Multiscale Analyses of Inland Tropical Cyclone–Midlatitude Jet Interactions: Camille (1969) and Danny (1997)

The purpose of this presentation is to document the synoptic background and underlying mesoscale processes that lead to inland flooding associated with TC Camille and the inland reintensification of TC Danny. Severe inland flooding occurred over west-central Virginia on 20 August 1969 as the remnants of TC Camille interacted with a lower-tropospheric baroclinic zone beneath the equatorward entrance region of an upper-level jet over mountainous terrain. The worst of the flooding occurred over West Nelson Country, Virginia, where 690 mm of rain were recorded. On 24 July 1997, TC Danny reintensified unexpectedly back to tropical storm strength over northeastern North Carolina as it interacted with a lower-tropospheric baroclinic zone beneath the equatorward entrance region of an upper-level jet. Sustained winds around TC Danny increased from 10 m s⁻¹ to 20 m s⁻¹ and the central pressure decreased from 1012 hPa to 1000 hPa before it emerged over the Atlantic Ocean east of Virginia. This increase in sustained wind speed and decrease in central pressure while TC Danny was inland is the greatest when compared to nine other TCs in a 61-year list (1950–2010) of inland reintensifying TCs over the eastern United States.

ERA-40 and the NCEP CFSR (Climate Forecast System Reanalysis) global gridded datasets, available at 1.125° and 0.5° resolution, are used to construct multiscale analyses for the TC Camille and TC Danny cases, respectively. Detailed surface analyses are employed to identify and document the life cycles of underlying mesoscale features associated with both storms. Hourly radar summary charts are used in the TC Camille case study to track the evolution of the precipitation over west-central Virginia. WSR-88D radar datasets and satellite imagery are used to identify structural changes in the convective and stratiform precipitation around TC Danny as it reintensified. A PV perspective is employed to facilitate the interpretation of the multiscale analyses.

Results suggest that the inland flooding associated with the passage of the remnants of TC Camille across the Appalachians can be attributed to: (1) enhanced tropospheric-deep ascent beneath the equatorward entrance region of the downstream upper-level jet; (2) a moist, lower-level southerly flow that ascended over the lower-tropospheric baroclinic zone; (3) a weak surface baroclinic zone that served as a focus for frontogenesis and mesoscale ascent; and (4) heavy upslope precipitation in the mountains. The inland reintensification of TC Danny can be attributed to: (1) frontogenesis along the lower-tropospheric baroclinic zone and associated tropospheric-deep ascent beneath the equatorward entrance region of the upper-level jet; (2) deep convection that provided a source of diabatic heating and reinforced the ascent near the storm center; (3) upper-tropospheric PV reduction north of the storm center that strengthened the meridional PV gradient and the associated jet; and (4) lower-tropospheric vorticity growth in an environment that favored enhanced ascent near the storm center.