Doppler Velocity Signatures of Idealized Elliptical Vortices

Abstract

Doppler radar observations have revealed a class of atmospheric vortices (tropical cyclones, tornadoes, dust devils) that possess elliptical radar reflectivity signatures. One famous example is Typhoon Herb (1996) that maintained its elliptical reflectivity structure over a 40-hour period. Theoretical work and dual-Doppler analyses of observed tropical cyclones have suggested two physical mechanisms that can explain the formation of two types of elliptical vortices observed in nature, namely, the combination of a circular vortex with either a wavenumber two vortex Rossby wave or a deformation field. The characteristics of these two types of elliptical vortices and their corresponding Doppler velocity signatures have not been previously examined.

Idealized elliptical vortices consisting of a Rankine vortex combined with wavenumber two disturbances are constructed and sampled by a hypothetical Doppler radar. The Doppler velocity patterns of all wavenumber two cases shown in this study are visually similar. The characteristics of the two-dimensional flow patterns and the corresponding Doppler velocities produced by an idealized vortex Rossby wave and deformation field are illustrated. The ground-based velocity track display (GBVTD) single Doppler vortex wind retrieval technique was performed on these two types of elliptical vortices. The GBVTD technique can retrieve wavenumber two components in the vortex Rossby wave case outside the RMW but failed to retrieve wavenumber two structure of the deformation case for all radii. It can be seen that when the phase of the wavenumber two VT is ahead of VR by π/4 with same amplitude, GBVTD coefficients A3 and B3 are near zero; hence, GBVTD technique cannot retrieve the wavenumber two amplitude in these conditions. However, by knowing the elliptical shape of the vortex as a priori these two mechanisms can still be distinguished by the characteristics of the corresponding GBVTD coefficients.

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