On the interaction of tropical - cyclone - scale vortices . 11 : Discrete vortex patches

The interaction of vortex patches in relation to the observed scales and features reported in Part I of this paper is investigated. It is found that the initial approach of compound vortices, such as tropical cyclones, arises from distortion of their, weaker, outer vorticity fields. Care needs to be taken in applying simple concepts, such as the propagation of each vortex on the gradient of its neighbour. For similar vortices, the initial interaction consists of mutual orbit and approach with only minor fluctuations in shape. Once the vortices approach to within a critical separation distance, rapid merger occurs. A simplified expression for this critical distance is derived and it indicates that binary tropical cyclones must approach to within 150-300 km before merger of the cores can occur. When vortices have substantial differences in size or intensity, the smaller/ weaker system shears into the outer circulation of the other and no core merger occurs. It is suggested that this is typical of the interaction between tropical cyclones and both the monsoon trough and embedded mesoscale convective complexes. The motion of interacting vortices consists largely of a mutual orbit, which scales according to their relative sizes and intensities. An important finding, however, is that higher-frequency meanders may develop. This indicates that a tropical cyclone interacting with a monsoonal shear zone may develop internal asymmetries and a trochoidal oscillation as a direct result of such interaction. Further, weak mesoscale vortices within the cyclone circulation may continue to cause a track meander long after they have been sheared beyond the resolving capacity of atmospheric observations.