to test the capability of a short-term, storm-scale, ensemble predictive system to resolve the cycling process, whether this process is physically representative of the current understanding of cyclic supercells, and if it can be used to provide useful forecasts of these storms. Two experiments are performed using forecasts generated by NSSL’s Warn-on-Forecast System (WOFS) – a short-term, convection-allowing ensemble – for four cyclic supercells occurring in May 2017. The first experiment tested the effects of changing the WOFS horizontal grid spacing from 3 km to 1 km. Rare cases of cyclic-like processes were identified at 3 km, but, as expected, cycling occurred far more frequently at 1 km. The second experiment analyzed the different environmental conditions of the WOFS forecasts. Object-based identification was used to identify the mesocyclones and extract environmental inflow parameters from a storm-relative 150°slice with a radius of 80 km from the center of each mesocyclone. Lower magnitudes of storm-scale parameters like 0 – 1 km SRH, 0 – 3 km SRH, and STP are present for rapid-cycling supercells and higher values are present for slow-cycling cases. These results provide initial evidence that high-resolution WOFS forecasts can potentially provide useful guidance on the likelihood and cycling frequency of cyclic supercells.