Observed Processes Underlying the Favorable Vortex Repositioning Early in the Development of Hurricane Dorian (2019)

Abstract

Dorian’s evolution from a weak, disorganized tropical storm to a rapidly intensifying hurricane is documented through a unique multi-platform synthesis of NOAA’s P-3 tail-Doppler radar, airborne in situ data, and Meteo-France’s Martinique and Guadeloupe ground radar network. Dorian initially struggled to intensify with a misaligned vortex in moderate mid-tropospheric vertical wind shear that also allowed detrimental impacts from dry air near the inner core. Despite vertical wind shear eventually decreasing to less than 5 m/s and an increasingly symmetric distribution of stratiform precipitation, the vortex maintained its misalignment with asymmetric convection for 12 hours. Then, as the low-level circulation (LLC) approached St. Lucia, deep convection near the LLC’s center dissipated, the LLC broadened, and precipitation expanded radially outwards from the center temporally coinciding with the diurnal cycle. Convection then developed farther downtilt within a more favorable, humid environment and deepened appreciably at least partially due to interaction with Martinique. A distinct repositioning of the LLC towards Martinique is induced by spin-up of a mesovortex into a small, compact LLC.It is hypothesized that this somewhat atypical reformation event and the repositioning of the vortex into a more favorable environment, farther from detrimental dry mid-tropospheric air, increased its favorability for the rapid intensification that subsequently ensued. Although the repositioning resulted in tilt reducing to less than the scale of the vortex itself, the pre-existing broad mid-upper level cyclonic envelope remained intact with continued misalignment observed between the mid-level center and repositioned LLC even during the early stages of rapid intensification.

How Do Weak, Misaligned Tropical Cyclones Evolve Toward Alignment? A Multi‐Case Study Using the Hurricane Analysis and Forecast System

“…Tang & Zhang, 2016), this study also reveals the potential usefulness of local time as a predictor of tilt (in the context of other environmental factors). This is also consistent with recent observational cases, Dorian (2020) and Sally (2020), during which convective bursts led to center reformations during the early morning hours (Alvey et al, 2022;Brown, 2020). Section 3.5 will utilize customized high resolution output from a HAFS simulation of Ida ( 2021) to investigate the vortex-scale processes responsible for alignment and their relationships to the precipitation evolution.…”

“…Weak TCs typically become misaligned in the presence of moderate (i.e., 10–20 kt) to strong vertical wind shear (VWS, i.e., >20 kt). Several recent studies, however, have documented some cases that become aligned and undergo rapid intensification, despite the presence of moderate VWS (Alvey et al., 2020, 2022; Hazelton et al., 2020; Rios‐Berrios & Torn, 2017; Rios‐Berrios et al., 2016; Rogers et al., 2020). In these cases, the vortex overcomes its tilted configuration and asymmetric precipitation distribution, and it transitions toward a more aligned vortex with increases in symmetry and areal coverage of precipitation (and sometimes also the intensity of precipitation).…”

“…Several observational and modeling studies have shown the importance of the areal coverage of deep convection to encourage reformation or center repositioning processes (Alvey et al., 2022; Nguyen & Molinari, 2015; Rogers et al., 2016; Schecter, 2020); however, more uncertainty surrounds the importance of convective intensity. Case studies of weak, sheared TCs by Molinari et al.…”

“…Ground-based radar data are processed into Cartesian grids for 2020-2021 Atlantic basin TCs with best track center positions less than 300 km from coastal WSR-88D and Cayman Islands National Weather Service radars. Single Doppler radial velocities and multi-Doppler wind retrievals (for multiple overlapping radars; Bell et al, 2020) allow for TC center determination at different heights and vortex tilt estimates using methodologies described in Alvey et al (2022) and Section 2.3. Aircraft reconnaissance Vortex Data Messages also provide complementary low-level center position estimates when a ground radar's lowest elevation angle exceeds the height of the lower troposphere, typically at greater distances from the radar site.…”

“…Several observational and modeling studies have shown the importance of the areal coverage of deep convection to encourage reformation or center repositioning processes(Alvey et al, 2022;Nguyen & Molinari, 2015; …”