Sang-Ho Lee*, Hong-Bae Moon, Chang-Soo Kim Dept. Oceanography, BK21 Team, Kunsan Nat’l Univ., Korea Accuracy of currents measured by HF radar in the coastal sea off the Keum River estuary (South Korea) ROW8, Apr. 27-May 2, 08, Hawaii

Outline Outline 1.Study area and Purpose 2.Data 3.Comparisons of current measured by HF-radar and current meter 4.Discussions and Summary 1

DMZ CHINA 1. Study area and Objectives Topography in meter

 Natural conditions Complex coast line with islands Macro-tidal environment : 6 m in spring tide, U > 50 cm/s Shallow water with small bottom slope: < 40 m/50 km => Broad tidal flat. Runoff : ~7 x 10 9 m 3 /y Keum River (78%) + Mankyong R. (13%) + Dongjin R. (9%) Asian monsoon: strong northerly in winter weak southerly in summer 1987

 Coastal development (40,100ha) since 1992 : tide dyke 33 km long for reclamation of estuary mouth area => changes tidal current, river plume & circulation Saemangeum Tide dyke Gogunsan Island chain KNU

2. Data HF radar: Site 1 & 2 ■ : Mid-point of baseline - Mooring stations ★ : winter: N1, K1, K2, K3 ● : summer: M1, M2 -CTD surveys - Winds: Mal-do, AWS - Runoff: Keum River weir (

HF Radar: 25 MHz, dR= 1.5 km, d 5 deg; Ideal Antenna Pattern Facing radar radials are compared on the mid-point of baseline because the southern part of baseline is partly blocked by land and a seawall. CTD survey : Nov , Dec , 2006 Jun , Jul , Aug , 2007 To mid-point

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Data processing Data processing Extract hourly current from mooring data. Regressions by PCA (Yoshioka et al. 2006) Principal component analysis is used to obtain the regression line and root-mean-square (RMS) deviation of comparison. Radial velocity comparison : Current measured by mooring was projected into radar radial direction. (U,V) comparison : Radar-derived current at the mooring station is obtained by the interpolation from four near-grid point data using inverse distance weight. Tidal current ellipse and sub-tidal current: Harmonic Analysis and low-pass filter

3. Current Comparison 3.1 Facing radial velocities at mid-point of baseline Solid (dashed) line of regression is obtained by PCA (ordinary analysis) RMS deviation in winter (4.4 cm/s) is smaller than that in summer (5.4cm/s)

3.2 Radial velocities at mooring stations Scatter plots of the current measured by RCM and RDCP moorings. winter summer asymmetry rotary

N1 K2 Site 1Site 2 HF radar radial velocity Blue line By PCA

K1 K3 HF radar radial velocity Site 1Site 2

M1 M2 Site 1Site 2

RMS tends to increase with distance from radar to mooring sites. Large RMS in summer stations may be mainly produced by a vertical shear in stratified water column. => We need a check for this, especially M2 station. K3 near islands looks to be no good station for current comparison. Why RMS deviation in K1 is large for radials from Site 1 despite of the shortest distance?

3.3 Current component comparison N1 K1 K2 K3 M1 M2

 RMS deviation in current component comparison tends also to increase with distance from mid-point of baseline to mooring stations.  RMS deviations in K1 larger than that in K2 are produced by large RMS in the comparison of the radial velocity from site 1.  Radial velocities at M2 station have small intersection angle and are not adequate to resolve V component.

Apportionment of RMS deviations using GDOP  Errors of current comp. by radar (  h ) are less than 3.6 cm/s in N1, K1 & K2.  GDOP is much large in the V direction at M2. => V component may be poorly resolved. => V component may be poorly resolved.  For M1 station, (3) produces imaginary error (σ h ) when we input slightly different α, θ value. Close values in σ a and GDOP component made this result. We can not believe the values of σ h and σ p obtained in M1. => We can not believe the values of σ h and σ p obtained in M1. (Chapman et al., 1997)

3.4 Tidal current comparison : M 2 and O 1 Tidal current ellipses: good agreement except for K3 station. In M2 station,=> direction and strength of major axis of ellipses are largely different between surface and 2 m depth because of improperly resolved V component.

3.5 Sub-tidal current comparison Radar-derived surface currents Comparison of current variation between surface and 2 m depth at N1 station Surface currents response well to strong wind forcing.

Opposite directions Main source of large RMS deviation Is this real? CTD survey period Subtidal surface current is stronger than that in winter despite of weak wind.

Geostrophic current  Observed current was very close to geostrophic current below 3 m depth.  U w = U r -U g =>Upper layer current difference in section A can be explained by the wind-drift current in stratified water. =>Upper layer current difference in section A can be explained by the wind-drift current in stratified water. (Jul. 27, 08) Wind Solid line : HF + Mooring current Dashed line: Geostrophic current relative to bottom

4. Discussions and summary  RMS deviation of 4.4 cm/s in facing radar radial velocities in a distance of 10.5 km is smaller than typical error of 7~8 cm/s suggested by CODAR.  Local shear effects on the flow around island will be a main source producing large RMS deviation in K3.  RMS deviation for K1 at 10.4km was larger than those for K2 at 20km. We suspected Antenna pattern of radar site 1 and need pattern measurement.  RMS deviations showed a tendency to increase with distance from Site 2. 1) increase of radar-cell size => increase of horizontal shear (Lipa, B., 2003; Lipa et al., 2006; Ohlmann et al., 2007) 2) decrease of S/N ratio as the distance increases (Lipa et al., 2006).  From apportionment of RMS deviation, accuracy of radar-derived current becomes less than 5 cm/s except for K3.  Radar-derived current resolved well tidal and sub-tidal wind drift current. 4.1 Comparisons in Winter

4.2 Comparisons in Summer  RMS deviation of 5.4 cm/s in facing radar radial velocities is slightly increased compared with that in winter.  Larger RMS deviations from comparison of radar-derived and moored currents might be due to large radar ell by long distance and vertical shear with stratification.  In spite of large RMS deviations in current comparison for M1 station, tidal and subtidal current looks to be well resolved by radar. We can explain current discrepancy in upper layer in terms of superposition of wind-drift and geostrophic currents. Why subtidal surface currents are stronger compared with that in winter?  Why subtidal surface currents are stronger compared with that in winter?  Apportionment of deviation using GDOP can produce imaginary deviations. deviations. Surface Ekman current : Ekman depth De becomes shallow with stratification, and then Us increases.

<Summary> 1)When facing radar's radial vectors at the mid-point of baseline are compared, RMS deviation is less than 5.4 cm/s. 2)When HF radar-derived currents are compared with the currents measured by moorings, RMS deviations increase with distance from radar site, near the islands. 3)After apportionment of RMS deviations using GDOP, the accuracy of current vector becomes less than 5.1 cm/s in winter season except for K3 station. 4)We found that separation of RMS deviations using GDOP value can produce an uncertain accuracy of HF radar-derived current. 5)We can examine tidal current characteristics and variation of subtidal current using radar-derived data in our study area.

Thanks ! A gate in the SEAMANGEUM tide dyke