Synoptic-scale environments and dynamical mechanisms associated with predecessor rain events ahead of tropical cyclones
In order to investigate their preferred synoptic-scale environments, PREs occurring during 1988–2008 are stratified into three distinct categories: “jet in ridge,” “southwesterly jet,” and “downstream confluence,” based upon the configuration of the upper-tropospheric flow within which the TC is embedded. PRE-relative composites are presented to elucidate key dynamical processes in each category. While the composites indicate the importance of a poleward-moving stream of deep moisture from the TC, a low-level baroclinic zone, and an upper-level jet streak, the location, orientation, and magnitude of these key synoptic-scale features differ markedly among the three PRE categories. Our results therefore are suggestive of three distinct flow configurations favoring PRE development.
Dynamical and physical mechanisms associated with PREs are elucidated through a case study of a high-impact “downstream confluence” category PRE (event rainfall totals >100 mm), which occurred during 30–31 August 2006 in advance of TC Ernesto. This PRE developed in central and eastern North Carolina and Virginia as a poleward stream of deep tropical moisture (precipitable water values ~50 mm) interacted with a stalled cold front and a region of cold-air damming beneath the equatorward entrance region of a 200-hPa jet streak. Through the duration of the PRE, lifting within a mesoscale region of enhanced baroclinicity on the southeastern flank of an evaporationally generated cold pool provided a focus for the continuous development of stratiform and convective rainfall. Subsequently, significant flooding occurred as TC Ernesto (and its associated rain shield), aided by weak southwesterly steering flow, moved poleward over the region that had been affected by the PRE ~24 h earlier.