Title :
Side-looking ADCP and Doppler radar measurements across a coastal front
Author :
Trump, Clifford L. ; Allan, Nicholas ; Marmorino, George O.
Author_Institution :
Naval Res. Lab., Washington, DC, USA
Abstract :
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 profiler (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, Doppler radar mounted at a height of about 4 m on the starboard side. Both velocity 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 side of the front it was possible to make nearly simultaneous across-front measurements with both systems. Despite the differences in acoustic and radar scattering mechanisms, a combined backscatter intensity surface map could be made showing a continuous frontal signature about 10-m wide and 20 dB above background levels. Each system was also able to measure the same large-scale velocity change across the front, which was dominated by the discharging buoyant bay water flowing at about 50 cm/s relative to the ambient continental shelf water. However, within a 60-m wide zone, the radar system measured velocities up to 75 cm/s larger than the ADCP. Such large velocity differences arose from the radar´s sensitivity to motions associated with waves reflecting from the region of strongest across-front current convergence. This frontal convergence was resolved only by the ADCP, which showed a horizontal current change of about 25 cm/s over 10 m and appeared to extend over the upper 2 m or so of the water column. These results show that the combined information from the acoustic and radar systems provide a more complete picture of the frontal currents and wave-current interactions than either system could provide alone.
Keywords :
Doppler radar; acoustic applications; microwave measurement; oceanographic techniques; remote sensing by radar; 10 m; 120 m; 20 dB; 200 m; 25 cm/s; 300 kHz; 300-kHz acoustic Doppler current profiler; 50 cm/s; 60 m; 60 m wide zone; 75 cm/s; Chesapeake Bay; Doppler radar measurement; backscatter intensity; coastal front; combined backscatter intensity surface map; continuous frontal signature; depth of 0.6 m; discharge front; discharging buoyant; horizontal current; prototype X-band; radar sensitivity; research ship; side-looking ADCP; side-looking Doppler; surface current convergence; vertically polarized Doppler radar; wave current interaction; Acoustic measurements; Backscatter; Convergence; Doppler radar; Fault location; Mouth; Narrowband; Radar measurements; Sea measurements; Velocity measurement;
Journal_Title :
Oceanic Engineering, IEEE Journal of