A Numerical Study of Convective‐Scale Structures in the Outer Cores of Sheared Tropical Cyclones: 1. Updraft Traits in Different Vertical Wind Shear Magnitudes

Abstract

This study investigates the outer-core convective-scale updraft features of numerically simulated tropical cyclones under vertical wind shear of different magnitudes. Updrafts in the outer core were weighted in favor of downshear formation, and the increase in the shear value led to more short-lived updrafts. The tops of detected updrafts tended to cluster at either 4-to 6-km or 12-to 14-km height. Downshear-left updrafts mostly moved radially inward, and updrafts on the left side of the shear vector moved more rapidly azimuthally. Longer-lived updrafts gained greater vertical mass fluxes during their life cycles, with those in strongly sheared tropical cyclones having larger vertical mass fluxes for the same updraft life span. The height of stronger vertical velocity in convective bursts decreased as shear increased, whereas larger vertical velocity within nonconvective-burst cells held between 3 and 4 km. The values of several environmental parameters (such as the supercell composite parameter and the energy-helicity index) associated with updrafts in the downshear-left quadrant were larger than those in the downshear-right quadrant for the weakly sheared tropical cyclone, whereas they became much larger in the downshear-right quadrant for highly sheared vortices. There were no relationships between the parameters and updraft mean mass fluxes, so the use of these parameters for predicting the likelihood of updraft strength in tropical cyclones requires caution. Plain Language SummaryAs a tropical cyclone approaches the coasts, high-impact weathers, such as thunderstorms and tornadoes, are usually observed far away from the tropical cyclone center. These weathers are closely related to the activity of isolated convective cells in the outer rainbands. This study investigates the statistical characteristics of updrafts embedded in the convective cells to give insights into the behavior of the updrafts under environmental vertical wind shears. We find that the larger the shear, the shorter-lived updrafts become. The updrafts tend to crest at either 4-to 6-km or 12-to 14-km height. The height of larger vertical velocity in more intense convection decreases as shear increases. Several environmental parameters have been indicated to successfully predict midlatitude supercell development, which are also examined here. These environmental parameters associated with updrafts are much larger in the downshear-right quadrant under larger environmental shears. It is found that there are no relationships between the parameters and updraft strength, so these parameters are applied to predicting updraft strength in tropical cyclones with caution.

Evolution of convective characteristics during tropical cyclogenesis

“…The identification and tracking of cells focuses on kinematic fields, rather than on any microphysical aspects of convection. The aim of SPOUT is to provide “better statistical understanding about individual updraft characteristics [that] could allow researchers to form and test hypotheses on the structure, evolution, and microphysical properties of these convective towers.” A number of recent studies have utilized SPOUT to analyse convection in rainbands or outer‐region convection (e.g., Terwey and Rozoff, 2014; Li and Fang, 2018), or in the inner core during the rapid intensification phase (Harnos and Nesbitt, 2016).…”

Stronger Vertical Shear Leads to Earlier Secondary Eyewall Formation in Idealized Numerical Simulations

“…The model used is the fully compressible and nonhydrostatic model TCM4 (Wang, 2007). The model mesh and physical configurations are the same as that in Li and Fang (2018), with the finest mesh covering a 624 × 624 km area and an initial axisymmetric vortex with a maximum tangential wind speed of 20 m s −1 at a 90‐km radius near the surface, on an f plane at 18°N over the ocean with a fixed sea surface temperature of 29°C. The initial thermodynamic profile of the unperturbed model atmosphere is adopted from the moist‐tropical sounding of Dunion (2011) and periodic lateral boundary conditions are utilized.…”

Typhoon Track, Intensity, and Structure: From Theory to Prediction

“…Under environmental vertical wind shear (VWS), thermodynamic processes can help saturate the TC inner core before RI onset through an increase in the column-integrated moist static energy, highlighting the impact of vortex structure on TC intensification (Chen et al, 2019). The outer-core convective-scale updrafts are weighted in favor of downshear formation, and the increase in the magnitude of VWS leads to more short-lived updrafts and decreased height of strong vertical velocities within convective bursts (Li and Fang, 2018). A downshear-upshear contrast in outer-core conditional instability occurs in weakly sheared TCs, while an enhanced downshear-left-downshear-right difference exists in strongly sheared TCs, which helps to maintain azimuthally asymmetric convective activity in the outer core of TCs (Li and Dai, 2020).…”