Vad-Based Determination of the Nyquist Internal Number of Doppler Velocity Aliasing without Wind Information

Abstract

Doppler velocity dealiasing based on the continuity between adjacent gates in the azimuthal and/or radial directions cannot completely remove the ambiguity because the folding character of the initial value used to start the process is not always known. A method to determine the Nyquist interval number (n) that could affect this reference value, and hence the continuity-checked data ensemble, is proposed on the basis of the Velocity Azimuth Display analysis. It is shown that the associated zeroth-order Fourier coefficient (a₀) is a measure of an eventual global shift, which can be defined so that (a₀+2nV_a) fall within [-V_a, +V_a], V_a being the Nyquist velocity of Doppler radar. This is the case for a full 360° distribution of Doppler velocity, but for partial-azimuthal data samplings the Fourier modeling is strongly dependent both on the azimuthal extent of continuity-checked data and on the quality of data. Numerical and experimental data are used to investigate the reliability of the VAD-based determination of Nyquist interval number for various azimuth intervals. It is found that such methods could properly work for sampling intervals higher than 160°, and probably also for reduced intervals (130°) if non-linear wind components are negligible. A great advantage of the present method lies in performance without environmental wind information.