Side-looking ADCP and Doppler radar measurements across a coastal front

Measurements are reported from two side-looking Doppler systems, which were used to study the discharge front located off the mouth of Chesapeake Bay. One system was a commercial 300-kHz narrow-band acoustic Doppler current p rofiler (ADCP), which was mounted at a depth of 0.6 m on the port side of a research ship. The other was a prototype X-band, vertically polarized, Dop pler radar mounted at a height of about 4 m on the stal board side. Both ve locity and backscatter intensity were measured along two beams to ranges of 120 m (ADCP) and 200 m (radar), so that by sailing alternately on each sid e of the front it was possible to make nearly simultaneous across-front mea surements with both systems. Despite the differences in acoustic and radar scattering mechanisms, a combined backscatter intensity surface map could b e made showing a continuous frontal signature about 10-m wide and 20 dB abo ve background levels. Each system was also able to measure the same large-s cale velocity change across the front, which was dominated by the dischargi ng buoyant bay water flowing at about 50 cm/s relative to the ambient conti nental shelf water. However, within a 60-m wide zone, the radar system meas ured velocities up to 75 cm/s larger than the ADCP, Such large velocity dif ferences arose from the radar's sensitivity to motions associated with wave s reflecting from the region of strongest across-front current convergence. This frontal convergence was resolved only by the ADCP, which showed a hor izontal current change of about 25 cm/s over 10 m and appeared to extend ov er the upper 2 m or so of the water column, These results show that the com bined information from the acoustic and radar systems provide a more comple te picture of the frontal currents and wave-current interactions than eithe r system could provide alone.