There has been significant success in recent years in determining tropical cyclone maximum potential intensity, or MPI. Both theory and observation agree that sea surface temperature is a crucial ingredient in determining the MPI. Much less understood however, are the mechanisms responsible for the rapid intensity fluctuations often observed in tropical cyclones. Presently, intensification mechanisms are broadly classified into three categories: the air-sea interaction, internal storm interactions, and external storm interactions. The air-sea interaction, as the main energy source for the tropical cyclone, excites core convection through sea surface latent heat fluxes. Internal storm interactions are composed of both eye-eye wall processes and anomalous potential vorticity interactions. External influences, which result from the weakened inertial stability in the outflow layer associated with the decay of the primary circulation with height, allow the tropical cyclone to interact with the surrounding atmosphere.

This presentation will focus on the influence of outflow layer asymmetries on intensity fluctuations. In particular, do preexisting outflow channels in the tropical cyclone environment facilitate more rapid growth than would otherwise occur. Previous studies have focused on the role of angular momentum fluxes or potential vorticity superposition principles to explain strengthening. These techniques have yet to yield consistent results for the multitude of possible environmental interactions. Here, the idea is not so much to put a quantitative seal on environmental influences, as to explore, through idealized numerical simulations, the impact of changing environmental conditions on tropical cyclone intensity and core convection. The premise of this work is rooted in MPI theory. These theories do not incorporate the work done against the surroundings or the impact of the surroundings upon the tropical cyclone. Outflow channels can provide low inertial stability so that minimal work is required to vent the outflow against the radial pressure gradient and may provide regions of convergence to force subsidence against positive static stability. If there are preexisting outflow channels in the vicinity of the tropical cyclone it is reasonable to expect less energy expenditures on behalf of the cyclone to exhaust it's anticyclonic angular momentum.

To study these intensification changes and the role of tropical cyclones on the westerlies, idealized simulations of the tropical cyclone-jet interaction will be presented. Such an interaction allows for both the beneficial impact of upper tropospheric low inertial stability (associated with the anticyclonic shear side of the jet) as well as the detrimental effect of vertical shear. A three dimensional primitive equation model is used with an idealized vortex and upper tropospheric zonal jet with no along jet variation. Such a setup permits the study of outflow dynamics without the need to consider secondary circulations maintained by the jet. E-P fluxes are calculated to estimate the magnitude of angular momentum forcing and to study the impact of convectively forced wave-mean flow interactions. Potential vorticity diagnosis permits the study of upper tropospheric vortex-vortex interactions as well as the impact of the tropical cyclone on the westerlies. Finally, momentum and thermodynamic budget analysis will help elucidate the core response to external forcing.