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

Abstract

This study examines how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via downdraft ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS. A strong, positive, linear relationship exists between the low-level vertical mass flux in the inner core and TC intensity. The linear increase in vertical mass flux with intensity is not due to an increased strength of upward motions but, instead, is due to an increased areal extent of strong upward motions (w > 0:5 m s−1). This relationship suggests physical processes that could influence the vertical mass flux, such as downdraft ventilation, influence the intensity of a TC.The azimuthal asymmetry and strength of downdraft ventilation is associated with the vertical tilt of the vortex: downdraft ventilation is located cyclonically downstream from the vertical tilt direction and its strength is associated with the magnitude of the vertical tilt. Importantly, equivalent potential temperature of parcels associated with downdraft ventilation trajectories quickly recovers via surface fluxes in the subcloud layer, but the areal extent of strong upward motions is reduced. Altogether, the modulating effects of downdraft ventilation on TC development are the downward transport of low-equivalent potential temperature, negative-buoyancy air left-of-shear and into the upshear semicircle, 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.

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

“…By −2 hr, MFC remains maximized near the outflow boundaries and the region of most intense convection. Although many studies have shown that downdraft driven cold pools are often unfavorable for TC intensification (Alland et al, 2021a), this analysis highlights the nuanced nature of early stage TC development wherein downdrafts may not always deter TC intensification if they are weaker and/or have a smaller areal coverage. In fact, a previous idealized modeling study of TC genesis by Davis (2015) highlighted the instrumental role of downdraft-driven cold pools in initiating strong updrafts and contributing to convective organization.…”

“…Previous literature has described the ramifications of vertical wind shear for TC intensification: The differential wind can advect the mid‐upper level vortex away from the surface center causing a horizontal displacement of the upper vortex (Frank & Ritchie, 2001; Jones, 1995). This misalignment can increase susceptibility to dry air entrainment, which detrimentally impacts the thermodynamic environment, inhibits or reduces the precipitation near the inner core (Alland et al., 2021a, 2021b; B. Tang & Emanuel, 2010, 2012), and makes it more difficult for the TC to intensify. A misaligned vortex also induces mid‐tropospheric thermal anomalies (cold downtilt; DeMaria, 1996; Frank & Ritchie, 1999; Jones, 1995; Ryglicki et al., 2018) that help force asymmetric precipitation concentrated in the downtilt quadrants (Boehm & Bell, 2021).…”

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

“…Furthermore, a misaligned vortex increases susceptibility of the TC to entrainment of drier environmental air. Dry air not only dilutes convective updrafts but can also entrain into convective downdrafts that flush the boundary layer with unfavorable, low u e air (Riemer et al 2010;Finocchio et al 2016;Nguyen et al 2017Nguyen et al , 2019Chen et al 2019;Alland et al 2021a).…”

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

“…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). From the viewpoint of the maximum potential intensity theory (Emanuel, 1986), ventilation acts to lower the efficiency of the TC heat engine and thus hinders TC intensification.…”