CONTACT INFORMATION: P.I.s Clayton Paulson and W. Scott Pegau COAS 104 Ocean Admin Bldg Corvallis OR 97331 phone: 541 737-5229 fax : 541 737-2064 email: cpaulson@oce.orst.edu spegau@oce.orst.edu DATA VERSION: The data was initially processed by W. Scott Pegau in October of 1999. The data supplied is version 1. This data is expected to be in its final format. This document contains notes on the collection and processing of data related to the SBE-25 CTD used in profiling mode as part of the summer leads portion of SHEBA. Further information and pictures of the equipment is available on the Sheba Main Page. Most of the data was collected from a small lead about 1 km WNW from the ship. EQUIPMENT: This data set includes data from a SBE-25 CTD and a Wetlabs ac-9. SBE-25 notes: A SeaBird SBE-25 Conductivity-Temperature-Pressure (CTD) sensor was used to make physical measurements of uppermost 60 m of the water column. The instrument was normally mounted to profile from within about 40 cm of the surface to 15 m depth with occasional profiles to 60 m. The sensors were calibrated before and after the experiment. Data was logged internally during each sampling period at a rate of 8 Hz. The data was downloaded after 2 hours of sampling because of the limited internal memory. Ac-9 number 117 was used. This instrument measures the beam attenuation and absorption coefficients relative to a pure water blank (wavelengths 412 440 488 510 532 555 650 676 715). Calibrations were performed before deployment. Filtered water measurements were used to check for drift in the calibrations during the experiment. No drift in calibrations was observed. The instrument is a flow-through style instrument with water being pulled through the instrument by a pump. A 0.2 micron pore-size filter was occasionally installed on the absorption side to measure the absorption by dissolved materials. A modular oceanographic data and power system was used to power and log data from all of the instruments. This unit also provides a time stamp to the data stream, which is used to synchronize the data files. PROCESSING NOTES: The SBE-25 data was processed using the processing software supplied by the manufacturer (seasoft). The calibration file used is sheba25p.con from post calibrations. No significant differences were found between the pre and post calibrations of the temperature and conductivity sensors. For the ctd processing we used the following seasoft routine: Datcnv - to insert calibrations output (T, C, P,time(seconds),fluorometer voltage) Calibration coefficients were applied to the ac-9 data using software from the manufacturer. The ac-9 data was merged with the CTD data based on the time records from each instrument. The absorption and attenuation data was corrected for temperature and salinity effects in accordance with the operations protocols. To correct for scattered light not collected by the absorption meter, the a715 measurement was subtracted from the measurements at the other wavelengths. A deep downcast was then selected and the data was binned into 1 m intervals. The binning interval was centered around the reported pressure (depth) and the median value of each variable within that bin is reported. The optical data processed to this level includes apg(lambda) and cpg(lambda). The a stands for absorption coefficient, c for beam attenuation coefficient, p for particle contribution, and g for dissolved component contribution. The absorption and beam attenuation coefficients of water were added to the apg and cpg to determine the total absorption (at) and beam attenuation coefficients (ct). The pure water values used are provided in table 1. The total scattering coefficient (bt) was calculated as the difference between ct and at. When consecutive filtered and unfiltered casts were obtained the dissolved absorption coefficient (ag) measurements were added to the unfiltered data file. The absorption and beam attenuation coefficients of the particulate materials (ap and cp) were then calculated. Chlorophyll-a concentrations were estimated from the ac-9 data using (apg(676)-apg(650))/0.014. Table 1. Pure water absorption and attenuation coefficients used. Wavelength 412 440 488 510 532 555 650 676 Aw .0046 .0064 .0146 .0325 .0444 .0596 .3400 .4558 Cw .0097 .0103 .0171 .0345 .0461 .0610 .3407 .4564 IMPORTANT NOTES We have not tried to correct the pressure signal for changes in atmospheric pressure. We have not compared the estimated chlorophyll concentrations to the values measured by the biologists present during this experiment. Extreme care must be used when interpreting the fluorometer signal. We found that false subsurface maximums were evident because of changes in the fluorescence efficiency as the phytoplankton and instruments went under the ice. These measurements require corrections for temperature and salinity effects. The gradient in these properties at the base of the fresh water layer was much stronger than any of our previous observations. Especially when a filter was attached I am concerned about poor results near this interface. The strong density gradient also causes Shlering effects in the optics that will cause the meters (especially beam attenuation) to be high. These effects may be present for a couple of meters below the interface depending on the flushing of the sensors. DATA FORMAT: The data is supplied in ascii, tab-delimited, columnar format files. Each file covers a 2 to 4 hour sampling period within a day. The columns include the date as YYYYMMDDHHmmss, the julian date with January 1 being day 1, pressure, temperature, salinity, sigma-T, fluorometer voltage, and the optical properties described above. The units of position are in meters, pressure in decibar (1 decibar is ~ 1 meter), temperature in degrees C, salinity in PSU, and sigma-T is the density - 1000 with units of kg/m^3, all optical parameters have units of 1/m.