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Juliane Schwendike and Jeffrey D. Kepert

Abstract

This paper describes the boundary layer wind structure and dynamics of Hurricanes Danielle (1998) and Isabel (2003), based on the analysis of high-resolution global positioning system dropwindsonde data and simulation of the flow by a three-dimensional boundary layer model produced by Kepert and Wang. The observations show that the hurricane boundary layer has a complex three-dimensional structure with large variability over small distances. The analysis emphasizes three aspects: the degree of gradient-wind balance, the radially varying depth of the boundary layer, and the strength of the near-surface wind speed relative to that at a higher level. Each aspect is compared both with results obtained in a simulation of the individual storm by Kepert and Wang’s model and with theoretical predictions. The observations show that the boundary layer depth decreases toward the center of the storm, consistent with theoretical arguments. The strongest azimuthal winds occur near the top of, but still within, the frictional inflow layer. These strong azimuthal winds are marginally supergradient in Hurricane Danielle but strongly so in Hurricane Isabel, where the imbalance amounts to approximately 10 m s−1 near the radius of maximum winds and is statistically significantly nonzero. This layer of supergradient flow is surmounted by a layer of outflow, in which the flow returns to gradient balance. The maximum storm-relative azimuthal wind occurs in the left front of Hurricane Danielle, and the strongest inflow is located in the right front. These asymmetries rotate anticyclonically with height, but there is also a clear wavenumber-2 asymmetry superimposed, which shows less rotation with height and is possibly forced by environmental factors associated with the storm’s impending recurvature. In Hurricane Isabel, the azimuthal wind maximum is located in the left rear and the inflow maximum in the left front, with neither showing much tendency to vary in azimuth with height. The ratio of the near-surface wind speed to that farther aloft increases toward the storm center for both storms. The largest values are located near the radius of maximum wind, and in general higher values are found on the left of the storm’s track than on the right. Simulations of the two storms with the boundary layer model are able to explain several of these factors; they also show some ability to reproduce individual dropsonde wind observed profiles. Important is that the model predicts weakly supergradient flow in Danielle and strongly supergradient flow in Isabel, in excellent agreement with the observational analysis. Based on these simulations, physical arguments, and earlier studies, the authors conclude that the differences between these storms in this respect result from their differing radial profiles of gradient wind and argue that the occurrence of supergradient flow in the upper boundary layer of individual hurricanes should be readily predictable.

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Jeffrey D. Kepert, Juliane Schwendike, and Hamish Ramsay

Abstract

Plausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed, but that the main cause of this unstable layer is heating from turbulent dissipation. The top of the well-mixed layer is thus distinct from the top of the boundary layer in tropical cyclones. The top of the inflow layer is a better proxy for the top of the boundary layer but is not without limitations. These results may have implications for boundary layer parameterizations that diagnose the boundary layer depth from thermodynamic, or partly thermodynamic, criteria.

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Sam Hardy, Juliane Schwendike, Roger K. Smith, Chris J. Short, Michael J. Reeder, and Cathryn E. Birch

Abstract

The key physical processes responsible for inner-core structural changes and associated fluctuations in the intensification rate for a recent, high-impact western North Pacific tropical cyclone that underwent rapid intensification [Nepartak (2016)] are investigated using a set of convection-permitting ensemble simulations. Fluctuations in the inner-core structure between ringlike and monopole states develop in 60% of simulations. A tangential momentum budget analysis of a single fluctuation reveals that during the ringlike phase, the tangential wind generally intensifies, whereas during the monopole phase, the tangential wind remains mostly constant. In both phases, the mean advection terms spin up the tangential wind in the boundary layer, whereas the eddy advection terms deepen the storm’s cyclonic circulation by spinning up the tangential wind between 1.5 and 4 km. Calculations of the azimuthally averaged, radially integrated vertical mass flux suggest that periods of near-constant tangential wind tendency are accompanied by a weaker eyewall updraft, which is unable to evacuate all the mass converging in the boundary layer. Composite analyses calculated from 18 simulations produce qualitatively similar results to those from the single case, a finding that is also in agreement with some previous observational and modeling studies. Above the boundary layer, the integrated contribution of the eddy term to the tangential wind tendency is over 80% of the contribution from the mean term, irrespective of inner-core structure. Our results strongly indicate that to fully understand the storm’s three-dimensional evolution, the contribution of the eddies must be quantified.

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