The role of environmental inertial stability in tropical cyclone intensification: Asymmetric environment

Upper tropospheric resistance to tropical cyclone outflow impacts the energy drain of the system and influences tropical cyclone intensification. It is hypothesized that regions where the environmental angular momentum (potential vorticity) is nearly equal to the tropical cyclone's outflow angular momentum (potential vorticity) the energetic drain of outflow expansion into these regions is minimized and consequently tropical cyclone intensity is maximized. To explore the role of asymmetric environmental distributions of inertial stability on a developing idealized tropical cyclone, a set of experiments in which a jet of varying intensity was placed to the north of the developing tropical cyclone. Results of these experiments show that, despite an imposed weak vertical shear due to the jet, the most rapid intensifications occurred in the presence of the weakest environmental inertial stabilities. It is shown that a large fraction of the TC outflow (an outflow jet) was ventilated in the direction where both the radial angular momentum (potential vorticity) gradient is minimized across the outflow-environment boundary and the work done to expand outflow is minimized.

Asymmetric outflow is not intrinsic to the system but rather the result of interactions with an evolving environmental flow field. While environmental forcing is consistent with diagnostics such as eddy momentum fluxes and potential vorticity superposition in which energy is supplied to the system, our viewpoint is that the environment is largely detrimental to intensification and that outflow jets are the result of interactions with regions of weak environmental inertial stability which act to minimize the tropical cyclone outflow layer energy sink. Comparison of this viewpoint with the good trough - bad trough viewpoint will be made.