---TITLE: UAF SBI Mooring Data ---AUTHOR: Thomas J. Weingartner ---FUNDING SOURCE AND GRANT NUMBER: ONR grant #N00014-02-1-0308 ---DATA SET OVERVIEW: Two year-long moorings were deployed on the Chukchi Sea shelf between 8/2002-9/2003 and 9/2003-9/2004. Each mooring was instrumented with an upward looking Acoustic Doppler current profiler and T/C recorders. Mooring name delimiters CC and BC denote Central Channel and Barrow Canyon. For the 2002-2003 deployments: CC1 was located at 70 40.39N,167 3.85W, 51m of water BC1 was located at 71 3.09N, 159 32.82W, 78m of water For 2003-2004 deployments: CC1 was located at 70 40.63N, 167 3.50W, 52 m of water BC1 was located at 71 3.12N, 159 32.54W, 79 m of water See attached folder with data plots Filenames associated with this dataset: BC_ADCP0487_0203_final.txt BC_ADCP3327_0304_final.txt BC_RCM7_11112_0203_final.txt BC_RCM7_9639_0304_final.txt BC_SBE16_2827_0304_final.txt BC_SBE16_2828_0203_final.txt BC_SBE37_0453_0203_final.txt BC_SBE37_1131_0304_final.txt CC_ADCP0622_0203_final.txt CC_ADCP2234_0304_final.txt CC_RCM7_11120_0304_final.txt CC_RCM7_9513_0203_final.txt CC_SBE16_1717_0203_final.txt CC_SBE37_0450_0304_final.txt ---DATA COLLECTION and PROCESSING: Temperature/Conductivity Calibration procedures: SBE and RCM file variables: Press ... water pressure (decibars) Temp ... water temperature (degrees C) Sal ... water salinity (PSU) Sig-t ... water density (kg per m^3 - 1000) DIR ... water current direction (degrees T) MAG ... water speed (cm/s) U ... east-west component of water current vector (cm/s) V ... north-south component of water current vector (cm/s) V0 ... External voltage channel 0 (V) V1 ... External voltage channel 1 (V) V2 ... External voltage channel 2 (V) V3 ... External voltage channel 3 (V) Seabird (SBE) instruments were calibrated using linear interpolation between pre- and post- deployment factory-derived calibration coefficients. Aanderaa (RCM) instruments only had pre-deployment calibration coefficients applied. All records were visually inspected and de-spiked by hand to remove obviously bad data points. Significant non-physical offsets remained after the above initial procedures were implemented. Apparent density inversions between instruments separated by less than 15m in the vertical direction were typical manifestations of these errors. In addition, records during winter months also displayed (often prolonged) instances of water below the freezing point. Three possibilities exist to explain data below the freezing point: 1. The water was supercooled. Although supercooling probably occurs at times, we are unable to assess its frequency with respect to water actually at the freezing point. We thus make the assumption that all water was at the freezing point and at the surface when it acquired its T/S signature. We further assume that the newly created water mass is colder and saltier than the ambient water. Since temperature diffuses faster than salt, in the absense of mixing we would expect that this water mass would tend to warm up faster than it would freshen. In any event, both mixing and diffusion processes will have the effect of moving freezing point water parcels away from being supercooled. So, while recognizing that supercooling probably is a factor at some times, we assume that measurements which depict water below the freezing point is due to actual instrumental error. 2. The temperature probe was in error. We use the SeaBird temperature probes to correct for offsets in the closest RCM temperature probe. Error in the Seabird temperature probes is less than 0.005 deg C, resulting in a maximum salinity error of about 0.1 due to temperature probe error at the freezing point. Error of Aanderaa temperature probes is ~ 0.01, resulting in a possible salinity error 0.2. We have no way of assessing temperature probe error other than comparing two nearby temperature probes. 3. The conductivity probe was in error. Typically, conductivity sensors that fail due to fouling are affected by displaying erroneously low salinity readings. In the euphotic zone, one might expect that instruments higher in the water column would exhibit more biofouling, particularly during summer months. Near the bottom, one might expect that instruments closer to the bottom are more prone to biofouling due to BBL resuspension of seafloor material. To correct for conductivity cell error, we located the data that reside below the freezing point and computed the salinity deficit (the salinity value required to move those water parcels back to the freezing point). Assuming the temperature probe is reading correctly, we encountered water up to 0.5 psu below the freezing point; typical salinity deficits were less than 0.2. Histograms of the salinity deficit from each winter month were plotted for each T/C probe set. Allowing for some noise factor in the measurements, we determine the salinity offset required to move 95% of the data back to or above the freezing point. Subjective evaluation of the offsets thus determined were then made, to either apply or not apply the offset. Inspection of shipboard CTD casts in conjunction with the mooring data aided this evaluation. Application of these offsets acted to remove or reduce the noted density inversions, which gives us some faith in our method. Of the 6 SeaBird instruments, we applied 5 offsets, ranging from 0.05 to 0.36. Of the 4 Aanderaa instruments, we applied 3 offsets, ranging from 0.1 to 0.2. We believe that the offsets applied are correct to within 0.1. Thus, combining salinity uncertainty associated with temperature probes, we feel that the final salinities are known to better than 0.2 and 0.3 for the SeaBird and Aanderaa instruments, respectively. Some persistent salinity inversions of up to about 0.1 remain after all of this, but it is not clear if the error comes from ione or both of the instruments. Coherence between T and S fluctuations from all instruments on a single mooring (SeaBird, Aanderaa and RDI) is extremely high. No quality control procedures were implemented upon the external voltage channel records. ADCP Processing details and description of variables: ADCP file variables: U ... east-west component of water current vector (cm/s) V ... north-south component of water current vector (cm/s) BinDepths ... nominal depths below the sea surface of each bin level (m) BTDepth ... Bottom Track distance from transducer head (m) Temperature = temperature measured by the ADCP, corrected via nearby Seabird T/C recorder (degrees C) Data quality of most of these instruments appears generaally good but problems with the Barrow Canyon 2002-2003 instrument persist and is described below. Aanderaa mechanical current meters were deployed about 10 meters below each ADCP instrument and serve as good cross-reference points. Coherence in the vertical direction between the ADCP and RCM instruments is very high. The Barrow Canyon deployments employed SUBS floatation housings... "torpedo" shaped floats that, like a vane, were free to swivel and follow the current. The SUBS did not ride trim in the water... there were often about 10 degrees of tilt. In itself, this should not create a problem for the data. The RDI firmware compensates for travel time by using the pitch/roll data so that even though the data from opposing beams were sampling different areas, they are reporting data from the same depth level. The major instrumental problem occured in the 2002-2003 Barrow Canyon deployment. It appears that there exists a compass error that is not a linear function of the direction the current was moving. In particular, the down-canyon flow appears to be oriented slightly too far to the east and the upcanyon flow is oriented much too far to the south. We have discussed this at length with the RDI personnel. At the moment there appear to be two leading possibilities for the compass error. 1) the compass calibration performed in Seward prior to deployment was not in the same magnetic environment during operation. This may have been caused by a battery pack changing orientation, etc. If this was the problem, we will probably never know for sure. 2) There could be a problem with the compass in ADCP # 0487. We will explore this possibility further by making tests with the instrument. It appears that the velociy magnitude data is good from 0487, so we have taken the following measures to make the dataset usable: Other Barrow Canyon deployments show that ~98% of the variance is aligned with the principal axis of variation, the along-canyon direction. In the 2002-2003 deployment only ~92% of the variance is along the principal axis of variation. We assume that the eastward heading flow is in fact moving down-canyon (this is supported by the nearby RCM instrument). The direction sign is then applied to the magnitude time series. This has the effect of placing 100% of the variance in the along-canyon direction. The following screening techniques, parameters and corrections were applied to the ADCP data: Error Amplitude limit = 5 cm/s Correlation limit = 64 Percent-good of 4-beam solution limit = 30% Bins within 6% of the distance of the reflecting surface are blanked. Velocities and distances (bin cell distances and bottom track reflections) were corrected by the speed of sound using salinity = 33 and temperature as recorded by the ADCP. Temperature was corrected by comparison to nearby SeaBird temperature probes. Bottom track distances were corrected for pitch and roll of the ADCP. In the case of the skewed Barrow Canyon SUBS orientation, this improved the estimates significantly. Error screening of the bottom track data was performed by using low-pass filtered time series, checking for double reflections and other out-of-expected range readings. Bin depths were referenced to the mean distance to reflection based on the first two months of data collection (we thus avoid errors due to ice growth later in the deployment). Velocities were corrected for the local magnetic deviation based on the mid-point of the deployment. Barrow Canyon 2002-2003 = 19.75 degrees Barrow Canyon 2002-2003 = 19.42 degrees Central Channel 2003-2004 = 14.73 degrees Central Channel 2003-2004 = 14.48 degrees