SCICEX'93 Hydrographic Data Processing Description CTD Data from USS Pargo 1993 Dr. James Morison morison@apl.washington.edu (206) 543-1394 Roger Andersen roger@apl.washington.edu (206) 543-1258 Polar Science Center, Applied Physics Lab, University of Washington 1013 NE 40th, Seattle, WA 98105-6698 USA FAX (206) 543-3521 _______________________________________________________________________ During SCICEX'93, the Pargo sampled hydrographic data with casts while surfaced using a Sea-Bird CTD, expendable CTD probes dropped while submerged, and a Sea-Bird Seacat mounted on the submarine's sail continuously recording. Twenty surface CTD casts and 34 potentially usable expendable drops were recorded, while the CTD on the sail logged a sample every 12 minutes. The surface cast CTD was Sea-Bird SBE19 serial number 114, except for two casts using backup CTD SBE19 serial number 117; the Seacat on the sail was Sea-Bird SBE16 serial number 914. The Mark 12 expendable CTD probes were a batch with the manufacturer's serial number beginning 93060000. Due to the high quality of the SBE19's, the surface casts were taken to be the measurement standard. A depth correction for SBE19 serial number 114 of +1.746 decibars was obtained from post-cruise pressure sensor calibration. In general, the accepted profile for a given station was the average of the down and up casts. In a few cases, the top portion of the down cast appeared seriously corrupted in a manner suggesting plugging of the pumped orifice or tubing by ice, either from ingesting slush in the surface layer or submerging a cold-soaked instrument. This generally cleared up in a few tens of decibars. In these regions selected by eye, the up cast alone was used. As an initial examination of the quality of the expendable probe data, seven expendables were found close enough to seven surface casts in both position and time to allow direct comparison. It was apparent that the expendables differed substantially from the surface casts, particularly in depth. Point comparisons were obtained by matching depths of vertical temperature gradient peaks in each expendable profile with the same temperature gradient peaks in the corresponding surface cast profile. The expendable depth was taken to be accurate at 15 decibars, immediately after launch. One of these seven expendables contributed a disproportionate share of the variance of depth compared with the surface casts, and it was eliminated from further consideration. The resulting scatter of expendable pressure against surface cast pressure appears somewhat nonlinear, particularly at shallow depths. After some consideration, a practical correction to expendable pressure was accepted, consisting of three connecting line segments. For expendable pressure down to 40.6773 decibars: Corrected_Pressure = 0.3979 * Expendable_Pressure + 9.0314 For expendable pressure from 40.6773 down to 146.8042 decibars: Corrected_Pressure = 0.7533 * Expendable_Pressure - 5.4253 For expendable pressure below 146.8042 decibars: Corrected_Pressure = 0.9050 * Expendable_Pressure - 27.6955 To examine expendable temperature and conductivity, pressure-corrected expendable profiles were accumulated in 5 decibar bins and scattered against similarly binned surface cast profiles. Fitting lines then provided small corrections. For expendable temperature (¡C): Corrected_Temperature = 1.0092 * Expendable_Temperature + 0.0385 For expendable conductivity (Siemans/meter): Corrected_Conductivity = 1.0023 * Expendable_Conductivity + 0.0020 These corrections were then applied to all of the expendable CTD profiles, and salinity and sigma-theta recalculated. The quality of the expendable probe data remained a problem because of the evident variability in both accuracy and qualitative behavior between different probes. To further refine the expendable data, it was hoped to tie individual probes other than the six used in the surface comparison to the CTD on the submarine's sail. Before this was attempted, a check was made for consistency between the surface CTD casts and the sail CTD. Although both are high quality instruments, this comparison was complicated by circumstances involving the operation of the submarine. Each surface cast, naturally, was made immediately following a sequence of maneuvers during which the submarine rose above it's 103 meter cruise depth, sought out a thin or open feature in the ice cover, and surfaced. There the submarine remained for typically 3-5 hours, part of which time was used for the CTD cast. Diving back to cruise depth, there then was a delay before the sail CTD returned to thermal equilibrium, although this effect was less than it would have been in winter or spring conditions. All of the maneuvering disturbed the smooth sampling of water the sail CTD could ordinarily perform while smoothly cruising along at uniform depth. This issue, like others involving the sail CTD, was further complicated by the fact that 103 meters depth was often near the region of steepest gradients, aggravating the effects of small errors. The question was which sail CTD samples to use for comparison with the 20 surface CTD casts. These were objectively chosen to be just before the submarine began maneuvering to surface by departing more than a few meters from its routine cruise depth. Several had to be examined by eye to ensure the selection of a truly undisturbed sample. The result was a correction applied to the sail CTD pressure of +2.9404 decibars. Left out were three of the surface casts for which the best choice of a sail CTD sample remained clearly unreasonable. Proceeding to try to tie the expendable probe profiles to the corrected sail CTD, it was necessary to repeat the exercise of choosing the best sail CTD points, this time for the expendable drops. Several were dropped shortly after a surfacing; others were disturbed by maneuvers visible as depth excursions in the sail depth record. The resulting comparisons showed the expendables retaining too great a variability to be usable. At this point, taking expendable profiles uncorrected by the surface casts and stretching the depth to match the sail CTD's salinity was tried. This proved a dead end, as the resulting profiles did not resemble adjacent surface casts. Returning to the surface-cast corrected expendables, eight of these were then thrown out for either being too far from their corresponding sail CTD sample for reasonable correction or for exhibiting excessive noise or improbable depth behavior. This left 26 expendable profiles, including the six in close proximity to a surface cast. Contour plots of temperature and salinity were then produced for several vertical slices across this region of the Arctic Ocean, notably including two long parallel transects between the Lomonosov Ridge and the Beaufort Sea just north of Alaska. Although the contours are usable, the quality is compromised by certain not believable features clearly introduced by individual expendables. Errors in temperature and conductivity compared with the sail CTD were then tallied. The six expendables used to produce the correction to the surface casts show mean errors of +0.015 Siemans/meter and +0.082¡C. Separate constant corrections were then applied to each of the other expendables to make their errors in temperature and conductivity equal to the mean error of the six. This was called correction alternative (A). The mean errors of all 26 expendables relative to the sail CTD were +0.009 Siemans/meter and +0.035¡C. Correction alternative (B) applied separate constant corrections to each expendable to make the errors in each equal to the mean errors from all. For each alternative, salinity and sigma-theta were recalculated and the long transect contour slices were redrawn. Both (A) and (B) seemed to somewhat over-correct expendable salinity, but also introduced unbelievable features in the temperature contours, especially near the bottom of the slices. Unfortunately, it seems likely that the sail CTD, by providing only a point sample within a vertical region of high vertical gradients and consequently magnifying small errors, is unable to substantially correct the expendable data. As a final attempt to improve the expendable profiles, it was noted that at the bottom of the surface casts the salinity and temperature varied quite slowly across the long contour slices. Deep averages of temperature and conductivity, for a range 450-500 decibars chosen for its general availability, were calculated for the surface casts in these slices, and interpolated values for these quantities were calculated for the spatial positions of the intervening expendables. Comparing these with the 450-500 decibar means from the expendables provided an offset that made the expendable profiles of temperature and conductivity look very reasonable, at the bottom. Applying these temperature and conductivity offsets to the expendables, recalculating salinity and sigma-theta, and redrawing the contours produced the cleanest contours so far. Although not every funny feature introduced by an expendable is eliminated, the lower regions, of course, are quite clean and several features in the expendable profiles within the 50-200 decibar region are made consistent with adjacent surface cast profiles. _______________________________________________________________________