Tropical Cyclone Outer Size Impacts the Number and Location of Tornadoes

Abstract

Landfalling tropical cyclones (TCs) often produce tornadoes in addition to other hazards. Most tornadoes occur during the afternoon within 100-500 km of the TC center during the 48-h period before and after landfall (Novlan & Gray, 1974;Schultz & Cecil, 2009). Compared to their non-TC counterparts, TC tornadoes typically: 1) are less damaging (Edwards, 2010;Edwards, 2012), 2) are produced by "miniature" supercells (Spratt et al., 1997;Edwards et al., 2012), and 3) occur in strong vertical wind shear and sufficient thermodynamic instability concentrated over a shallow lower-tropospheric layer (McCaul, 1991;McCaul & Weisman, 1996). These favorable kinematic conditions are due to the TC warm-core structure and surface friction (Novlan & Gray, 1974;Gentry, 1983), while favorable thermodynamic environments are associated with weak convective inhibition and CAPE typical of the tropics (McCaul, 1991;Molinari et al., 2012). Moreover, recent work has shown that ambient deep-tropospheric vertical wind shear (VWS; i.e., the difference between the 850-and 200-hPa ambient winds) is a key factor controlling the number and TC-relative azimuthal location of tornadoes (Schenkel et al., 2020;Schenkel et al., 2021). However, we lack a complete understanding of the factors controlling the number and TC-relative radial location of tornadoes. One understudied factor that may impact the number and TC-relative radius of tornadoes is the outer size of the TC (McCaul, 1991). Indeed, TC outer size is crucial in describing the scale and magnitude of hazards (Lin et al., 2014;Chavas et al., 2017), and is the focus of this study.

Lightning and Radar Characteristics of Tornadic Cells in Landfalling Tropical Cyclones

“…We also examine the association of lightning with: (a) distance of the tornadic and non‐tornadic cells from the center of the TC and (b) TC intensity at the time of tornadic and non‐tornadic cell occurrence as shown in Figures 4a and 4b, respectively. Although other factors were explored based upon prior work (e.g., synoptic‐scale deep‐tropospheric vertical wind shear, tornado damage rating; Schenkel et al., 2020; Paredes et al., 2021), these factors did not have strong relationships with lightning and are not shown here in the interest of brevity. Figure 4 shows two key results: (a) both tornadic and non‐tornadic cells tend to be associated with lightning more frequently as the distance from the TC center increases and (b) tornadic cells in hurricane‐strength TCs are associated with lightning more often than non‐tornadic cells.…”

“…This large variability may be associated with differences in the distance of tornadic and non‐tornadic cells from the TC center among TCs as well as the intensity of the TC. These factors may also be proxies for other factors like TC outer size, especially since larger TCs tend to be associated with more tornadoes that occur farther from the TC center due to a broader set of TC radii with favorable kinematic environments (McCaul, 1991; Paredes et al., 2021). The location of flashes also differs strongly between the two subsets with lightning concentrated to the northeast of tornadic cells, whereas lightning is distributed more symmetrically around non‐tornadic cells.…”

“…These conditions lead to enhanced charge separation between graupel and ice particles and, consequently, a greater frequency of IC and negative polarity CG lightning compared to the eyewall (Black & Hallett, 1999; Molinari et al., 1999). The larger CAPE in the TC outer region, concentrated in the lower troposphere, compared to the inner core (Bogner et al., 2000; Molinari et al., 2012) together with strong lower‐tropospheric vertical wind shear typically also provides the most favorable kinematic and thermodynamic environments for tornadic convection in the TC (McCaul, 1991; Paredes et al., 2021).…”