Near-Surface Current Profile Measurements Using Time Series Optical Imagery

Accurate measurements of near-surface current profiles are difficult to obtain using ADCPs and CODARs alone. The same is true of current retrievals using our airborne remote time series EO imagery method. Thus, we designed an experiment to calculate the velocity profile to the surface using optical imagery-based measurements of the Doppler spectrum of wide-band gravity waves under a variety of conditions, and compared the results with moored ADCPs. A location at Castle Point on the Hudson River was chosen because the channel is wide and quasi-linear, and there is a tall building for overlooking the site from Castle Point, which itself is elevated. The surface gravity wave Doppler spectrum was measured with cameras in our aircraft and rigidly mounted on top of the building, the latter being especially interesting because they exhibit wide-band waves that are resolved down to ~50 cm scales, thereby potentially enabling current estimates very close to the surface. The 3D wave spectrum is measured by co-registering the imagery time series and mapping the image data to rectilinear coordinates on a geodetic surface at the mean water level. The resulting 3D (x-y-t) data cubes are Fourier transformed to compute 3D (k_x-k_y-omega) spectra. Most of the variance in these spectra is seen to lie along the dispersion surface for linear gravity waves. In the past, we have computed a value for the local mean current velocity by fitting the theoretical form of the wave dispersion surface to these 3D spectra, under the assumption that the current vector and water depth have constant values over the space-time scales of the data cube. In most cases, the application of this technique to image data collected from an aircraft along exposed coastlines and inlets has resulted in very good fits to this curve, enabling successful retrievals of the current with results typically within 10 cm/sec in magnitude and 10deg in direction of the data from the top uncontaminated de- pth bin of an ADCP located near the center of the data cube. However, the presence of high current shear or use of an ADCP bin that is more than 2 m removed from the surface reduces the agreement. The goal of this investigation is to extend the spectral measurements to shorter waves than typically resolved from the aircraft, and determine the current velocity profile throughout the top several meters above the ADCPs. This paper describes our initial results, including an overview of the experiment and description of two example data sets in cases of high shear. The resulting spectra exhibit a sheared gravity wave dispersion surface and also an advective surface likely associated with water surface film and/or bubbles. These both provide current estimates, and these are compared with upward-looking ADCP profiles and surface-track returns.