Shearing Wind Helicity and Thermal Wind Helicity

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.

Topographic Effect on the Energetics of Geostrophic Adjustment

In this work, the impact of topography on the geostrophic adjustment processis discussed with a simple two-layer shallow water model, in which the lower-layer fluid isinitially stationary while the upper-layer is perturbed by the impulsive injection of momentum.During the geostrophic adjustment process of this ideal model, the initial kinetic energy isreleased and a fraction of it is converted into potential energy of the final geostrophicallyadjusted state. Thus, after the geostrophic adjustment, the kinetic energy of the system is reducedwhile the potential energy is enhanced. As the topographic effect is considered, it is found thatthe decrease of the kinetic energy (ΔKE) and the increase of the potential energy (ΔPE) of thesystem are weakened as compared to the case that the lower boundary is flat. Furthermore, thetopographic effect on APE is less pronounced than that on ΔKE, which implicates that the topographytends to inhibit the energy dispersion of the inertio-gravity wave. The numerical simulation of thegeostrophic adjustment process shows that, due to the impact of topography, the convergence anddivergence in the low layer are reduced and the undulation of the interface between the lower-layerand upper-layer is weakened. This means that the amplitude of the inertio-gravity wave is decreased,and thus, the energy radiated by the inertio-gravity wave is lessened.