Wavelet analysis of surface current vector fields measured by high frequency Doppler radar

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Abstract
Fourier spectral methods have been widely applied to coastal zone current measurements. However in cases such as riverine tides or estuarine outflow currents exhibit non-stationary characteristics which invalidate the basic assumptions of these methods. Wavelet analysis techniques can be used to determine the temporal evolution of current variance over a range of frequency scales and therefore can provide an improved understanding of event-driven dynamics. Morlet continuous-wavelet transforms were applied to multiple vector time-series measurements from a High Frequency (HF) Doppler radar and moored ADCPs near the mouth of Chesapeake Bay in 1996 and 1997 as well as wind measurements at the Chesapeake Light tower. The time-varying clockwise (CW) and counter-clockwise (CCW) wavelet spectra were computed from each vector time-series. The horizontal, vertical and temporal evolution of high energy scales could then be visualized. Significant short-term intensifications of 30-60 hour CW energy in the region of the outfall plume were observed that were highly coherent with local wind forcing.