An official website of the United States government
Abstract The dynamics of an asymmetric rainband complex leading into secondary eyewall formation (SEF) are examined in a simulation of Hurricane Matthew (2016), with particular focus on the tangential wind field evolution. Prior to SEF, the storm experiences an axisymmetric broadening of the tangential wind field as a stationary rainband complex in the downshear quadrants intensifies. The axisymmetric acceleration pattern that causes this broadening is an inward-descending structure of positive acceleration nearly 100 km wide in radial extent and maximizes in the low levels near 50 km radius. Vertical advection from convective updrafts in the downshear-right quadrant largely contributes to the low-level acceleration maximum, while the broader inward-descending pattern is due to horizontal advection within stratiform precipitation in the downshear-left quadrant. This broad slantwise pattern of positive acceleration is due to a mesoscale descending inflow (MDI) that is driven by midlevel cooling within the stratiform regions and draws absolute angular momentum inward. The MDI is further revealed by examining the irrotational component of the radial velocity, which shows the MDI extending downwind into the upshear-left quadrant. Here, the MDI connects with the boundary layer, where new convective updrafts are triggered along its inner edge; these new upshear-left updrafts are found to be important to the subsequent axisymmetrization of the low-level tangential wind maximum within the incipient secondary eyewall.
more »
« less
This content will become publicly available on August 28, 2026
The Role of Outflow‐Layer Inertial Stability in Governing the Radial Location of Secondary Eyewall Formation in Tropical Cyclones
Abstract Secondary eyewall formation (SEF) in tropical cyclones (TCs) emerges from a complex interplay of internal dynamics and environmental influences. Motivated by observations linking low inertial stability in the TC outflow layer to eyewall replacement cycles, we investigate how variations in outflow‐layer inertial stability control both the initiation and radial position of SEF. Idealized simulations reveal that reduced outflow‐layer inertial stability enhances upper‐level divergence and updraft in the TC outer core, fostering the growth of stratiform rainbands. By averaging secondary circulation over the domain grids featuring stratiform precipitation, it is explicitly shown that the strength of the mesoscale descending inflow (MDI) is greater within the widespread and more developed stratiform clouds. Such stratiform‐induced MDI can dynamically and thermodynamically broaden the tangential wind field in the lower altitudes. As a result, the ensuing increase in boundary‐layer inertial stability and inflow supplies greater absolute vorticity influx in the outer‐core region, making the tangential wind tendency peaks and the secondary eyewall intensifies at a larger radius. This study highlights the role of MDI in the coupling between the upper‐ and lower‐tropospheric dynamics.
more »
« less
- Award ID(s):
- 1834300
- PAR ID:
- 10652992
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 130
- Issue:
- 16
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- e2025JD043708
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract This study examines axisymmetric and asymmetric aspects of secondary eyewall formation (SEF) in tropical cyclones (TCs) by applying a nonlinear boundary layer model to tangential wind composites of observed TCs with and without SEF. SEF storms were further analyzed at times prior to and after SEF, as defined by the emergence of a secondary maximum in axisymmetric tangential wind. The model is used to investigate the steady‐state boundary layer response to the free‐tropospheric pressure forcing derived from observed tangential wind fields. The axisymmetric response to the Post‐SEF wind field displayed a secondary updraft maximum associated with a mature secondary eyewall; the model correctly produced no secondary updraft for non‐SEF storms. The Pre‐SEF response also exhibited a secondary updraft associated with an incipient secondary eyewall largely due to the broadened outer tangential wind field that commonly precedes SEF events. The asymmetric wind fields and model response were analyzed relative to the 850–200 hPa environmental wind shear vector. In Pre‐SEF storms, the tangential wind field displayed a broadened tangential wind structure in the downshear quadrants. The boundary layer response shows a downwind shift toward the left‐of‐shear quadrants, exhibiting the clearest secondary maxima in updrafts, tangential wind, and radial inflow. This left‐of‐shear response was the leading contributor to the secondary eyewall signals in the Pre‐SEF axisymmetric response. Sensitivity analyses confirmed the robustness of these asymmetric signals. These findings suggest that enhanced tangential wind and boundary layer updrafts in the left‐of‐shear sectors may be early indicators and critical features of SEF in sheared TCs.more » « less
-
Abstract Hurricane Patricia (2015), the most powerful tropical cyclone (TC) on record, formed its secondary eyewall when its center was about 113 km offshore before its landfall at the southwestern coast of Mexico at around 2300 UTC 23 October. The ARW-WRF Model reproduced well the main features, allowing for a detailed investigation of the secondary eyewall formation (SEF). Our results show that the secondary eyewall developed from a stationary banding complex (SBC), originating from the intersection of two outer rainbands (OR1 and OR2) on the western side of the TC. This process was largely regulated and enhanced by the coastal terrain through the orographic channel effect. The results from sensitivity experiments show that increasing terrain height amplified the channel effect, accelerating airflow between the TC vortex and the terrain, strengthening convergence into OR1, and promoting midlevel descending inflow conducive to convective enhancement downstream in the SBC. While the terrain weakened low-level moisture transport, it also positioned OR2 closer to OR1, facilitating the formation of the SBC and accelerating the moat development. Backward trajectory analysis revealed that the inflows below the upper-level outflow layers of both the primary and secondary eyewalls contributed to moat development. With increasing terrain height, dry air transported into the moat region by the upper-level inflows from the secondary eyewall significantly increased, further suppressing convection in the moat. These findings offer novel insights into the understanding of SEF processes and underscore the importance of the topographic effects in shaping outer rainband organization, contributing to the moat and SEF.more » « less
-
Abstract This study revisits the issue of why tropical cyclones (TCs) develop more rapidly at lower latitudes, using ensemble axisymmetric numerical simulations and energy diagnostics based on the isentropic analysis, with the focus on the relative importance of the outflow-layer and boundary layer inertial stabilities to TC intensification and energy cycle. Results show that although lowering the outflow-layer Coriolis parameter and thus inertial stability can slightly strengthen the outflow, it does not affect the simulated TC development, whereas lowering the boundary layer Coriolis parameter largely enhances the secondary circulation and TC intensification as in the experiment with a reduced Coriolis parameter throughout the model atmosphere. This suggests that TC outflow is more likely a passive result of the convergent inflow in the boundary layer and convective updraft in the eyewall. The boundary layer inertial stability is found to control the convergent inflow in the boundary layer and depth of convection in the eyewall and thus the temperature of the energy sink in the TC heat engine, which determines the efficiency and overall mechanical output of the heat engine and thus TC intensification. It is also shown that the hypothesized isothermal and adiabatic compression legs at the downstream end of the outflow in the classical Carnot cycle are not supported in the thermodynamic cycle of the simulated TCs, implying that the hypothesized classical TC Carnot cycle is not closed. It is the theoretical maximum work of the heat engine, not the energy expenditure following the outflow downstream, that determines the mechanical work used to intensify a TC.more » « less
-
Abstract The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥20 kt (24 h)−1, where 1 kt ≈ 0.5144 m s−1], weakening [WE; intensity tendency <−10 kt (24 h)−1], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3091 dropsondes were composited for analysis below 2.5-km elevation—1086 during IN, 1042 during WE, and 963 during SS. In nonintensifying hurricanes, the low-level tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability (I2) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I2) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Nonintensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures (θe) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θe is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.more » « less
