OPTICAL MEASUREMENTS OF FRAZIL CONCENTRATION

The concentration of ice particles in the water column has been measur ed with two optical instruments, both of which are commercially availa ble. The first instrument used is a single wavelength beam transmissom eter with a pathlength of 25 cm. We calibrated the transmissometer in a laboratory flume and applied this calibration to field measurements made in an Arctic lead. An observed vertical profile of frazil concent ration, which we believe is the first field observation to be reported , was dynamically consistent with the observed temperature and salinit y profiles. This consistency provides indirect support for the validit y of the laboratory calibrations. The transmissometer is capable of me asuring frazil concentrations as low as 2 X 10(-3) kg/m(3). The second instrument tested is a three-wavelength absorption meter with the sel ected wavelength bands in the near infrared portion of the spectrum. I n this region of the spectrum, water and ice have greatly different ab sorption characteristics. To remove the effect of non-ice particles on the measurement of frazil concentration we used the difference betwee n the absorption coefficient measured at two wavelengths. The differen tial absorption technique can measure frazil concentrations as low as 5 x 10(-2) kg/m(3). Both instruments are capable of rapid and sensitiv e in situ measurements of frazil concentration necessary for field and laboratory studies of frazil formation and evolution within the water column.