Comparison of airborne synthetic aperture radar imagery with in situ surface-slope measurements across Gulf Stream slicks and a convergent front

Synthetic aperture radar (SAR) imagery showing two types of features near t he inshore edge of the Gulf Stream are compared with nearly simultaneous es timates of the sea surface slope held derived from optical shipboard measur ements. One class: of feature consists of a set of narrow, dark lines havin g radar signal modulations of about -10 dB at L band. These modulations are comparable to those observed in the in situ wave-slope data over wavenumbe rs corresponding to the radar-resonant Bragg waves. These modulations are a lso predicted by a simplified equilibrium wave spectrum model using a surfa ce elasticity of about 25 mN/m (as determined from in situ measurements of surface tension) and a wind friction velocity of about 10 cm/s (from buoy a nd shipboard measurements). These results support Lyzenga and Malmorino's [ 1998] inference that the Gulf Stream dark lines represent relatively passiv e, biogenically derived surfactant slicks that are advected and strained by the large-scale surface flow field. The second class of feature consists o f a pair of bright-dark signatures over the shallow continental shelf regio n near Cape Hatteras, North Carolina. These features are characterized by s ignal modulations of about +4 and -4 dB in bath the L band SAR and in situ data. These modulations appear to be due to the interaction of surface wave s with the current convergence and divergence regions associated with a rot ary circulation in a shallow convergent front. Model calculations of this i nteraction by Lyzenga [1998] indicate that the positive perturbations assoc iated with current convergence should be relatively independent of the wave number, which is in approximate agreement with the in situ measurements of the present paper. The model also indicates that the negative perturbations associated with the region of current divergence should decrease for waven umbers higher than L band. That trend is observed in the SAR data but not i n the in situ data, which show significant perturbations at higher wavenumb ers. It is conjectured that this is due to the effects of surfactants and e ither along-front or time variability in the frontal dynamics, which are no t included in the model. Additional comparisons df this kind should be usef ul to further test and improve physical models of near-surface phenomena.