Generalized Empirical Formulas of Threshold Distance to Characterize Cyclone–Cyclone Interactions

This paper presents a framework of using remote sensing imagery and image processing techniques to analyze the interaction among typhoons or tropical cyclones (TCs) with and without the involvement of mid-tropical depressions (TDs) or tropical storms (TSs). The cyclone-cyclone interaction (named the Fujiwhara effect) could happen when two TCs are located at a distance within 1400 km. However, when there exists a mid-TD between the two TCs even with a distance of more than 1400 km, but usually within a distance of 1500-2700 km, the mid-TD may induce impacts on the two cyclones, defined as a TD-inlaid cyclone-cyclone interaction. The existence of the mid-TD significantly complicates weather forecasting due to difficulty in modeling its influence on the two cyclones. In this paper, we proposed innovative empirical formulas of threshold distance to characterize cyclone-cyclone interactions either with or without the involvement of the TD(s). We generalize empirical formulas for interaction distances between TCs and TD/TS related to current-intensity number, size factor, height difference, and rotation factor. To validate the appropriateness of the generalized empirical formulas, we choose and analyze seven sets of two-typhoon events. It is shown that the generalized formulas successfully predicted the impact of mid inlaid TC/TS quantitatively. The mid-TD's effects on the two cyclones represent a new aspect and a significantly improved understanding for the prediction of more complicated cyclone-cyclone interactions. Note that the empirical formulas are derived from a very small set of events and that they must be verified with a sufficient data set for future practical use.