عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In the study of atmospheric phenomena, different types of parameters are generally considered. The type of parameter applied depends mostly on the scale and severity of the phenomena under consideration. Weather systems with fast rotating wind, such as tropical storms or tornadoes, are among the most destructive weather-related phenomenon. Study of these kinds of phenomena and recognition of the causes of their intensification or weakening are very important. Quantities that are measured in this type of system vary and the calculations yield different results and properties depending on the method used. There is a direct relationship between type of system and severity to the thermodynamic instability of the atmosphere in the region of the occurrence. It is typical to use certain parameters for studying atmospheric instabilities, such as vorticity, potential vorticity. However, these parameters are gauges for measuring the power of a system related to the value of the instability of a system suitable for a straight flow. To measure the energy of activity of system with a severely rotating flow it is suggested to use another quantity, that of helicity. Thus, surveying sources and sinks of helicity is highly important in dealing with vortices and hurricanes and rotating systems in general. Besides helicity, helicity flux is also a very valuable tool for studying rotating flows. The mathematical relationship between helicity and helicity-flux are presented in this study. Also, a new method of computing helicity flux is discussed and the values of helicity in two methods are compared.
In theory, the dissipation of helicity occurs in the boundary layer. In fact, the dissipation aspect of helicity involved in the frictional process in the boundary layer is much higher than the production of it in buoyancy process. Consequently, there is a separating surface in the boundary layer above the surface of which the production is higher than the loss by friction effect, and below it the friction is much higher than that produced by buoyancy. The helicity produced in the upper layer moves downward into the area wherein the turbulent viscose force of the surface is dissipated due to friction. Since the stationary condition of the vortex is used in calculating helicity,, the downward flux of helicity is a highly accurate parameter of the stationary turbulent vortex.
This study calculates the downward helicity flux, sources and sinks of helicity of the hurricane Gonu. Data of hurricane Bonnie were utilized as both a comparison and reference for the study of hurricane Gonu. The results show that dynamical buoyancy is the main factor in producing helicity and surface friction and the main dissipating factor. Therefore, an increase in dynamical buoyancy or reduction in dissipative friction results in the regeneration or enhancement of a rotating system. Furthermore, the increase (or decrease) of the magnitude of helicity results in an increase (or decrease) in the height of the layer at which the maximum helicity flux occurs. It is suggested that the maximum helicity flux occurs at the top of viscous turbulence boundary layer.