Polar lows and tropical hurricanes: Their energy and sizes and a quantitative criterion for their generation |
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| Authors: | G S Golitsyn |
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| Institution: | (1) A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevskii per. 3, Moscow, 119017, Russia |
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| Abstract: | Similarity and dimension considerations applied to convection in a rotating fluid allows one to estimate the sizes and horizontal velocities of generated vortices. To do this, it is necessary to know the buoyancy flux in the fluid and the angular velocity of fluid rotation 1, 2]. The author’s preliminary efforts 3] have shown that the sizes, wind speeds, and total kinetic energy can thus be estimated correctly for tropical cyclones (TCs), as well as for polar lows (PLs) (which are often called explosive mesocyclones because they take just a few hours to develop). In this study, the sensible and latent heat fluxes for U = 33 m/s and the related buoyancy fluxes are estimated on the basis of climatology, bulk formulas, and the velocity scale of convection in a rotating fluid. In the tropics, at hurricane wind speeds U ≥ 33 m/s and climatological air humidity r = 80%, the total heat flux at the water surface temperature T s ≥ 26°C becomes equal to or greater than 700 W/m2. Due to the Clausius-Clapeyron equation, the latent heat flux to the atmosphere (the main part of the flux in the tropics) decreases substantially at lower values of T s. Thus, an energy flux from the ocean to the atmosphere of 700 W/m2 or greater should be regarded as the first necessary condition for TC genesis instead of the temperature T s. Low static stability, which must be at least half its climatological value as estimated here, is another necessary condition 4]. In polar regions, total fluxes roughly twice those in the tropics are needed for the formation of explosive mesocyclones, PLs, which is explained by the much smaller role of latent heat, greater geostrophicity, and stronger static stability of the atmosphere there. Enthalpy fluxes and wind speeds are interrelated: the larger the flux is, the stronger the convection, the higher the concentration of angular momentum in an ascending convective air column, and the greater the azimuthal velocity in the vortex are, which in turn enhances the transfer of energy from the ocean. Considering the problem with the use of simple analytic relations makes it possible, for the first time, to find a numerical criterion for their generation. It is hoped that this material may be useful for educational purposes as well. |
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