Asymmetric Hurricane Boundary Layer Structure during Storm Decay. Part I: Formation of Descending Inflow

In this first part of a two-part study, the three-dimensional structure of the inner-core boundary layer (BL) is investigated in a full-physics simulation of Hurricane Irma (2017). The BL structure is highlighted during periods of intensity change, with focus on features and mechanisms associated wi...

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Bibliographic Details
Published in:Monthly weather review 2021-11, Vol.149 (11), p.3851-3874
Main Authors: Ahern, Kyle, Hart, Robert E., Bourassa, Mark A.
Format: Article
Language:English
Subjects:
Advection
Amplification
Angular momentum
Asymmetry
Azimuth
Boundary layer structure
Boundary layers
Convection
Decay
Entropy
Hurricanes
Identification
Inflow
Momentum
Motion stability
Physics
Simulation
Storm motion
Storms
Vertical motion
Vertical wind shear
Wind
Wind shear
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Summary:In this first part of a two-part study, the three-dimensional structure of the inner-core boundary layer (BL) is investigated in a full-physics simulation of Hurricane Irma (2017). The BL structure is highlighted during periods of intensity change, with focus on features and mechanisms associated with storm decay. The azimuthal structure of the BL is shown to be linked to the vertical wind shear and storm motion. The BL inflow becomes more asymmetric under increased shear. As BL inflow asymmetry amplifies, asymmetries in the low-level primary circulation and thermodynamic structure develop. A mechanism is identified to explain the onset of pronounced structural asymmetries in coincidence with external forcing (e.g., through shear) that would amplify BL inflow along limited azimuth. The mechanism assumes enhanced advection of absolute angular momentum along the path of the amplified inflow (e.g., amplified downshear), which results in local spinup of the vortex and development of strong supergradient flow downwind and along the BL top. The associated agradient force results in the outward acceleration of air immediately above the BL inflow, affecting fields including divergence, vertical motion, entropy advection, and inertial stability. In this simulation, descending inflow in coincidence with amplified shear is identified as the conduit through which low-entropy air enters the inner-core BL, thereby hampering convection downwind and resulting in storm decay.
ISSN:0027-0644
1520-0493
DOI:10.1175/MWR-D-21-0030.1