An unusual case of rapid cyclogenesis in the northeast Pacific basin

Abstract

In late November 2019, a record-setting extratropical cyclone rapidly intensified in the northeast Pacific basin over a relatively cold sea surface and near steep topography along the United States West Coast. Cyclogenesis began as a lower-tropospheric diabatic Rossby wave (DRW) spawned from intense,differential lower-tropospheric frontogenesis at the intersection of a zonally-oriented baroclinic zone and warm, moist tropical air plume, with the cyclone then propagating eastward through a pre-existing expansive anticyclone. Then, an intensifying upper-level jet/front system (UJFS) borne from a downward surge of stratospheric air plunged equatorward from Alaska and became favorably aligned with the DRW to result in deepening rates as high as 6 hPa hr-1 before the cyclone made landfall near the California-Oregon border. The November 2019 (NV19) storm is of particular interest as the cyclone followed an unusual storm track for explosively-deepening cyclones, intensified at an extremely rapid rate based on two climatologies, and underwent its maximum deepening in a location that was the furthest east based on three climatologies. This dissertation provides a comprehensive analysis of this explosive cyclogenesis event, which includes a detailed synoptic overview and is motivated by three central research questions which explore distinct aspects of this unusual storm. Using piecewise potential vorticity (PV) inversion, the evolution ofthe storm is shown to follow a rare bottom-up development whereby most of the intensification period was driven by the DRW with additional intensification provided by the UJFS just prior to landfall. Further application of the piecewise PV inversion results reveals that the UJFS strengthened primarily due toupper-tropospheric PV with minimal influence from PV associated with the DRW, suggesting independent intensification of the UJFS and the DRW. Ensemble-based sensitivity analysis highlighted that ensemble members which forecasted a stronger NV19 storm also forecasted a later interaction between the UJFS and the DRW, which further supports the idea that these two structures primarily intensified independently. Future analysis of explosive cyclogenesis events involving a DRW and UJFS is needed, including further investigation into the NV19 storm, comparing the NV19 storm to similar explosive cyclogenesis events, and simulating the behavior of DRWs within a warming climate.