Boundary Layer Recovery and Precipitation Symmetrization Preceding Rapid Intensification of Tropical Cyclones under Shear

Abstract

This study investigates the precipitation symmetrization preceding rapid intensification (RI) of tropical cyclones (TCs) experiencing vertical wind shear by analyzing numerical simulations of Typhoon Mujigae (2015) with warm (CTL) and relatively cool (S1) sea surface temperatures (SSTs). A novel finding is that precipitation symmetrization is maintained by the continuous development of deep convection along the inward flank of a convective precipitation shield (CPS), especially in the downwind part. Beneath the CPS, downdrafts flush the boundary layer with low-entropy parcels. These low-entropy parcels do not necessarily weaken the TCs; instead, they are “recycled” in the TC circulation, gradually recovered by positive enthalpy fluxes, and develop into convection during their propagation toward a downshear convergence zone. Along-trajectory vertical momentum budget analyses reveal the predominant role of buoyancy acceleration in the convective development in both experiments. The boundary layer recovery is more efficient for warmer SST, and the stronger buoyancy acceleration accounts for the higher probability of these parcels developing into deep convection in the downwind part of the CPS, which helps maintain the precipitation symmetrization in CTL. In contrast, less efficient boundary layer recovery and less upshear deep convection hinder the precipitation symmetrization in S1. These findings highlight the key role of boundary layer recovery in regulating the precipitation symmetrization and upshear deep convection, which further accounts for an earlier RI onset timing of the CTL TC. The inward rebuilding pathway also illuminates why deep convection is preferentially located inside the radius of maximum wind of sheared TCs undergoing RI.

Strong Diurnal Pulsing of Cold Clouds in Rapidly Intensifying Tropical Cyclones

“…Second, the diurnal pulse may represent a more symmetric convective structure (or symmetrization process) of the TC, as it is a ring‐like (symmetric) feature that manifests at the cloud top and possibly exists through the deep convective layer (Ditchek, Corbosiero, et al., 2019; Ditchek, Molinari, et al., 2019; Dunion et al., 2014, 2019). It is now widely recognized that symmetric structures of convection and precipitation within the inner core are favorable for RI (Alvey et al., 2015; Chen et al., 2020; Rogers et al., 2015; Shimada et al., 2017; Tao & Jiang, 2015). Last but not least, strong outward propagating pulses will enhance the upper‐level radial outflow, as well as subsidence in the outer TC region (Chen & Zhang, 2013; Dunion et al., 2019; Ryglicki et al., 2019).…”

Driving Forces of Extreme Updrafts Associated With Convective Bursts in the Eyewall of a Simulated Tropical Cyclone

“…The fourth method for vertical force decomposition that is less dependent on a local reference state can be accomplished via (Chen et al, 2021;Davies-Jones, 2003;Doswell & Markowski, 2004;Gu et al, 2019;Jeevanjee & Romps, 2015):…”

Modulation of Asymmetric Inner‐Core Convection on Midlevel Ventilation Leading up to the Rapid Intensification of Typhoon Lekima (2019)

“…As a reference, the SST at the model initial time in SSTA0 is shown in Figure 1c. Previous studies showed that the reduced SST acts to retard the recovery of low‐entropy air entering the boundary layer from aloft and therefore suppress the development of convection in the upshear‐left quadrant (Chen et al., 2021; Nguyen et al., 2019). This is also the case for the simulated Lekima and will be shown in Section 3.1.…”

“…The causality between the boundary layer recovery and convective strength in the upshear‐left quadrant can be explained by the “inward rebuilding” paradigm proposed by Chen et al. (2021). Specifically, the low‐ θ e boundary‐layer parcels recover their entropy when passing through the upshear‐left and upshear‐right quadrants, then being lifted in the shear‐forced downshear convergence zone, and finally develops into mature convection when returning the upshear‐left quadrant.…”