Relating Tropical Cyclone Steering Flow to Tropical Cyclone Intensity

Previous studies have suggested that relatively weak tropical cyclones (TC's) are steered by a shallow lower troposheric mean layer wind, while more intense tropical cyclones are steered by a deeper tropospheric layer mean wind. While numerous studies making use of Potential Vorticity(PV) diagnostics have explored how changes in the large scale flow effect the TC track, the have not resolved the problem of why the steering may vary with TC intensity.

Analyses of well developed TC's based on high resolution dropsonde and aircraft doppler data, reveal the existence of a tower or annular column of PV in the vicinty of the TC center. As the PV tower is advected by the large scale flow the tropical cyclone moves(neglecting propogation effects). Also, as the tropical cyclone intensifies, the PV gradients around the TC center also intensify. Based on these ideas, the hypothesis of this study is that portion of the TC PV column characterized by the largest magnitude (horizontal gradients of) PV will define the steering layer.

This study will make use of data sets derived from a high resolution numerical simulation of a TC that undergoes significany shearing to test the above hypothesis. Also to be addressed will be to : quantify the relationship between shear rates in the vicinity of the TC associated with the large scale flow and the TC intensity and determine the effects of cumulus parametriaztions and microphysical schemes on TC PV structure and TC steering.