On the sensitivity of TC intensification under upper-level trough forcing

A previous study on TC Dora (2007) in the southwest Indian Ocean gave some insight on the pathway to storm intensification under upper-level trough forcing. The French operational hydrostatic limited-area model at 8-km resolution, Aladin-Reunion, allowed diagnoses of trough impacts in the TC core. The main mechanisms identified for vortex intensification were (a) PV superposition (associated with radial convergence of angular momentum) at mid and upper levels where unusual mean radial inflow was observed, followed by (b) secondary eyewall formation induced by eddy momentum flux convergence, eddy PV (or absolute vorticity) fluxes, and vertical velocity forcing from the trough, which contributed to mean and eddy vertical advection of the tangential wind in the outer eyewall. The predicted intensity evolution was consistent with the observed eyewall replacement cycle and intensification of TC Dora.

The sensitivity of the interaction to the respective positions and strengths of the TC and the trough is now assessed. A hierarchy of sensitivity experiments is run with the same model in which the initial location and/or intensity of the storm are modified (through the assimilation of pseudo-wind observations). How robust is the formerly proposed paradigm for intensification under upper-level forcing when the geometry of the interaction is modified? How much does a weaker storm intensify under upper-level forcing compared to a stronger storm? Track and intensity verifications will be used as well as diagnostics regarding the interaction of the main two cyclonic PV anomalies associated with the trough and the TC at both mid and upper levels. PV budget and Eliassen–Palm fluxes will be compared to the reference simulation to investigate the robustness of intensification with respect to the relative locations and intensities of the storm and upper-level trough.