
Environmental Sciences
Seminar Abstract
Tropical Cyclones and Polar Lows: Velocity, Size and Energy Scales, 26corigin criteria
The final goal is to quantitatively formulate some necessary conditions for development of intense atmospheric vortices: tropical cyclones (TC) and polar lows (PL). In this way we use the bulk formulas for the fluxes of momentum, sensible and latent heat between ocean and atmosphere. The velocity scale is used in two forms: (i) as in convection of rotating fluids where it is expressed through the buoyancy flux, , and the Coriolis parameter, , and (ii) through the formula expressing the total dissipation of kinetic energy in the atmospheric boundary layer as a cube of velocity times the drag coefficient. In the quasistationary case the dissipation equals the generation of the energy. In both cases the velocity scale can be expressed through the temperature and air humidity difference between the ocean and the atmosphere using the notion of the reduced gravity. Both forms produce quite comparable velocity scales. Using parameters and one can form scales of the area and, by adding the mass of a unit air column, also a scale of the total kinetic energy. These scales nicely explain the much smaller size of a PL, comparing with a TC, and the total kinetic energy of a TC of the order J. It will be shown that the total enthalpy fluxes between the ocean and the atmosphere to produce wind of 33 m s are about 700 W m for TC and 1700 W m for PL, which is explained by much larger role of the latent heat in the first case and stricter geostrophic constraints and larger static stability in the second case. This explains the mystical role of 26 C for a TC origin. The buoyancy flux, a product of the reduced gravity and the wind speed, together with the atmospheric static stability determines the rate of the penetrating convection. It is known from observations that the formation time for a PL reaching altitude of 5  6 km can be only few hours, and a day, or even half a day, for a TC reaching 15 18 km. These two facts allow us to construct curves on the plane of and determining possibilities for forming an intense vortex. Here is the atmospheric temperature at height m. It is necessary for a PL to have C in accordance with the observations and numerics. The conditions for TC are not so straightforward but our diagram shows that the temperature difference of a few degrees, or possibly even a fraction of a degree, might be sufficient for a TC development for a range of static stabilities and development times. Last updated: 03/02/2009 