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

   https://doi.org/10.1175/JAS-D-20-0054.1  

   Alland, Joshua J. ; Tang, Brian H. ; Corbosiero, Kristen L. ; Bryan, George H. ( March 2021 , Journal of the Atmospheric Sciences)

   null (Ed.)

   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. 

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2. Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change—Lower‐Layer Shear Versus Upper‐Layer Shear

   https://doi.org/10.1029/2019JD030586  

   Fu, Hao ; Wang, Yuqing ; Riemer, Michael ; Li, Qingqing ( June 2019 , Journal of Geophysical Research: Atmospheres)

   In this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower‐layer and upper‐layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper‐layer shear than in the lower‐layer shear, which is found to be related to the stronger storm‐relative asymmetric flow in the middle‐upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm‐relative flow in the former imposes a greater ventilation of the warm core in the middle‐upper troposphere, leading to a more significant weakening of the storm. The storm in the lower‐layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear‐induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper‐layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.

    

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

   https://doi.org/10.1029/2021JD035718  

   Barron, Nicholas R. ; Didlake, Jr., Anthony C. ; Reasor, Paul D. ( March 2022 , Journal of Geophysical Research: Atmospheres)

   Ten years of airborne Doppler radar observations are used to study convective updrafts' kinematic and reflectivity structures in tropical cyclone (TC) rainbands. An automated algorithm is developed to identify the strongest rainband updrafts across 12 hurricane‐strength TCs. The selected updrafts are then collectively analyzed by their frequency, radius, azimuthal location (relative to the 200–850 hPa environmental wind shear), structural characteristics, and secondary circulation (radial/vertical) flow pattern. Rainband updrafts become deeper and stronger with increasing radius. A wavenumber‐1 asymmetry arises, showing that in the downshear (upshear) quadrants of the TC, updrafts are more (less) frequent and deeper (shallower). In the downshear quadrants, updrafts primarily have in‐up‐out or in‐up‐in secondary circulation patterns. The in‐up‐out circulation is the most frequent pattern and has the deepest updraft and reflectivity tower. Upshear, the updrafts generally have out‐up‐in or in‐up‐in patterns. The radial flow of the updraft circulations largely follows the vortex‐scale radial flow shear‐induced asymmetry, being increased low‐level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) quadrants. It is hypothesized that the convective‐scale circulations are significantly influenced by the vortex‐scale radial flow at the updraft base and top altitudes. Other processes of the convective life cycle, such as bottom‐up decay of aging convective updrafts due to increased low‐level downdrafts, can influence the base altitude and, thus, the base radial flow of the updraft circulation. The findings presented in this study support previous literature regarding convective‐scale patterns of organized rainband convection in a mature, sheared TC.

    

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4. The Role of Low-Level Flow Direction on Tropical Cyclone Intensity Changes in a Moderate-Sheared Environment

   https://doi.org/10.1175/JAS-D-20-0360.1  

   Lee, Tsung-Yung ; Wu, Chun-Chieh ; Rios-Berrios, Rosimar ( September 2021 , Journal of the Atmospheric Sciences)

   The impact of low-level flow (LLF) direction on the intensification of intense tropical cyclones under moderate deep-layer shear is investigated based on idealized numerical experiments. The background flow profiles are constructed by varying the LLF direction with the same moderate deep-layer shear. When the maximum surface wind speed of the simulation without background flow reaches 70 kt (36 m s⁻¹), the background flow profiles are imposed. After a weakening period in the first 12 h, the members with upshear-left-pointing LLF (fast-intensifying group) intensify faster between 12 and 24 h than those members (slow-intensifying group) with downshear-right-pointing LLF. The fast-intensifying group experiences earlier development of inner-core structures after 12 h, such as potential vorticity below the midtroposphere, upper-level warm core, eyewall axisymmetrization, and radial moist entropy gradient, while the inner-core features of the slow-intensifying group remain relatively weak and asymmetric. The FI group experiences smaller tilt increase and stronger midlevel PV ring development. The upshear-left convection during 6–12 h is responsible for the earlier development of the inner core by reducing ventilation, providing axisymmetric heating, and benefiting the eyewall development. The LLF of the fast-intensifying group enhances surface heat fluxes in the downshear side, resulting in higher energy supply to the upshear-left convection from the boundary layer. In all, this study provides new insights on the impact of LLF direction on intense storms under moderate shear by modulating the surface heat fluxes and eyewall convection.

    

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5. Thermodynamic Characteristics of Downdrafts in Tropical Cyclones as Seen in Idealized Simulations of Different Intensities

   https://doi.org/10.1175/JAS-D-21-0006.1  

   Wadler, Joshua B. ; Nolan, David S. ; Zhang, Jun A. ; Shay, Lynn K. ( August 2021 , Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye-eyewall mixing below 2-km altitude. Additionally, a layer of higher θ e between 1 and 2 km altitude associated with low-level outflow that extends 40 km outward from the eyewall region creates a “thermodynamic shield” that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-θ e air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean θ e deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their θ e by 10-14 K, showing that air-sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur ~5 km radially outward from an updraft and transport low-θ e mid-tropospheric air towards the inflow layer. Here, the low-θ e air entrains into the updraft in less than five minutes with a mean θ e deficit of 8.2 K. In general, θ e recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution. 

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