An alternative method for inferring winds from spaced-antenna radar measurements

Authors

  • Richard J. Lataitis,

  • Steven F. Clifford,

  • Christopher L. Holloway


Abstract

One variation of the spaced-antenna technique for measuring atmospheric winds using a pulse Doppler radar is based on a calculation of the complex temporal cross-correlation function for the backscattered fields detected by a pair of spaced receiving antennas. The delay to the peak of the cross-correlation amplitude, adjusted for the temporal evolution of the backscattered field pattern through an extended analysis, is related to the so-called trace velocity along the antenna pair baseline, with two nonparallel baselines yielding the full horizontal wind vector. We suggest an alternative method for inferring the horizontal wind from the temporal cross correlation. We demonstrate that the slope of the cross-correlation amplitude at zero lag, normalized by the level of the cross-correlation amplitude at zero lag, is directly proportional to the component of the wind velocity along the antenna pair baseline. We illustrate that this measure of the horizontal wind is insensitive to the temporal evolution of the backscattered field pattern. Therefore no extended analysis is needed to estimate the horizontal wind vector. The advantage of this approach is that it is a very simple and direct method of retrieving the horizontal wind. Our theory indicates that for many cases of interest the proportionality constant relating the normalized slope to the wind component along the antenna pair baseline depends only on the transmitting and receiving antenna diameters and on the separation between the receiving antennas. This constant can be calculated, or the system can be calibrated against a rawinsonde retrieval. A disadvantage to the slope technique is that it is somewhat sensitive to the degree of vertical anisotropy of the scattering medium. We loosely define the degree of vertical anisotropy r as the ratio of the vertical to horizontal correlation length of refractive index irregularities having a spatial scale of the order of the Bragg wavelength λ/2, where λ is the radar wavelength. In the neutral atmosphere, r typically satisfies 0 < r ≤ 1. We find that for a horizontally isotropic, Kolmogorov-type refractive index spectrum with λ/2 < L0z, where L0z is the vertical outer scale, the slope method is useful provided r is greater than the effective system beam width λ/De, where De is an effective system antenna diameter (approximately equal to the transmitter antenna diameter). We note that this same restriction ensures that the correlation scale of the backscattered field (i.e., the pattern scale) is solely a function of the antenna diameters. The slope method is therefore limited to scattering from isotropic to moderately anisotropic turbulence (i.e., λ/De < r ≤ 1) provided λ/2 < L0z and is perhaps best suited for wind profiling in the atmospheric boundary layer. A comparison of horizontal winds obtained by applying the slope technique to data from a 915-MHz spaced-antenna radar and winds from a rawinsonde retrieval showed good agreement.

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