Central Surface Pressure Research Articles

The Cyclostrophic Adjustment of Vortices with Application to Tropical Cyclone Modification

The cyclostrophic and hydrostatic adjustment of simple one-layer and multilayer vortex flows to the local removal and/or redistribution of mass and angular momentum are studied, and a detailed physical interpretation of the dynamics of adjustment is given for the one-layer model. The calculations provide insight into possible responses of tropical cyclones to modification by cloud seeding and facilitate an appraisal of the Simpson-Malkus modification hypothesis. Calculations for two- and three-layer models show that the maximum tangential velocity is increased whether or not mass transfer takes place predominantly inside or outside the radius at which the maximum occurs, and the central surface pressure decreases due to subsidence at one or both interface levels. However, the magnitude of these effects are comparatively small in relation to the strengths of the induced meridional circulation and corresponding changes in tangential wind speed outside the core, at, or beyond, the radii at which mass transfer occurs. Moreover, the estimated maximum change in tangential wind speed that might be produced in a tropical cyclone by following the seeding procedure suggested by Simpson and Malkus is small compared with observed natural variations.

Budget Analysis of a Tropical Cyclone Simulated in an Axisymmetric Numerical Model

A tropical cyclone simulated in an axisymmetric numerical model is analyzed in detail from various aspects in order to deepen the understanding of the basic mechanisms of its evolution. Namely, the budget equations of temperature, moisture, relative angular momentum, vorticity, radial-vertical circulation, and kinetic energy are investigated for the different stages of the development of a tropical cyclone. The spatial distributions of each term in the budget equations are shown and their role in the following processes are discussed. In the pre-deepening stage of a large weak vortex in a conditionally unstable atmosphere, a solenoidal field is formed as a result of a delicate heat budget which depends on the static stability and the moisture content. The baroclinicity field thus established drives the system into a deepening stage. A positive feedback process builds up a warm moist core, accelerates the radial-vertical circulation, and intensifies the moist convection. A net outflow of mass from the central region and a resultant drop of central surface pressure take place during this period. The relative angular momentum of the inner column as a whole increases through convergence of relative angular momentum. In terms of relative vorticity, intensification and shrinking of the vortex is due to the combined effects of advection, horizontal convergence and twisting. At the end of the deepening stage, conditional instability in the central region is neutralized. The moment due to Coriolis force acting on the intensified azimuthal flow counterbalances the baroclinicity vector, so that the acceleration of radial-vertical flow ceases. Concentration of relative angular momentum and vorticity in the central region also levels off. In the budget of these quantities, the role of both vertical and lateral steam becomes important. In the troposphere, except the upper part and the boundary layer, the gradient wind relationship is established between the pressure field and the azimuthal flow. In the mature stage, the status in the inner region is quasi-stationary while that of the outer area keeps changing slowly. The importance of evaporation at the central area for the maintenance of an intense tropical cyclone is demonstrated in an additional experiment.

Structure of a Tropical Cyclone Developed in a Three-Dimensional Numerical Simulation Model

Abstract A three-dimensional, 11-level, primitive equation model has been constructed for a simulation study of tropical cyclones. The model has four levels in the boundary layer and its 70×70 variable grid mesh encloses a 4000-km square domain with a 20-km resolution near the center. Details of the model, including the parameterization scheme for the subgrid-scale diffusion and convection processes, are described. A weak vortex in the conditionally unstable tropical atmosphere is given as the initial state for a numerical integration from which a tropical cyclone develops in the model. During the integration period of one week, the sea surface temperature is fixed at 302K. The central surface pressure drops to about 940 mb, while a warm moist core is established. The azimuthal component of mean horizontal wind is maximum at about 60 km from the center at all levels. A strong in-flow is observed in the boundary layer. At upper levels, a secondary radial-vertical circulation develops in and around the regi...