Interaction between a midlatitude upper-level trough and tropical cyclone Dora (2007) over the southwest Indian Ocean

An example of a favourable interaction between a tropical cyclone (TC) and an upper-level trough is examined from a numerical modelling perspective. In relation with a Rossby wave breaking event taking place in the southwest Indian Ocean, an upper-level potential vorticity (PV) anomaly, associated with a midlatitude trough, approached TC Dora (2007). The storm rapidly intensified as the PV anomaly, clearly identified on the ECMWF operational analyses, was entrained toward and merged with high-PV values associated with the TC circulation.

The aim of this presentation is to identify the processes leading to the rapid intensification of Dora, in relation with the PV exchanges between the cyclone and the trough at the synoptic scale. Numerical simulations have been conducted using the French hydrostatic limited-area 8-km resolution model Aladin-Réunion. This model, which includes its own 3D-Var assimilation scheme, has been used for operational purposes over the basin since 2006. TCs are initialized with pseudo-observations from a bogus vortex.

Horizontal and vertical resolution of Aladin-Réunion allows a detailed examination of the complex three-dimensional asymmetric tropopause folding and its deformation during the interaction with the storm. The highest PV values approach the storm center as close as 200 km at upper levels (250-150 hPa) where the inertial stability is the lowest. Meanwhile, a smaller negative PV anomaly that detaches from the main trough at mid-levels in the south-southeastern quadrant of the TC center, is advected toward the storm. It interacts closely with high-PV zones emanating from the TC-core at mid-levels (this mechanism could involve the generation of vortex Rossby waves).

Eliassen-Palm fluxes computed in a cylindrical and isentropic framework (following Molinari et al. 1995) show the import of cyclonic angular momentum and PV at inner radii at high and mid-levels, spinning up the cyclonic circulation over the whole troposphere. Low-level import of angular momentum at outer radii strengthens the circulation outside the radius of maximum wind at a time when Dora's intensification was temporarily stopped. The radius of maximum winds rapidly contracts afterwards, in correlation with a rapid increase of the maximum winds. This is consistent with the observed eyewall replacement cycle and intensification of TC Dora.