The Subtropical Jet Stream Delivers the Coup de Grâce to Hurricane Felicia (2009)

Abstract

he NOAA Gulfstream IV (G-IV) routinely deploys global positioning system dropwindsondes (GPS sondes) to sample the environment around hurricanes that threaten landfall in the United States and neighboring countries. Part of this G-IV synoptic surveillance flight pattern is a circumnavigation 300?350 km from the circulation center of the hurricane. Here, the GPS sondes deployed over two consecutive days around Hurricane Felicia (2009) as it approached Hawaii are examined. The circumnavigations captured only the final stages of decay of the once-category-4 hurricane. Satellite images revealed a rapid collapse of the deep convection in the eyewall region and the appearance of the low-level circulation center over ~8 h. Midlevel dry air associated with the Pacific high was present along portions of the circumnavigation but did not reach the eyewall region during the period of rapid dissipation of the deep clouds. In contrast, the subtropical jet stream (STJ) enhanced the deep-layer vertical shear of the horizontal wind (VWS; 850?200 hPa) to greater than 30 m s?1 first in the northwest quadrant; ~6 h later the STJ was estimated to reach the eyewall region of the hurricane and was nearly coincident with the dissipation of deep convection in the core of Felicia. Felicia?s demise is an example of the STJ enhancing the VWS and inhibiting intense hurricanes from making landfall in Hawaii. The authors speculate that VWS calculated over quadrants rather than entire annuli around a hurricane may be more appropriate for forecasting intensity change.