A Numerical Study of Convective-Scale Updraft Traits in the Outer Core of Sheared Tropical Cyclones: Updraft Traits in Different Vertical Wind Shear Magnitudes

The characteristics of outer-core convective-scale updrafts in numerically simulated tropical cyclones are investigated in environmental vertical shears of 5, 10, 15, and 25 m s^-1, which represent weak, moderate, strong, and extreme shears with respect to tropical cyclones, respectively. For shears of 5, 10, and 15 m s^-1, downshear occurrence of principal rainbands in the outer core is more striking with the increased shear magnitude. In contrast, the outer-core clouds in shear of 25 m s^-1 tend to organize into a so-called outer mesoscale convective system. The Statistical and Programmable Objective Updraft Tracker (SPOUT) is used to identify the convective-scale updrafts in the outer core. Convective-scale updrafts populate downshear in principal rainbands and outer mesoscale convective systems, with long-lifetime updrafts dominantly originating downshear left. In all experiments, the frequency of updrafts with the lifetime < 30 min is greatest and increases with the shear magnitude. The top of updrafts is concentrated between 4-6- and 12-14-km heights. The majority of updrafts move at a speed of 10-20 m s^-1, travelling faster at smaller radii. The contoured frequency by altitude diagrams for vertical mass transport-weighted vertical velocity of the updrafts shows that much of the upward vertical mass transport is attributed to vertical velocity around 2 m s^-1 near the 2.5-km altitude, and the preferred height of very large vertical velocity tends to drop with increased shear. In addition, the buoyancy associated with updrafts decreases with the radius from the storm center, and the averaged buoyancy value decreases with increased shear. Larger buoyancy is concentrated in downshear quadrants, fitting into development of intense updrafts therein. We classify the convective-scale updrafts into convective bursts and non-convective bursts, find that the former predominantly occurs downshear right for shear of 5 m s-1 but downshear left for shears of 10, 15, and 25 m s^-1. The supercell composite parameter and the energy–helicity index are examined between non-convective bursts and convective bursts, indicating that statistical differences in the two empirical indices exist.