Impacts of smallscale vortices distributed stochastically on tropical cyclone motion

Impacts of smallscale vortices on tropical cyclone (TC) track in a three component system (an idealized subtropical high ridge, a TC, and smallscale vortices) were examined numerically using a barotropic primitive equation model and idealized initial fields. Two smallscale vorticity fields were generated stochastically. In the two fields, the number, size, structure, as well as the total kinetic energy of the smallscale vortices are the same, except the coordinates of the small scale vortex centers distributed stochastically are different. Two experiments, named ExpA and ExpB, with 56h, integration time are performed.Comparison of the integration outputs from the two experiments indicates that the various smallscale vorticity field involved could produce both the various horizontal wind in the TC outer area and the various intensity of the environmental steering for TC. The values of the environmental current velocity along eastwest direction are 7.8, 8.2, and 8.7 m/s in ExpA and 8.3,9.5, and 9.7 m/s in ExpB at t=24,36,and 48 h, respectively. The values along southnorth direction are 0.9,1.8, and 2.5 m/s in ExpA and 2.3, 2.3, and 5.9 m/s in ExpB, respectively. The various intensity of the environmental steering could lead to various TC center positions, for example, the difference between the coordinates of TC centers in the two experiments reaches 120 km at t=48 h, which is about 56% of the prediction error of TC track in the northwestern Pacific region derived from eight operational models.Dynamics of the interplays between a subtropical high ridge and a TC in previous studies has shown that when the TC located equatorward of the subtropical ridge, the relationship between the TC propagation and the local absolute vorticity gradient consists of points scattered around their mean value. After introducing the stochastic vorticity field into the model, the interplays among the subtrop ical high ridge, the TC, and small scale vortices might make the relationship mo re complex, that is, besides the scattered points mentioned above, there also ex ists a high correlation between the TC propagation and the local absolute vorticity gradient.