Summer Leads Data CONTACT INFORMATION: P.I.s Clayton Paulson and W. Scott Pegau COAS 104 Ocean Admin Bldg Corvallis OR 97331 phone: 541 737-3504 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 August 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-19 CTD used to make surface measurements as part of the summer leads portion of SHEBA. Further information and pictures of the equipment is available at http://photon.oce.orst.edu/ocean/projects/sheba/sheba.htm Almost all of this data was collected in a lead ~1km WNW of the ship. EQUIPMENT: This data set includes data from three instruments 2 Garmin 12XL GPS units and a SBE-19 CTD. GPS notes One Garmin 12xl gps was left on the ice to provide a reference point. The second unit was place on the port edge of the skiff for determining the position of the boat. Most of the lead circumference surveys were conducted keeping the port side of the skiff towards the ice. A 1 m open water buffer was maintained between the boat and the ice edge. We tried to follow the edge wherever we could navigate the 3m boat. The gps units were internally recorded at 15 second intervals except on a couple occasions early on in the experiment. The time synchronization between units drifted over time. SBE-19 notes A SeaBird SBE-19 Conductivity-Temperature-Pressure (CTD) sensor was used to make physical measurements of the surface waters. The instrument was slung under the bow of the boat with the sensors in front of the boat measuring undisturbed water. The depth of the sensor was measured and found to be 15 cm +- 5 cm. The depth varied dependent on which operator was in the boat and where that person stood. Under all conditions the sensors were within the top 25 cm of the lead. Since the person was in the rear of the boat the sensors position tends to err on being less than 15 cm. The sensors were calibrated before and after the experiment. Data was logged internally during each sampling period at a rate of 2 Hz. The data was downloaded after 2 to 4 hours of sampling because of the limited internal memory. The CTD's internal time stamp was synchronized with the gps time. PROCESSING NOTES: The position from each gps unit was interpolated to 1 second intervals using a piece-wise cubic spline and the two interpolated records were merged. The position of the gps on the ice was subtracted from the position on the boat to provide a relative position. The relative position was converted from degrees to meters using 111,120 m/degree latitude. The longitude component was multiplied by the cosine of the latitude and then the meters/degree constant. The SBE-19 data was processed using the processing software supplied by the manufacturer (seasoft). The calibration file used is sheba19f.con it includes a compensation slope for the conductivity cell that is the average of the slope from pre and post calibrations (1+post)/2. The correction applied changed the slope of the conductivity by <0.2% compared to the pre-cruise calibration. No changes were applied to the temperature cell calibration that drifted by a millidegree between calibrations. The pre-cruise calibration was combined with the conductivity slope to develop the final configuration file (sheba19f.con). For the ctd processing we used the following seasoft routines: Datcnv - to insert calibrations output (T, C, P,time(seconds from start)) Wildedit - to try and identify when the conductivity cell was not in the water Filter - lowpass conductivity using a 0.5 second filter by the software manual recommendation Derive - to determine salinity and ?-t Binavg - make 1 second bin averages of the 2 Hz data Asciiout - change from binary to ascii and remove header except for column headers The ascii version of the CTD data was then merged with the gps data. The two records were merged based on time. The logged starting time and date was added to the time interval output from Datcnv. The position data was then linearly interpolated to the time of the CTD to merge the two data streams together. The data was cropped at each end to remove data collected while the boat was moored to the ice. No such cropping has occurred for periods when the boat was moored during the middle of the record. THE PRESSURE RECORD HAS NOT BEEN COMPENSATED FOR ATMOSPHERIC PRESSURE WHICH CAN CAUSE UP TO A 0.3 DECIBAR ERROR IN PRESSURE. 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, relative position EW, relative position NS, pressure, temperature, salinity, and sigma-T. 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.