Abstract
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.
Significance Statement
This is the first part of a two-part study that uses simulations to analyze the cylindrical structure of the lowest 2.5 km of the atmosphere in two major hurricanes: Hurricane Irma in 2017 and Hurricane Earl in 2010. The structure at times when these hurricanes were weakening is highlighted. During those times, the wind and thermal fields had more-variable azimuthal structure, which was linked to the state of the environment that contained the hurricane. The research finds that these azimuthal structures could be physically linked to how the studied hurricanes weaken, and it provides motivation for considering the lower-atmospheric azimuthal structure of hurricanes when analyzing their intensities and changes in their intensities.
Ahern’s current affiliations: NOAA/AOML/Hurricane Research Division and Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida.
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This article has a companion article which can be found at http://journals.ametsoc.org/doi/abs/10.1175/MWR-D-21-0247.1
