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Kevin C. Prince
and
Clark Evans

Abstract

A tropical cyclone (TC) that recurves into the midlatitudes can lead to significant downstream flow amplification by way of a favorable interaction with the midlatitude waveguide. Current conceptualizations emphasize the role of the meso-α- to synoptic-scale diabatically enhanced vertical redistribution of potential vorticity in facilitating downstream flow amplification following the interaction of a TC with the midlatitude waveguide. Less understood, however, is the extent to which this downstream flow amplification may be facilitated by the convective-scale diabatically enhanced horizontal redistribution of potential vorticity. Consequently, this study aims to diagnose the role that deep, moist convection in an associated predecessor rain event north of the TC played in influencing the midlatitude waveguide and potentially the downstream evolution. A convection-allowing numerical simulation is performed on a predecessor rain event that precedes the interaction of North Atlantic TC Irma in September 2017 with the midlatitude waveguide. Horizontal gradients in microphysical heating result in intense convective-scale potential vorticity dipoles aligned perpendicular to the vertical wind shear vector, with the negative anomaly poleward (and thus closer to the midlatitude waveguide) of the large-scale southwesterly vertical wind shear vector. Regions of intensely negative potential vorticity persist for multiple hours after their formation as they become deformed by the large-scale strain field that is aligned parallel to the background vertical wind shear vector. The deformation-driven thinning of the negative potential vorticity is associated with a transfer of energy to the large-scale flow, suggesting a nonnegligible impact to the TC–midlatitude waveguide interaction by the collection of convective cells embedded in the predecessor rain event.

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Clark Evans
,
Morris L. Weisman
, and
Lance F. Bosart

Abstract

In this study, the dynamical processes contributing to warm-core meso-β-scale vortex formation associated with the 8 May 2009 “super derecho” are examined utilizing two complementary quasi-Lagrangian approaches—a circulation budget and backward trajectory analyses—applied to a fortuitous numerical simulation of the event. Warm-core meso-β-scale vortex formation occurs in a deeply moist, potentially stable environment that is conducive to the development of near-surface rotation and is somewhat atypical compared to known derecho-supporting environments.

Air parcels in the vicinity of the developing vortex primarily originate near the surface in the streamwise vorticity-rich environment, associated with the vertical wind shear of the low-level jet, immediately to the east of the eastward-moving system. Cyclonic vertical vorticity is generated along inflowing air parcels primarily by the ascent-induced tilting of streamwise vorticity and amplified primarily by ascent-induced vortex tube stretching. Descent-induced tilting of crosswise vorticity contributes to cyclonic vertical vorticity generation for the small population of air parcels in the vicinity of the developing vortex that originate to its north and west. No consistent source of preexisting vertical vorticity is present within the environment.

Cyclonic circulation on the scale of the warm-core meso-β-scale vortex increases in the lower troposphere in response to the mean vortex-scale convergence of cyclonic absolute vorticity and the local expulsion of eddy anticyclonic vertical vorticity generated within the system’s cold pool. Increased cyclonic circulation is partially offset by the system-scale tilting of horizontal vorticity associated with the low-level jet, rear-inflow jet, environmental vertical wind shear, and rotational flow of the warm-core vortex itself.

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