Impacts of Randomly Distributed Small-Scale Vortices on Typhoon Tracks

Impacts of small-scale vortices on typhoon or tropical cyclone (TC) tracks in a system of three components (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, where the number of initial small-scale vortices is 100. In the two fields, the number,size, structure as well as the total kinetic energy of the small-scale vortices were all the same, but their center coordinates were randomly distributed. Two experiments, named EXA and EXB, with the integration time of 56 h, were performed. Comparison of the integration outputs from the two experiments indicates that various small-scale vorticity fields involved could produce both various horizontal winds in the TC outer area and various intensities of the environmental steering for TC. The values of the environmental flow speed along the east-west direction are 7.8, 8.2, and 8.7 m s-1 in EXA and 8.3, 9.5, and 9.7 m s-1 in EXB at t = 24, 36, and 48 h, respectively. The values along the south-north direction are 0.9, 1.8, and 2.5 m s-1 in EXA and 2.3, 2.3, and 5.9 m s-1 in EXB, respectively. The various intensities of the environmental steering could lead to various TC center positions. For example, the difference between the coordinates of TC centers in EXA and EXB reaches 120 km at t = 48 h, which is about 56% of the prediction error of TC tracks in the northwestern Pacific region derived from eight operational models. Dynamics of the interplays between a subtropical high ridge and a TC in previous works has shown that when the TC was located equatorwards of the subtropical ridge, the relationship between the propagation of TC and the local absolute vorticity gradient was characterized by points scattered around their mean values. After introducing the stochastic vorticity field into the model, the interplays among the subtropical high ridge, the TC, and small-scale vortices might make this relationship more complex, that is, besides the scattered points, there also exists a high correlation between the propagation of TC and the local absolute vorticity gradient.