Abstract

Landfalling Tropical Cyclones (TCs) may induce tornadoes, while less frequent than their midwestern counterparts, are still able to produce the damage potential leading to additional billion-dollar costs. This research seeks to find some common ingredients for tornadogenesis associated with landfalling TCs impinging on synoptic frontal boundaries, during which the TC’s circulation has been strongly affected on both the vorticity, upward velocity, and other surrounding environmental, convective, and tornadic parameters. This research found that by increasing the vorticity and vertical velocity associated with a cold frontal boundary, moving or stalled, and placing it into a moist convectively primed environment surrounding the TC increases the chance for a tornado outbreak to occur. The most frequent outbreaks occur when a cold frontal boundary has a direct interaction with the circulation of an approaching TC, such as seen in Hurricane Michael (2018) when it was moving through North Carolina (NC) Piedmont into the Virginian Coastal Plains. In this case, the vorticity, surface moisture, vertical velocity, and surface convergence are all observed to have modestly large increases leading to the environment conducive to tornadogenesis. This prefrontal environment also has an area of moderate to large Convectively Available Potential Energy (CAPE) with values over 500 J kg^-1 as well as Potential Instability (PI) index with the vertical gradient of potential temperature (∂θ/∂z) becoming negative. Storms interacting with a stalled frontal boundary, such as Hurricane Florence (2018), also have this increase in vorticity though to a lesser extent. During the interaction with Florence vorticity peaked in southeast NC and northeast South Carolina associated with the outer band that was located near the stalled front. This area was in the favorable front right quadrant and experiencing a boost in CAPE from the flow of warm moist air off the Gulf Stream. The main driver of vorticity in this location was mainly due to the vertical vorticity stretching generated by the low-level flow convergence associated with the interaction of the flow around the TC and the stalled front, which occurred over the southern coastal areas of NC. In the case of tornado outbreaks not associated with a frontal boundary, such as Hurricane Allen (1980), the largest driver tends to be the TC itself with Allen strong vorticity advection, abundant moisture, and large CAPE, from the moist and unstable airstream from the Gulf of Mexico, into southern Texas, a region that is typically favorable for tornado development being near the crossroads of Tornado Alley and Dixie Alley (Klemp, 1987), a zone that frequently combines the conditions needed for tornadogenesis with hot dry air from the north and west meeting warm moist air from the south and east.

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