Combined Effects of Midlevel Dry Air and Vertical Wind Shear on Tropical Cyclone Development. Part II: Radial Ventilation

Abstract

This study demonstrates how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via radial ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS environments. Two radial ventilation structures are documented. The first structure is positioned in a similar region as rainband activity and downdraft ventilation (documented in Part I) between heights of 0 and 3 km. Parcels associated with this first structure transport low-equivalent potential temperature air inward and downward left-of-shear and upshear to suppress convection. The second structure is associated with the vertical tilt of the vortex and storm-relative flow between heights of 5 and 9 km. Parcels associated with this second structure transport low-relative humidity air inward upshear and right-of-shear to suppress convection. Altogether, the modulating effects of radial ventilation on TC development are the inward transport of low-equivalent potential temperature air, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development.

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

“…In this study, we focus on the radial ventilation at midlevels, while the relationship between the downdraft ventilation and upshear convection is left for future research. More specifically, we will show that the radial ventilation at altitudes of 5-10 km upshear, which is analogous to the midlevel radial ventilation pathway discussed in Alland et al (2021b), are impacted by the convectively induced overturning circulation. Analysis will be carried out based on the numerical simulation of a real RI case under moderate VWS (defined as the deep-layer VWS of 5-10 m s −1 ), Typhoon Lekima (2019).…”

“…It is interesting to note that the negative HADV within the boundary layer (1,000-850 hPa) decreases with time in both experiments, which is likely due to the lessened boundary-layer cooling along with the reduction in tilt. Previous studies (Alland et al, 2021b;Tang & Emanuel, 2010) suggested that the radial ventilation in the boundary layer is coupled with the downdraft ventilation. That is, the low-entropy air parcels flushed into the boundary layer by downdrafts can be transported inward and lower the entropy in the inner-core.…”

“…Two types of ventilation have been proposed in previous studies. At the midtroposphere, the storm‐relative flows associated with the environmental VWS and vortex tilt directly transport the low‐entropy environmental air into the inner core of TC (Alland et al., 2021b; Ge et al., 2013; Tang & Emanuel, 2010). In the boundary layer, the low‐entropy air is brought from aloft by the tilt‐induced convective downdrafts (Alland et al., 2021a; Riemer et al., 2010).…”

Electrically Active Diurnal Pulses in Hurricane Harvey (2017)

“…Storm-centered fields were computed by centering each field on the smoothed mean sea level pressure (MSLP) minimum in the HRRR for each analysis time, where a 19-point moving average was used to achieve the smoothing. Tracking minimum MSLP, an integrated quantity, was found by Alland (2019) to find the storm center more accurately than other storm-centering methods such as those detailed in Nguyen et al (2014).…”

Statistical Analysis of Convective Updrafts in Tropical Cyclone Rainbands Observed by Airborne Doppler Radar

“…Rainbands can act as a local source of potential vorticity (PV) that spirals into the PV central core, which can help strengthen the storm (Franklin et al., 2006; May & Holland, 1999). Oppositely, storm weakening can occur when rainband downdrafts, induced either by precipitation forcing or compensating subsidence, reduce the θ e of boundary layer air feeding into the eyewall (e.g., Alland et al., 2021a, 2021b; Barnes et al., 1983; C. K. Yu & Chen, 2011; Li & Dai, 2020; Li & Wang, 2012a; Powell, 1990a, 1990b; Sawada & Iwasaki, 2010a, 2010b). Diabatic heating within the strong rainband convection can also produce a local pressure minimum due to hydrostatic adjustment, which reduces the radial flow into the eyewall and possibly weakens the TC (Powell, 1990a; Wang, 2009).…”