Coastal, Quasi-stationary Cold Pools and their Influence on Deep Moist Convection during the 2018 Kauai Rainfall Event

The Hawaiian Islands' complex and coastal terrain features are integral to convective initiation and the development of storms that produce torrential rainfall and flash flooding. However, the complicated coastal terrain geometries can also influence the propagation and location of cold pools, further exacerbating the initiation and intensity of convection and the produced rainfall. A high-resolution WRF model analysis of the 2018 Kauai rainfall event indicates that the coastal terrain geometries of Northern Kauai favorably contributed to cold pool damming in response to the intense and persistent thunderstorms during this event. Cold pools are closely bounded between the north-facing slopes of Kauai’s coastal terrain and the ocean. Cold pool gust front boundaries oscillate zonally within this region. These quasi-stationary cold pools instigated multiple rounds of deep moist convection with high precipitation supercell characteristics that contributed to the record-breaking rainfall reported at Waipa Garden on April 15, 2018.

This study aims to understand the evolution of cold pools during the 2018 Kauai rainfall event and investigate how gust front boundaries modulated the intensity of rainfall observed over Waipa Garden. A pressure decomposition technique is applied within the WRF model to isolate the thermodynamic and dynamic vertical acceleration associated with cold pool gust fronts produced by these storms. Results suggest the interaction of the quasi-stationary cold pool boundaries with the environmental inflow imparts dynamic vertical accelerations within the boundary layer that often dominate over effective buoyancy, contributing to lower storm updraft bases, increased frequency of convective initiation, and the tilting/stretching of vorticity within these updrafts. The lofting of surface-level vorticity anomalies from cold pools also contributes to dynamic vertical acceleration that is well correlated to periods of enhanced rain rates. Altogether, the dynamic vertical acceleration lifts parcels above the level of free convection to heights where greater effective buoyancy is realized. Thereafter, effective buoyancy becomes the dominant means of vertical acceleration within the middle to upper levels of thunderstorms, contributing the greatest to record-breaking rainfall.