Shearing Wind Helicity and Thermal Wind Helicity

• Published in: Advances in atmospheric sciences Vol. 23; no. 4; pp. 504 - 512
• Main Author: 韩瑛 伍荣生 方娟
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01.07.2006; The Key Laboratory of Mesoscale Severe Weather, Nanjing University, Nanjing 210093
• Subjects: Atmospheric sciences; Atmospherics; Climatology; Dynamical systems; Dynamics; Helicity; Hurricanes; Latent heat; Shearing; Studies; Temperature; Weather; Wind; 温热效应; 热带气象; 螺旋性; hurricane; helicity; TWH; SWH; thermal wind helicity; shearing wind helicity
• ISSN: 0256-1530; 1861-9533
• DOI: 10.1007/s00376-006-0504-5

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.