A series of idealized numerical simulations of tropical cyclones is performed to enhance the knowledge on the mechanism of tropical cyclone (TC) motion in vertically sheared environments. The simulations are performed using an f-plane version of the typhoon track prediction model previously run at the Japan Meteorological Agency.
A wide variety of TC tracks show up when the model is integrated repeatedly with different formulations of convective parameterization from an identical initial field. To account for the diversity of TC motion in vertically sheared environment, a steering-weight concept is introduced. The steeringweight is a set of weighting factors, mathematically derived from the surface pressure tendency equation, and can be a measure of how sensitive the motion of a TC is, to the steering flow at each vertical level. The experiments with different cumulus parameterizations show that the vertical profile of steering weight tends to strongly depend on the cumulus parameterization scheme used, but much less on the environment specified. This result implies that the axisymmetric thermal structure of modeled TCs, is mostly determined by parameterization schemes rather than environment in the model. And the longterm (say, 72 h mean) TC motion is well explained by the combination of steering weight, and large-scale flow fields.
The result suggests that the differences in TC track among numerical prediction models, may be attributable to the differences in steering weight to some degree, given that there is no significant difference in the simulated large-scale flow fields among the models. Moreover, as long as the steering weight concept holds in nature, the steering-weighted deep layer mean flow, instead of the conventional and empirical pressure-weighted one, would work better in accounting for the motion of real tropical cyclones.