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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.
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Investigating Axisymmetric and Asymmetric Signals of Secondary Eyewall Formation Using Observations‐Based Modeling of the Tropical Cyclone Boundary Layer
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.
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- Award ID(s):
- 1810869
- PAR ID:
- 10448320
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 126
- Issue:
- 16
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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