Three-dimensional Fujiwhara Effect

This study hypothesizes that the motion of binary tropical cyclones (TCs) can be modified by diabatic heating (DH) asymmetry associated with self-induced vertical wind shear. To demonstrate this hypothesis in the quiescent environment, a set of idealized f-plane numerical experiments are conducted. Identical experiments show that the critical distance for separating, merging, or repelling motion is 8^o. The binary TC interaction shows that single large-scale cyclonic and anticyclonic circulations can develop as a result of the superposition of the circulation of the binary TCs in the lower and upper troposphere. These two different large-scale circulations result in a vertically sheared environment in each TC. Potential vorticity (PV) tendency diagnosis shows that the TC motion is consistent with a positive region of the local tendency of PV. To quantify the contribution of each PV diagnostic term to the TC motion, we calculate each vector of the PV tendency diagnostic equation. In the merger case, the horizontal advection vector (HADV) appears to be greater than the DH vector (DHV) due to attraction. In the repulsion case, the DHV is largely compensated by the HADV. Specifically, the strong shear-induced DH generated in the lower troposphere and downshear or downshear-left quadrant can modify the TC motion in collaboration with the transport of PV to the middle and upper troposphere by offsetting the PV imbalance via the HADV. Overall, these results validate our hypothesis, which we refer to as the three-dimensional Fujiwhara effect. In short, a system of binary TCs induces vertical wind shear in each TC, which can subsequently make the tracks of TCs more separated and clockwise through asymmetric diabatic heating.

The three-dimensional Fujiwhara effect is validated in the western North Pacific using the best track and ERA5 reanalysis data. Based on the ERA5 reanalysis, the TC motion was found to deviate systematically from the steering flow. The direction of deviation is clockwise and repelling with respect to the midpoint of the binary TCs with a separation distance of more than 1000 km. The large-scale upper-level anticyclonic and lower-level cyclonic circulations serve as the VWS for each TC in a manner consistent with the idealized simulations. The VWS of a TC tends to be directed to the rear-left quadrant from the direction of the counterpart TC, where the maxima of rainfall and diabatic heating are observed. The potential vorticity budget analysis shows that the actual TC motion is modulated by the diabatic heating asymmetry that offsets the counterclockwise and approaching motion owing to horizontal advection when the separation distance of the binary TCs is 1000–2000 km. With a small separation distance (< 1000 km), horizontal advection becomes significant, but the impact of diabatic heating asymmetry is not negligible. The abovementioned features are robust, while there are some dependencies on the TC intensities, size, circulation, duration, and geographical location.

This research sheds light on the motion and structure of binary TCs that has not been previously explained by a two-dimensional barotropic framework. The results have been summarized in Lee et al. (2023, QJRMS) and Ito et al. (2023, MWR).