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ESRL/PSD Seminar Series

Aerodynamic Models for Hurricanes

Arkadii Leonov
Univ of Akron, Ohio


In spite of many impressive results obtained from observations and theoretical/numerical studies, there are several unresolved fundamental problems in understanding of hurricane functioning, propagation, and genesis/maturing. This report based on the author publications in -Y´arxiveĦ is aimed at resolving these problems using simple idealistic models, ignoring turbulent approaches.

Firstly, an aerodynamic quasi-steady model for the upper adiabatic layer of hurricane was developed. It allowed analytically describe the radial distributions of pressure and velocity fields. Then the vertical evolution of these distributions, along with the hurricane structure in adiabatic layer was described by a coupled set of equations for the vertical mass flux and vertical momentum balance, averaged over the eye wall cross section. After solving these equations, the 2D axisymmetric profiles of dynamic and thermodynamic variables can be analytically calculated, if their initial values at the bottom of adiabatic layer are known. The model described the observed change in the direction of hurricane rotation from cyclonic at the bottom to anti-cyclonic at the hurricane top, as well as the change in the direction of radial velocity.

Secondly, a model was developed for hurricane boundary layer (HBL). It described the basic physics in HBL, including evaporation, condensation and interaction of wind with oceanic waves. An airflow model was developed, the HBL structure evaluated, and the basic balance relations for thermodynamic and dynamic variables in HBL were established. They included: the mass balance, the balances of sensible and latent heat, as well as the entropy balance and dissipation for eye wall region. These balances determined the values for unknown parameters at the interface between two layers, as well as the hurricane translation speed. It was shown that the stable functioning of quasi-steady hurricane depends only on temperature of horizontal ´heat bandĦ in which direction the hurricane travels, and sailing wind. Evaluations made for ´standardĦ hurricane are consistent with data. Two observed and poorly explained effects were described by this model too. One is a high temperature increase at the top of HBL was explained and calculated by the sudden vapor condensation. Another one, increase in angular momentum in HBL, was explained by transfer of energy to the rotating wind from the oceanic waves propagating outside the eye wall region to the hurricane peripheries. Additionally, the radial distributions of wind velocity and surface pressure observed in the hurricane Frederic (1979) were calculated and compared with observations.

Finally, an analytical, semi-phenomenological model of hurricane genesis and maturing was developed. It was based on concept of emerging plumes in near tropical regions, which capture rotation from horizontally sheared wind. The model shows that the only cyclonically rotated plumes survive and mature. They grow to typical hurricane dimensions for about one month. The evaluations based on the model seem to be in accord with observations.

Wednesday, July 18th

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