The Role of Wind Shear, Vertical Vorticity, and Pressure Perturbations on a Terrain Anchored, High-Precipitation Supercell

The complex terrain features of Hawaii are an integral ingredient in flash-flooding events for the Hawaiian Islands. The majority of flash-flood events in Hawaii can be attributed to deep layer moisture associated with kona lows, cold fronts, upper-tropospheric troughs, or tropical systems. However, more rarely, Hawaii experiences flash-flood events with more dynamic storm modes, such as bow-echoes and supercells. One such event, the 2018 Kauai rainfall event that established a new U.S. 24-h record at Waipa Garden, consisted of three periods of prolonged, terrain anchored, deep moist convection. Observational evidence suggests that instability, strong upslope flow, and veering winds with height resulted in mesocyclone storm rotation within the convective cells that produced rain rates approaching 140 mm per hour. The regeneration, advection, and tilting of these highly efficient precipitating and rotating cells over and down shear of terrain, normal to the upslope flow, resulted in updraft/downdraft separation and prolonged periods of rainfall down shear and over Waipa Garden.

The goal of this study is to investigate in detail the role vorticity played in the prolonged and heavy period of rainfall over Waipa. To address this goal, a large eddy simulation using WRF at resolution of 100 meters was completed over Kauai for this event. The model evolution of vertical vorticity and its tendencies over time will be presented to elucidate the components contributing to vertical vorticity. Second, pressure perturbations associated with buoyancy and dynamic effects, including linear and non-linear processes, will be addressed. The overarching goal of the analysis is to clarify how the complex terrain interacted with veering winds to generate rotating updrafts, slow moving cells, and extreme rainfall rates that resulted in the record rainfall accumulation. The results of the investigation will be presented along with a brief discussion of their implications.