On the nature of TC and trough-jet interactions

On the nature of TC and trough-jet interactions

William A. Komaromi1 and James D. Doyle2

1. National Research Council, Monterey, CA

2. Naval Research Laboratory, Monterey, CA

The interaction between a tropical cyclone (TC) and a variety of configurable troughs and zonal jets is examined from an idealized framework using the COAMPS-TC model. Conventional understanding of the TC secondary circulation dictates that stronger low-level inflow is traditionally associated with a strengthening cyclone, while a strengthening of the outflow is often viewed as a balanced response to the strengthening due to conservation of mass in an “in-up-out” model. Here we test the opposite hypothesis in that a strengthening of the outflow will result in an intensification of the entire secondary circulation, ultimately strengthening the storm. This is referred to as the active outflow hypothesis. The effect of having tropical cyclone outflow interact with a variety of jet and trough structures on the cyclone's structure and intensity is examined via a large number of idealized COAMPS-TC simulations, run at 5km resolution with full moist physics on a beta-plane, with periodic boundary conditions in x.

It is found that enhancing upper-level divergence without simultaneously reducing inertial stability (or vice versa) is insufficient to lead to a meaningful intensification of the cyclone. It is also necessary under most circumstances to reduce both the tangential wind and the vorticity terms in the inertial stability equation in order to achieve having the cyclone's outflow to effectively phase with a neighboring synoptic-scale feature, whether it is a trough or a jet. Direct perturbation of the upper-level wind field via nudging the winds near the outflow region reveals that it is possible to enhance one term in the inertial stability equations while reducing another, resulting in a cancelation effect and having no meaningful effect on the cyclone. An upper-level trough has the advantage versus the zonal jet of having a more sharply defined inertial stability minimum, which by carefully placing the cyclone and the trough at the initial time, one can investigate the optimal spacing and timing that allows the TC's secondary-circulation to couple with the jet. It is found that the region of reduced inertial stability in a curved trough may actually be quite small, occupying less than a single quadrant of the cyclone outflow region while still acting to intensify the cyclone. The curvature of the trough also allows the additional mass evacuated in the outflow to propagate downstream and not return to its point of origin.

In these simulations, the TC is very sensitive to its exact initial placement relative to the trough or jet with which it is interacting. If the cyclone is too far from the jet, there will be minimal interaction and the cyclone will neither strengthen nor weaken. If the cyclone is too close to the jet, it will become subject to enhanced vertical wind shear and weaken. Lastly, if the cyclone is not optimally placed with respect to the trough axis, the trough will not effectively evacuate mass at the upper levels away from the cyclone. Instead, the outflow will eventually curve back towards the cyclone, consistent with real case studies which have used radiosondes and satellite AMVs to examine the environments of non-intensify TCs.