A Climatological Analysis of Ambient Deep-Tropospheric Vertical Wind Shear Impacts upon Tornadoes in Tropical Cyclones

Abstract

The cyclone-relative location and variability in the number of tornadoes among tropical cyclones (TCs) are not completely understood. A key understudied factor that may improve our understanding is ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS), which impacts both the symmetry and strength of deep convection in TCs. This study conducts a climatological analysis of VWS impacts upon tornadoes in TCs from 1995–2018, using observed TC and tornado data together with radiosondes. TC tornadoes were classified by objectively defined VWS categories, derived from reanalyses, to quantify the sensitivity of tornado frequency, location, and their environments to VWS. The analysis shows that stronger VWS is associated with enhanced rates of tornado production—especially more damaging ones. Tornadoes also become localized to the downshear half of the TC as VWS strengthens, with tornado location in strongly sheared TCs transitioning from the downshear left quadrant in the TC inner core to the downshear right quadrant in the TC outer region. Analysis of radiosondes shows that the downshear right quadrant in strongly sheared TCs is most frequently associated with sufficiently strong near-surface speed shear and veering aloft, and lower-tropospheric thermodynamic instability for tornadoes. These supportive kinematic environments may be due to the constructive superposition of the ambient and TC winds, and the VWS-induced downshear enhancement of the TC circulation among other factors. Together, this work provides a basis for improving forecasts of TC tornado frequency and location.

Tropical Cyclone Outer Size Impacts the Number and Location of Tornadoes

“…To examine convective‐scale TC environments, we examine radiosondes within 1.2 times r 6 for each North Atlantic TC from 1995–2019 in the NOAA Integrated Global Radiosonde Archive (IGRA), version 2 (Durre et al., 2006). In addition to the IGRA quality control algorithm (Durre et al., 2006), we employ the following vertical criteria (Molinari & Vollaro, 2008; Schenkel et al., 2020): 1) data present within 200 m of the surface and 2) data gaps must be ≤ 1 km. We exclude sondes within 75 km of the TC center given their large azimuthal advection by the TC (Molinari & Vollaro, 2008; Molinari & Vollaro, 2010) yielding 8586 radiosondes in 233 TCs (Figure ).…”

“…A portion of our analysis uses a VWS‐relative coordinate given its importance in determining the TC‐relative azimuthal location of tornadoes (Schenkel et al., 2020; Schenkel et al., 2021). VWS data are calculated using 850‐and 200‐hPa reanalysis winds by partitioning the ambient winds from the total wind field following the methods above (Davis et al., 2008).…”

“…Range rings are provided every 100 km for (a), (c), and (e), and every 0.1r 6 for (b), (d), and (f). vertical criteria (Molinari & Vollaro, 2008;Schenkel et al, 2020): 1) data present within 200 m of the surface and 2) data gaps must be ≤ 1 km. We exclude sondes within 75 km of the TC center given their large azimuthal advection by the TC (Molinari & Vollaro, 2008;Molinari & Vollaro, 2010) yielding 8586 radiosondes in 233 TCs (Figure S1).…”

“…A portion of our analysis uses a VWS-relative coordinate given its importance in determining the TC-relative azimuthal location of tornadoes (Schenkel et al, 2020;Schenkel et al, 2021). VWS data are calculated using 850- (r𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡∕r6) radial coordinate from the center of small, medium, and large tropical cyclones (TCs).…”

“…First, VWS vertically tilts the TC downshear yielding an acceleration of the TC transverse circulation in a failed attempt to adiabatically restore thermal wind balance (Jones, 1995; Frank & Ritchie, 1999). This downshear strengthening of the TC overturning circulation is associated with enhanced ascent, moisture, and vertical wind shear (Molinari & Vollaro, 2008; Schenkel et al., 2020). Second, a constructive superposition between the TC and ambient winds, yields a nonlinear enhancement of helicity in the downshear sector, particularly downshear right (Gu et al., 2018; Schenkel et al., 2020).…”