Synthetic aperture radar imaging of axial convergence fronts in Cook Inlet, Alaska

Axial fronts of tidal currents are observed in Cook Inlet, AK, on a RADARSAT-1 standard mode synthetic aperture radar (SAR) image taken at 16:31:47 coordinated universal time (UTC) on July 12, 2002. The longest front appears as a 100-km-long quasi-linear bright feature in the SAR image. This front is characterized by an increase in the normalized radar cross section (NRCS) of 7 dB in the C-band horizontal polarization (C-HH) RADARSAT-1 SAR image. Two other smaller fronts exist in the middle of the inlet. The NRCS modulations appear to be less, at about 5 dB. A diagnostic Cook Inlet tidal model is developed to calculate the current velocity fields of the inlet and to demonstrate that the variation in bottom friction caused by the bathymetry distribution generates axial convergence at different tidal stages. The model, using the actual bathymetry, is driven by predicted tides from six tidal stations along the inlet coast. The model results show that the tidal current flowed into the inlet at the time the SAR image was obtained. Tidal current along two transects in the inlet is extracted to show that there is a significant cross-channel convergence of the along-channel velocity component, with a magnitude of 4 to 6 times 10^-4 s^-1 near the observed front positions. In general, a higher velocity convergence from the model corresponds to higher NRCS return areas in this SAR image.