Steering Weight Concept and its Application to Tropical Cyclones Simulated in a Vertical Shear Experiment

Abstract

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.

Observational Analysis and Numerical Evaluation of the Effects of Vertical Wind Shear on the Rainfall Asymmetry in the Typhoon Inner-Core Region

“…As shown in the figure, the upper-level wind tends to point somewhere to the left (right) of the low-level wind in the lowlatitudes (mid-latitudes), suggesting that the hodograph tends to rotate cyclonically (anticyclonically) with height in the low-latitudes (mid-latitudes). This result, combined with the common understanding that the TC motion is a steering-type process, with the steering flow equal to a vertically integrated tropospheric flow around the storm (Ueno 2003), the relationship between vertical shear and storm motion may be illustrated schematically, as in Fig. 4.…”

“…The model used in the study is an f-plane version of the previous JMA typhoon model, and essentially the same as that used in Ueno (2003), except for increased horizontal and vertical resolutions. It is a three-dimensional, hydrostatic, primitive equation, spectral limitedarea model, with physical parameterizations of the boundary layer and cumulus convection.…”

“…The construction method of initial environment presented here, is an extension of that in Ueno (2003), which allows only for the environments with unidirectional shear. The initial TC environment, is specified from the JMA global analyses at the corresponding time.…”

“…They speculated that differential advection of the axisymmetric vorticity by the shearing storm-relative flow, induces upward vertical motion on the downshear side, and the ascending air rise in a cyclonic spiral on the left side of shear vector, producing maximum latent heat release and rainfall on that side. Ueno (2003) also performed a set of idealized simulations to investigate the impact of diabatic heating on the behavior of a typhoonlike vortex embedded in a unidirectional vertical shear. In the simulations strong wavenumber-one asymmetries in TC rainfall are produced, which are detectable, not only in the hourly model output, but also in 36-hour stormrelative composites.…”

“…Most theoretical and numerical modeling studies (e.g., Jones 1995;Bender 1997;Frank and Ritchie 2001;Ueno 2003;Wu et al 2006) of the influence of vertical wind shear on the azimuthal distribution of convection in TCs, have mainly focused on the directional relationships between vertical shear, vortex tilt, and convective asymmetries, with one exception of Rogers et al (2003), who showed that changes in convection, evaluated by the ratio between the wavenumber-one and wavenumber-zero Fourier coefficients of model-simulated reflectivity, are well correlated with changes in the vertical shear. In the latter part of the present study, to make the relationship between shear and rainfall asymmetry more quantitative, we will attempt to derive a simple formula for the magnitude of wavenumber-one vertical motion, due to AM1 based on the thermal wind balance consideration.…”