Current Meter Data Report Ulleung Basin of Japan/East Sea June 1999 to July 2001 by Yongsheng Xu Karen L. Tracey D.Randolph Watts Mark Wimbush William Teague Jeff Book GSO Technical Report 2003-02 GRADUATE SCHOOL OF OCEANOGRAPHY UNIVERSITY OF RHODE ISLAND NARRAGANSETT, RHODE ISLAND July 2003 ABSTRACT Observations were conducted from June-1999 to July-2001 to study shallow and deep current variability in the southwest Japan/East sea. Data were collected during the field experiment with a two-dimensional array of pressure-gauge equipped inverted echo sounders (PIES) and deep recording current meters (RCM). This report documents the current meter data, which were collected with an array of 12 moorings that was in place for the two-year period. Instrument preparation, calibration and deployment/ recovery procedures, plus data processing procedures are discussed. Basic statistics of the cleaned hourly data and forty-hour low passed filtered time series are presented, followed by time series of the currents and temperatures measured by the Aanderaa current meters. The final section shows plots of mean current vectors and variance ellipses as well as histograms of current speed and direction. Setting and Design of the Experiment Introduction This report documents current and temperature data collected from recording current meters (RCM) as part of field studies conducted in the Japan/East Sea under the sponsorship of Office of Naval Research(ONR). These RCMs were deployed during cruise HAHNARO-06 aboard R/V Melville (June 6 to June 16, 1999). The recovery cruise was Cook Leg 09 aboard R/V Melville (June 21 to July 4). This experiment was conducted to study shallow and deep current variability in the Southwestern Japan/East Sea between Korea and Japan. This is a joint program between University of Rhode Island (D.R. Watts and M. Wimbush) and the Naval Research Lab (W.S. Teague). Other data collected as part of this program include those from pressure-gauge-equipped inverted echo sounders (PIES), 4 current meter moorings deployed by Korean Ocean Research and Development Institute (KORDI, Dr. M-S. Suk) and an additional current meter mooring installed by the Research Institute for Applied Mechanics at Kyushu University (RIAM, Dr, J-H. Yoon). Detailed documentation about the KORDI and RIAM RCMs will be in separate reports. The principal objectives of the study were as follows: 1. Observe the time-varying transports of the branches of the Tsushima Current in the Ulleung Basin. 2. Produce daily maps, from observations in the Ulleung Basin, of the upper-layer circulation and path-variability of the Offshore Branch and East Korean Warm Current, with mesoscale resolution. 3. Understand the physical coupling between the shallow and deep currents and eddies within this region, where large-amplitude meanders and steep loop formations occur. 4. Quantify cross-frontal and vertical fluxes associated with mesoscale processes in the East Korean Warm Current. The current data from this array of current meters will be used to level the PIES pressure measurements. The combined instruments (23 PIESs and 17 RCMs) provide two-year time series of dynamic height, vertical shear, and deep current fields, enabling us to map the upper and deep absolute current and temperature structure on a daily basis. The Moored Instrument Array To address the experiment's objective, an array of PIESs and current meter moorings was deployed in the study area shown in Figure 1. The array, moored within the Ulleung Basin of the Japan/East Sea, consisted of 25 PIESs arranged in a roughly 5 by 5 array with 55-60 km spacing between sites. Deep RCMs were placed nominally midway between the PIES sites. The moorings maintained by scientists at KORDI and RIAM are also shown. Position, depth, instrument information, launch time and release time for the URI current meter moorings are given in Table 1. Of the 13 RCMs deployed by URI all were recovered except the one at site M3-4. Each mooring supported one current meter, an Aanderaa model RCM8. The instruments were positioned about 24 meters above the anchor. The mooring design is shown schematically in Figure 2. Table 1. Site Information Site SN Lat Lon Depth(m) Launch Recovery (N) (E) Bot Ins Date UT Date UT M1-1 7077 37 43.47 129 37.04 1220 1197 6/10/99 0251 6/29/01 0417 M1-2 9296 37 45.00 129 57.00 1662 1639 6/10/99 0713 7/1/01 0130 M1-3 9266 37 39.86 130 24.00 1570 1547 6/11/99 0640 7/1/01 2241 M1-4 9687 37 40.00 131 27.00 2381 2358 6/13/99 0055 6/23/01 0221 M1-5 9324 37 43.44 132 11.90 2448 2425 6/13/99 0450 6/22/01 2200 M2-1 9268 37 28.20 129 57.62 1557 1534 6/12/99 0605 7/1/01 0354 M2-2 7357 37 14.94 130 39.92 2227 2204 6/12/99 1008 6/30/01 0318 M3-1 7356 36 51.65 130 3.04 2211 2188 6/12/99 0133 7/02/01 1723 M3-3 9591 36 49.00 131 25.00 2051 2028 6/9/99 2345 6/24/01 2346 M3-4 12553 36 49.00 132 0.00 1825 1802 6/8/99 2022 Lost Lost M4-1 9325 36 7.70 130 8.48 1550 1527 6/7/99 0859 6/27/01 1822 M4-3s 7355 36 7.00 131 30.02 1546 1523 6/8/99 0106 6/26/01 0948 M4-3n 9685 36 16.92 131 29.98 1797 1774 6/14/99 0852 6/26/01 0800 Instrument Calibration Introduction The Aanderaa RCM8s in this experiment measured the temperature (T), direction (D), and speed (S) of the current. None included optional sensors for pressure or conductivity. Each measurement in an RCM8 is recorded as an 8-bit number N from 0 to 1023. The desired variable is found from a polynomial in N: X=A_{3}N^{3}+A_{2}N^{2}+A_{1}N+A_{0} where X is either T or D. The set of coefficients A_{n} are different for T and D and are unique to each instrument. Coefficients for T and D were individually calibrated for each instrument at the URI Technical Services facility by cycling them through a suite of accurate T and D measurements, recording the corresponding suite of N values, and fitting a cubic polynomial to the measurements. The coefficients for S are those recommended by Aanderaa for the response of their paddle-wheel rotor: S=A_{1}N+A_{0} where A_{0}= 1.0 and A_{1} = 0.2688 to obtain speed in cm/s for all instruments. A_{0} and A_{1} values are updated here based on June 25, 2001 communication to M. Wimbush from R. Butler of Aanderaa regarding results of tow-tank calibration at IFREMER on 20 RCMs with this paddle-wheel rotor design. Temperature Calibration Temperature calibration was performed on a group of five instruments at a time in a circulating-water controlled-temperature bath. Independent reference temperatures were measured by a SeaBird sensor (Model SEB3, with accuracy 0.0003 deg C). The bath was cycled through about a dozen temperatures from approximately 16 degC to below 1degC. The cubic polynomial was fitted to the values, and the coefficients for each RCM8 are listed in Table 2. Direction Calibration Compass direction calibration occurred on a rotating platform that is keyed to orient the RCM8 at a sequence of stops at 15 or 30 degree intervals through the full range (0 to 360 degrees) of compass headings. The exact reference magnetic headings at each stop were independently measured by a Manufacturer KVH Model C100 fluxgate compass (accuracy +/- 0.5 degree). The RCM8 was swung in both clockwise and counterclockwise directions to minimize effects of hysteresis in its compass readings. A cubic polynomial was fitted to the set of {N_{i}, D_{i}} data pairs, and the coefficients are listed for each RCM8 in Table 2. Table 2. Calibration Coefficients for T in degrees C, D in degrees (from magnetic north) towards which the current is flowing, S in cm/s. M1-4 had battery failure and is not included in the table. Site Variable A _{3} A_{2} A_{1} A_{0} M1-1 T 2.8368e-09 -2.5025e-06 2.3219e-02 -2.5423e+00 7077 D -2.0650e-08 3.1006e-05 3.4252e-01 -1.3799e+00 S --------- --------- 0.2688 1 M1-2 T 2.6405e-09 -2.3759e-06 2.3164e-02 -2.4951e+00 9269 D 1.0177e-08 -7.7594e-07 3.4231e-01 3.7483e-01 S --------- --------- 0.2688 1 M1-3 T 3.2911e-09 -3.0294e-06 2.3317e-02 -2.4908e+00 9266 D 3.4822e-09 -1.6619e-05 3.6423e-01 -4.1599e-01 S --------- --------- 0.2688 1 M1-5 T 2.2788e-09 -1.8379e-06 2.2927e-02 -2.4436e+00 9324 D 9.7878e-09 -9.2718e-06 3.5132e-01 1.5474e+00 S --------- --------- 0.2688 1 M2-1 T 3.1927e-09 -3.2264e-06 2.3505e-02 -2.5156e+00 9268 D 3.5636e-08 -3.9932e-05 3.5768e-01 3.3686e+00 S --------- --------- 0.2688 1 M2-2 T 3.2391e-09 -3.1641e-06 2.3456e-02 -2.5843e+00 7357 D -2.3028e-08 4.8663e-05 3.2665e-01 -1.1212e+00 S --------- --------- 0.2688 1 M3-1 T 2.9427e-09 -2.4920e-6 2.3134e-02 -2.5477e+00 7356 D -7.3359e-08 1.2991e-04 2.9023e-01 8.5507e-00 S --------- --------- 0.2688 1 M3-3 T 4.2251e-09 -4.7244e-06 2.4129e-02 -2.6797e+00 9591 D -1.0282e-08 3.8519e-05 3.2212e-01 1.8059e+00 S --------- --------- 0.2688 1 M4-1 T 1.9627e-09 -4.5327e-07 2.1929e-02 -2.3170e+00 9325 D -3.0166e-09 2.3360e-05 3.3077e-01 1.5145e+00 S --------- --------- 0.2688 1 M4-3n D 2.6364e-09 -2.4688e-06 2.3272e-02 -2.6138e+00 7355 D -1.0053e-07 1.9707e-04 2.4704e-01 1.1502e+01 S --------- --------- 0.2688 1 M4-3s T 5.1463e-09 -5.6318e-06 2.4309e-02 -2.6929e+00 9685 D 2.3894e-08 -2.6507e-05 3.5749e-01 -5.8127e-01 S --------- --------- 0.2688 1 Data Processing Raw Data Transcription We followed standard procedures for downloading the Data Storage Units (DSUs) using the Aanderaa software and DSU reader (Data reading program 5059. Version 1.00-Built81). Clock Drift Correction Prior to deployment, the current meters were set up to sample once per hour. However, clock drifts of up to 40 minutes occurred during the two year deployment. These were spread uniformly through the entire record at each current meter by adjusting the sampling interval. Table 3 shows the drifts and new sampling intervals. Table 3. Clock Drift Information Site Drift(min) Drift(min) Delta Time(hr) UTC Begin UTC End DSU End Sampling DSU Date & Time Date & Time Date & Time M1-1 -18 -9 1.00001655263745 6/5/99 5:23 6/29/01 9:41 6/29/01 9:32 M1-2 -22 -27 1.00002016979306 6/5/99 1:16 7/1/01 12:38 7/1/01 12:11 M1-3 -20 -12 1.00001832609443 6/5/99 4:09 7/2/01 1:29 7/2/01 1:17 M1-5 -24 -10 1.00002225436749 6/5/99 1:46 6/23/01 0:10 6/23/01 0:00 M2-1 -21 -27 1.00001926358079 6/5/99 5:05 7/1/01 6:26 7/1/01 5:59 M2-2 -24 -21 1.00002204464041 6/5/99 4:47 6/30/01 6:11 6/30/01 5:50 M3-1 +38 -24 0.99996523012175 6/5/99 4:23 7/3/01 2:45 7/3/01 2:21 M3-3 -18 -9 1.00001664632127 6/5/99 3:55 6/25/01 2:13 6/25/01 2:04 M4-1 -22 -9 1.00002026566439 6/5/99 3:07 6/28/01 0:29 6/28/01 0:20 M4-3n -22 -21 1.00002030494331 6/5/99 1:58 6/26/01 12:20 6/26/01 11:59 M4-3s -22 -7 1.00002030719256 6/5/99 2:48 6/26/01 11:10 6/26/01 11:03 Magnetic Correction Current speeds are measured by rotor revolutions and directions by vane orientation referenced to an internal compass magnetic north. Hence, the magnetic variation is added to the measured values to convert to true direction. Magnetic variations were computed at each mooring site using the worksheet provided by the US National Geophysical Data Center at website http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth2.pl. For the JES array, the magnetic variation ranged from -8.1 to -7.5 degrees. Table 4 gives the magnetic variation for each current meter calculated for July 1, 2000, approximately the middle of the deployment period. Values near the beginning and end of the deployment period and their difference are given for the four corner RCM sites, showing that the magnetic variation changes during the deployment by only 1/20 degree. Table 4. Magnetic variation in degrees. Values near the beginning and end of the deployment period are also given for the four corner RCMs. Site 7/1/99 7/1/01 Difference 7/1/00 M1-1 -7.917 -7.967 0.05 -7.95 M1-2 -7.983 M1-3 -7.983 M1-5 -8.067 -8.117 0.05 -8.083 M2-1 -7.883 M2-2 -7.867 M3-1 -7.7 M3-3 -7.75 M4-1 -7.433 -7.483 0. -7.467 M4-3S -7.500 -7.550 0.05 -7.533 M4-3N -7.583 Table 5. Flagged records and interpolation information. Site Percent record flagged Percent flagged record that was interpolated M1-1 23 38 M1-2 23 21 M1-3 68 5 M1-5 41 14 M2-1 44 12 M2-2 23 28 M3-1 29 34 M3-3 51 9 M4-1 30 44 M4-3S 22 49 M4-3N 23 44 Removal of Tides Speed and direction time series were converted to U and V time series, where the U velocity component is positive towards East and the V component is positive towards North. The eight main tidal constituents (M2, S2, N2, K2, K1, O1, Q1, and P1) were removed in the first step of processing the data. The harmonic analysis technique was used to determine the constituent amplitudes and phases by least squares fitting sinusoids for the U and V data. This procedure does not require equally spaced data and thus skips over the stall periods. The tide constituents, M2, S2, N2, K1, O1, and Q1, are well resolved with a minimum of one month of hourly data. The tide constituents, K2 and P1, require about six months of data. Stall and Bad Data Treatment Minimum resolvable speed was 1.4 cm/s and those speeds not exceeding this value were treated as stalls. All records identified as stalls were flagged. Additionally, a few of the records contained bad data even though the speeds exceeded the minimum resolvable speed. These data records were also flagged. After removing the tidal signals from both components of the current, splines were fitted to the U and V components individually, specifying knots at all non-flagged data values. Flagged data which extended for periods no longer than five consecutive samples (i.e., 5 hours) were replaced with the spline-fitted values. U and V were set to zero whenever more than five flagged records occurred in a row. Table 5 lists the percentage of records that were flagged and the percentage of flagged records that were interpolated. Almost all flagged records were due to stalls. Figure 3 shows the mean average velocity computed with the stalls spline-fitted and with the stalls excised. Lowpass Filtering After the stalls were interpolated, the velocity components were lowpass filtered using two different schemes. The current data were lowpass filtered using a 40-hour cutoff period in order to remove any shorter-period signals including tidal signals still remaining in the data. The data were also lowpass filtered using a 5-day (120 hour) cutoff to remove nongeostrophic signals. The 40-hour lowpassed records are superimposed on the unfiltered data in Figure 5 to Figure 15. The 120-hour lowpassed records are shown in Figure 16 to Figure 26. 40 hour lowpassed records (40~HRLP) Prior to filtering, the data were first interpolated to exactly 1 hour time intervals from the first point in each record. The standard library function, DIGITAL_FILTER (Robert, 1984), was used to create the convolution kernel for the 40-hour low-pass filter. The Gibbs parameter was set to a value of 50, a good choice for most filters. The Gibbs Phenomenon variations are oscillations which result from abrupt truncation of the infinite FFT series. Time bases of the filtered time series are identical to those of the unfiltered, hourly records. 120 hour lowpassed records (120~HRLP) The data were lowpass filtered using a 4th order Butterworth filter with a cutoff period of 120 hours using MATLAB [Krauss et al., 1992]. For this filtering routine, it was not necessary to interpolate the data to 1-hour intervals prior to applying the filter. The filter was passed forward and backward in time to avoid introducing phase shifts. Eighteen hours of data at each end of the filtered series were discarded to avoid startup transients. After filtering, the time series were subsampled at 12 hour intervals, centered on 0000 and 1200 UTC. Basic Statistics Data Recovery Twelve of the thirteen current meters were recovered after the two-year duration of the moored array program. M3-4 was lost because the anchor would not release on recovery. M1-4 was recovered but the battery failed a few hours after deployment due to a manufacturing fault. Nevertheless the data recovery was adequate to meet our objectives of mapping the current and eddy fields. The time line for the data returns is shown in Figure 4. First-Order Statistics The first order statistics of velocity are presented for the recovered instruments. Basic statistics for unfiltered hourly data, 120~HRLP, and 40~HRLP filtered time series data are presented separately in Table 6, Table 7, and Tables 10-25. Table 6 has the basic statistics of hourly data for both stalls spline-fitted (U,V) and stalls excised (U',V'). M3-4 and M1-4 are not included in the Table. The data set from M1-3 is divided in two parts because of the long period of bad data excised from the middle of data set. Acknowledgements We gratefully acknowledge the efforts of the captains and crews of the R/V Revelle and R/V Melville, which were essential to the success of launch and recovery of the instruments. Tom Orvosh was responsible for preparing instruments and dumping the initial data. Andrew Hollis and Jeff Book helped with the data processing and resolving the clock drifts. We appreciate receiving the RCM data supplied by Moon-Sik Suk (KORDI), K-I. Chang (KORDI) and J-H. Yoon (RIAM), which made our data report more inclusive. The work of D.R. Watts and M. Wimbush was supported by the Office of Naval Research Japan/East Sea DRI through grant N00014-98-10246, and the work of W.J. Teague supported by the Office of Naval Research as part of the Basic Research Projects Linkages of Asian Marginal Seas and Japan/East Sea DRI under Program Element 0601153N. References Krauss, T.P, Shure, L and Little, J. N, 1992, Signal Processing Toolbox: for use with MATLAB. The Math Works Inc. Robert W, 1984, Digital Filters, in Proceedings of the Digital Equipment User's Society, Department of Applied Science, University of California, Davis, CA 95616.