A multiscale analysis of the inland reintensification of Tropical Cyclone Danny (1997) within an equatorward jet-entrance region

In mid-July 1997, Tropical Cyclone (TC) Danny made landfall in southeastern Louisiana as a very small and compact tropical cyclone. Danny reached a minimum central pressure of 984 hPa after it reemerged over water and subsequently made final landfall in southern Alabama, where excessive rains were reported, including nearly 960 mm over Dauphin Island. Danny weakened into a tropical depression as it slowly moved northward across Alabama, turned eastward into Georgia on 23 July, and accelerated northeastward across the Carolinas. On 24 July, Danny reintensified unexpectedly back to tropical storm strength over northeastern North Carolina as it interacted with a low-level baroclinic zone beneath an equatorward entrance region of an upper-level jet. Sustained winds around Danny reached 50 knots and the central pressure decreased from 1012 hPa to 1000 hPa before it emerged over the Atlantic Ocean east of Virginia. This presentation will document the results of a multiscale analysis of the inland reintensification of TC Danny. Synoptic and mesoscale processes will be emphasized in this multiscale analysis.

A preliminary climatology of inland reintensifying TCs is being constructed from gridded NCEP/NCAR reanalysis datasets and archived NCDC North American surface charts. Synoptic analyses of individual inland reintensifying TCs also are being constructed from gridded NCEP/NCAR reanalysis datasets for storms in the climatological dataset. More detailed synoptic and mesoscale analyses are being constructed from the gridded 0.5° NCEP Climate Forecast System Reanalysis (CFSR) datasets. The higher-resolution CFSR datasets will be used to identify and document important mesoscale circulation features associated with the observed inland reintensification of TC Danny. WRF model simulations will be used to supplement the mesoscale analyses and to help quantify the relevant dynamical forcing. WSR-88D radar datasets will be used to facilitate the mesoscale analyses and to help identify structural changes in the convection and stratiform precipitation around Danny as it reintensified. A potential vorticity thinking perspective will be employed to help elucidate storm–jet interactions.

Preliminary results suggest that the inland reintensification of TC Danny can be attributed to: (1) frontogenesis along the low-level baroclinic zone and associated tropospheric-deep ascent beneath the equatorward entrance region of the upper-level jet; (2) deep convection around Danny that provided a source of diabatic heating, which reinforced the ascent near the storm center; and (3) low-level vorticity growth around Danny through vortex-tube stretching in an environment that favored enhanced ascent near the storm center. An analysis of the relative contributions of these processes to the inland reintensification of TC Danny is ongoing.