ON THE INTERACTION OF TROPICAL-CYCLONE-SCALE VORTICES .1. OBSERVATIONS

A detailed analysis is made of the observed behaviour of interacting, tropical-cyclone-scale vortices in the western North Pacific region. It is found that all multiple-vortex interactions can be broken down into a common model of binary interaction. The classical Fujiwhara model of converging cyclonic rotation about a centroid followed by merger is rarely followed. Capture tends to occur rapidly, within several hours, and is followed by a period of relatively stable cyclonic orbit. Cyclone merger occurs by one vortex experiencing a loss of convective organization, followed by strong horizontal shearing and incorporation into the outer circulation of the other vortex. However, a substantial proportion of interacting cyclones escape from the interaction, and the changeover from a stable orbiting configuration occurs rapidly. A modified model of binary interaction is presented. Cases where swarms of mesoscale vortices are formed in convectively active regions without tropical cyclones are also documented. These vortices have a much narrower range of influence (a few hundred kilometres) than that observed for tropical cyclones. When groups of vortices come within this range they are observed to conform to the same interaction model as observed for tropical cyclones.