Helicity is defined as H ： V ω, where V and ω are the velocity and vorticity vectors, respectively. Many works have pointed out that the larger the helicity is, the longer the life cycle of the weather system is. However, the direct relationship of the helicity to the evolution of the weather system is not quite clear. In this paper, the concept of helicity is generalized as shearing wind helicity （SWH）. Dynamically, it is found that the average SWH is directly related to the increase of the average cyclonic rotation of the weather system. Physically, it is also pointed out that the SWH, as a matter of fact, is the sum of the torsion terms and the divergence term in the vorticity equation. Thermal wind helicity （TWH）, as a derivative of SWH, is also discussed here because it links the temperature field and the vertical wind field. These two quantities may be effective for diagnosing a weather system. This paper applies these two quantities in cylindrical coordinates to study the development of Hurricane Andrew to validate their practical use. Through analyzing the hurricane, it is found that TWH can well describe the characteristics of the hurricane such as the strong convection and release of latent heat. SWH is not only a good quantity for diagnosing the weather system, but also an effective one for diagnosing the development of the hurricane.
To study the potential effect of sea spray on the evolution of typhoons,two kinds of sea spray flux parameterization schemes developed by Andreas (2005) and Andreas and Wang (2006) and Fairall et al. (1994) respectively are incorporated into the regional atmospheric Mesoscale Model version 3.6 (MM5V3) of Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) and the coupled atmosphere-sea spray modeling system is applied to simulate a Western Pacific super ty-phoon Ewiniar in 2006. The simulation results demonstrate that sea spray can lead to a significant increase in heat fluxes at the air-sea interface and the simulated typhoon’s intensity. Compared with the results without sea spray,the minimum sea level pressure reduces about 8hPa after taking account of sea spray by Fairall et al.’s parameterization (1994) and about 5hPa by Andreas’ (2005) and Andreas and Wang’s (2006) parameterization at the end of the model integration,while the maximum 10m wind speed increases about 17% and 15% on average,respectively,through the entire simulation time period. Taking sea spray into account also causes significant changes in Tropical Cyclone (TC) structure due to an enhancement of water vapor and heat transferred from the sea sur-face to the air; therefore,the center structure of the typhoon becomes more clearly defined and the wind speed around the typhoon eye is stronger in numerical experiments. The simulations show that different sea spray flux parameterizations make different modi-fications to the TC structure.
