Numerical Simulations of the Extratropical Transition of Floyd (1999): Structural Evolution and Responsible Mechanisms for the Heavy Rainfall over the Northeast United States

Abstract

This paper examines the extratropical transition (ET) of Hurricane Floyd along the U.S. East Coast on 16?17 September 1999 using the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) down to 1.33-km horizontal grid spacing. The 36-h MM5 simulation reproduced the basic features of the ET event such as the track of Floyd, the development of a deep and intense baroclinic zone along the coast and its associated precipitation evolution, and the tendency for the heavy (>30 cm) precipitation to fall in a relatively narrow (30?40 km wide) band just inland of the coast; however, the MM5 overpredicted the moderate (10?20 cm) precipitation amounts near the coast by 40%?50% as the horizontal grid spacing was reduced to 1.33 km. The MM5 was used to diagnose the evolution of the enhanced baroclinic zone and associated heavy precipitation to the north of Floyd. A deep layer of deformation frontogenesis extended from the surface to 400 mb as a result of confluence between the southeasterlies to the northeast of Floyd at all levels and the inland northeasterlies and southwesterlies at low and midlevels, respectively. A combination of strong frontogenesis, moist symmetric instability below 800 mb, and slantwise neutrality aloft resulted in the narrow and intense band of precipitation just inland of the coast. A separate simulation without the Appalachians and coastal terrain had little effect on Floyd's wind and temperature evolution, and heavy precipitation (>30 cm) still developed just inland of the coast; therefore, terrain played a relatively minor role in the devastating flooding for this particular event over the Northeast. Frontogenesis calculations revealed that the upper-level baroclinic zone over the northeast United States was enhanced by a horizontal gradient in midlevel latent heating between the heavy precipitation near the coast and the lighter precipitation farther inland. This was also verified by completing a simulation without latent heating, which resulted in much less baroclinicity and downstream ridging aloft. In addition, without latent heating, the central pressure of Floyd was 25 mb weaker than the full-physics (control) run, and the storm only slowly moved up the coast. Without evaporative effects from precipitation, the low-level front was 10%?20% weaker than the control, and Floyd's central pressure was about 4 mb weaker. Another simulation without surface heat fluxes resulted in a 4?5 mb weaker cyclone, and 20%?30% less precipitation shifted 100?150 km farther eastward than the control.