Forecasting Tropical Cyclone Intensity Change during Trough Interaction

The interaction between tropical cyclones (TCs) and upper-tropospheric troughs is a common occurrence in the North Atlantic basin. The favorability of such interactions for TC intensity change, as well as the mechanisms by which TC intensity is affected by the trough, however, remain unclear. These open questions are investigated in this presentation through the use of reanalyses, satellite observations, and idealized numerical modeling.

A climatology of TC–trough interactions in the North Atlantic basin was compiled from the European Centre for Medium Range Weather Forecasts ERA-Interim (ERA-I) reanalysis. The climatology indicates that TC–trough interactions are more unfavorable for TC intensification compared to the climatology of the basin as a whole and TCs that do not interact with troughs. Although troughs are most often unfavorable for TC intensification, there are still TC–trough interactions that result in intensification.

Analog composites of favorable and unfavorable TC–trough interactions from satellite observations and the ERA–I were made to assess the roles of both the trough morphology and TC convection on intensity change. Relative to the unfavorable TC–trough interaction composite, favorable trough interactions are associated with weaker, shallower, and longitudinally-narrower troughs, which are associated with reduced vertical wind shear. Favorable troughs are also associated with reduced midlevel entropy deficits, especially upshear of the TC, which, combined with the smaller shear values, results in less ventilation of the TC and strengthening convection. During favorable TC–trough interactions, the strengthening convection is able to wrap upshear of the TC center, which is a favorable configuration for TC intensification.

Idealized numerical modeling simulations were used to better assess the physical mechanisms that affect intensity change during TC–trough interaction. Two model simulations were investigated: a simulation of a TC interacting with a westerly jet and a simulation interacting with a trough embedded in a westerly jet. Despite experiencing larger vertical wind shear, the TC–trough simulation TC was more intense for most of the simulation due to the increased dynamic and thermodynamic favorability imparted by the upstream trough. The presence of the trough resulted in moistening of the environment through three mechanisms associated with large-scale lift and divergence: reduced inertial stability resulting in enhanced outflow, the transition of the synoptic-scale flow from subgeostrophic to supergeostrophic, and Q-vector forcing for ascent.