Definition of the mean environmental steering flow for TCM-90 tropical cyclones
Bohner, Richard H., Jr.
Elsberry, Russell L.
Carr, L. E.
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A definition for the environmental steering flow in the vicinity of the TCM-90 tropical cyclones is sought by low-pass filtering the Naval Operational Global Atmospheric Prediction System deep-layer mean u- and v- velocity and geopotential height fields. One-dimensional and two-dimensional Fourier decompositions in a limited region (120(o) long) and in a global region are compared with six wavenumber low-pass filters (1-6, 1-9,...,1-21). The measure of goodness of the environmental steering flow interpolated to the storm position was to determine the minimum standard deviation of the propagation vector (defined as difference between storm motion and the steering estimate) for all six storms and the ensemble. The best results were found for either the limited and global region one-dimensional Fourier analyses of the u and v wind fields with a low-pass filter that included only wavenumbers 1 -15. The sic TCM-90 tropical cyclones were subsequently analyzed using this definition of the steering flow to estimate the propagation vectors and to examine the linear shear and relative vorticity gradients of the environmental flow. Except for early stages of storms in low latitude, the low-pass filtered analyses provided steering vectors consistent with the changing translation directions. However, the translation speeds tended to exceed the storm motion and lead to more westward propagation vectors than expected. These propagation vectors tended to be almost perpendicular to the absolute vorticity gradient vector.
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