Spectral light absorption and attenuation measurements from a towed undulating vehicle

Measurements from a nine-wavelength light absorption and attenuation instru ment mounted on a towed, undulating vehicle (SeaSoar) capable of rapidly pr ofiling the water column complete up-down cycles in 1.5-12 min depending on the maximum sampling depth - are used to characterize strong spatial varia tions in the distributions of upper-ocean biooptical properties. Water samp led from adjacent to the conductivity and temperature sensors located in th e nose of the vehicle is pumped through the 25-cm dual optical flow tubes o f a Western Environmental Technology Laboratories (WET Labs) ac-9 instrumen t mounted on top of SeaSoar. A three-stage algorithm for post-processing th e optical data to insure high-quality measurements concurrent with conducti vity-temperature-depth data is described. After synchronizing the optical, navigational and conductivity-temperature-depth data streams, the method re lies on finding the optimal time delay between when a water parcel is sampl ed first by the temperature and conductivity sensors and then by the absorp tion and attenuation optics. After applying the calculated rime-dependent l ag, a correction for the dependence of light absorption on temperature and salinity and a scattering correction to absorption are made. The final proc essed optical data from SeaSoar compare well with the same parameters sampl ed by a slowly lowered vertical profiling package deployed from a nearby st ationary vessel. A 2-h, 30-km long cross-shelf section consisting of 184 ve rtical profiles separated by 150-200 m reveals strong horizontal variations on short spatial scales (1 km or less) of the vertical distributions of bi o-optical properties. (C) 1999 Elsevier Science Ltd. All rights reserved.