The role of environmental inertial stability in tropical cyclone intensification.

摘要

The University of Wisconsin - Nonhydrostatic Modeling System was used to test the hypothesis that regions of environmental potential vorticity (PV) close in value to tropical cyclone outflow PV provide a preferred region for hurricane outflow. The low inertial stability of such environments provide little resistance to convective outflow, reducing the energy expense to the tropical cyclone needed in the ventilation of it's mass away from the storm core and therefor leading to a more rapid intensification to the maximum potential intensity (MPI).; A variable, symmetric distribution of environmental inertial stability in the upper troposphere was generated by varying the Coriolis parameter in a quiescent environment. Simulations at two latitudes, 10° N and 30° N were considered. Both the simulations attain the same overall intensity as there was no environmental flow to counter the intensification to the thermodynamically defined MPI. The low latitude simulation obtained MPI in a shorter time period as the high latitude simulation generated a significantly stronger anticyclonic jet due to the reduced Rossby radius of deformation of the higher latitude. A significantly larger amount of kinetic energy drained from the tropical cyclone in producing the strong anticyclonic jet was necessary to spin-down the ambient rotation of the environment so that outward expansion of the outflow could continue.; An additional set of idealized experiments were performed with a uniform jet placed north of the tropical cyclone such that the degree of inertial stability to the north was controlled by the jet strength. During the rapid intensification phase of the evolution, the weak shear (2ms -1) inhibited strengthening relative to the simulation with no jet, the degree of the inhibition being related to the jet strength and therefore to the magnitude of the environmental shear. Once an inertially stable eyewall developed, all simulations were resilient to the weak imposed shear and further intensification proceeded, the magnitude of intensification being related to the degree of inertial stability. The lower the inertial stability on the anticyclonic shear side of the jet, the more rapid the intensification to MPI. A beta-plane simulation showed that in the absence of environmental shear, the asymmetry in upper tropospheric inertial stability produced by the planetary vorticity gradient produced a tropical cyclone at it's MPI more rapidly than an f-plane simulation centered at the same latitude.; These results show that asymmetries in the environmental inertial stability field produce asymmetries in tropical cyclone outflow. Ventilating outflow in one or two preferred outflow regions, as opposed to a more symmetric outflow pattern, minimizes the energy drain for outflow expansion and provides more energy for intensification. Furthermore, these results are consistent with observations which show the tendency for the strongest tropical cyclones in nature to form one or two outflow jets.