Near-Surface Frontogenesis and Atmospheric Instability along the U.S. East Coast during the Extratropical Transition of Hurricane Matthew (2016)

Hurricane Matthew locally generated more than 400 mm of rainfall on 8–9 October 2016 over the eastern Carolinas and Virginia as it transitioned into an extratropical cyclone. The heaviest precipitation occurred along a swath situated up to 100–200 km inland from the coast and collocated with enhanced low-tropospheric frontogenesis. Analyses from version 3 of the Rapid Refresh (RAPv3) model indicate that rapid frontogenesis occurred over eastern North and South Carolina and Virginia on 8 October, largely over a 12-h time period between 1200 UTC 8 October and 0000 UTC 9 October. The heaviest rainfall in Matthew occurred when and where spiral rainbands intersected the near-surface front, which promoted the lift of conditionally unstable, moist air. Parallel to the spiral rainbands, conditionally unstable low-tropospheric warm, moist oceanic air was advected inland, and the instability was apparently released as the warm air mass rose over the front. Precipitation in the spiral rainbands intensified on 9 October as the temperature gradient along the near-surface front rapidly increased. Unlike in Hurricane Floyd over the mid-Atlantic states, rainfall totals within the spiral rainbands of Matthew as they approached the near-surface front evidently were not enhanced by release of conditional symmetric instability. However, conditional symmetric instability release in the midtroposphere may have enhanced rainfall 200 km northwest of the near-surface front. Finally, although weak cold-air damming occurred prior to heavy rainfall, damming dissipated prior to frontogenesis and did not impact rainfall totals.

Assessment of Conditional Symmetric Instability from Global Reanalysis Data

Slantwise convection, the process by which moist symmetric instability is released, has often been linked to banded clouds and precipitation, especially in frontal zones within extratropical cyclones. Studies also suggest that the latent heat release associated with slantwise convection can lead to a spinup of surface frontogenesis, which can enhance the rapid intensification of extratropical cyclones. However, most of these studies considered only local areas or short time durations. In this study, we provide a novel statistical investigation of the global climatology of the potential occurrence of slantwise convection, in terms of conditional symmetric instability, and its relationship with precipitating systems. Using the 6-hourly ERA-Interim, two different indices are calculated, namely, slantwise convective available potential energy (SCAPE) and vertically integrated extent of realizable symmetric instability (VRS), to assess the likelihood of occurrence of slantwise convection around the globe. The degree of association is quantified between these indices and the observed surface precipitation as well as the cyclone activity. The susceptibility of midlatitude cyclones to slantwise convection at different stages of their life cycle is also investigated. As compared to the nonexplosive cyclone cases, the time evolution of SCAPE and VRS within rapidly deepening cyclones exhibit higher values before, and a more significant drop after, the onset of rapid intensification, supporting the idea that the release of symmetric instability might contribute to the intensification of storms.