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

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.