Comparison of Observed (HF Radar and ADCP Measurements) and Computed Tides in the North Channel of the Irish Sea
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1764 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 Comparison of Observed (HF Radar and ADCP Measurements) and Computed Tides in the North Channel of the Irish Sea ALAN M. DAVIES,PHILIP HALL,M.JOHN HOWARTH,PHILIP J. KNIGHT, AND ROSE J. PLAYER Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside, United Kingdom (Manuscript received 8 March 1999, in ®nal form 12 September 2000) ABSTRACT A three-dimensional high-resolution (grid of order 1 km) model of the North Channel of the Irish Sea, incorporating a one equation turbulence energy submodel to parameterize vertical mixing, is used to compute the M2, S2, N2, K1, and O1 tides. Elevations and currents are compared with observations, with particular emphasis on a detailed comparison with current pro®les recorded by two acoustic Doppler current pro®lers (ADCPs) and HF radar measurements of the surface current. The comparison with the HF radar shows small-scale spatial variations in both modeled and observed currents superimposed upon the larger scale tidal currents. These small-scale changes appear to be associated with variations in bottom topography and can only be resolved by using ®ne-grid models or detailed measurements. Computed M2 current pro®les derived from a multiconstituent calculation are in excellent agreement with pro®les measured by the ADCPs. However, in a single constituent M2 calculation, the magnitude of the current is overpredicted. Increasing the bottom friction coef®cient to compensate for the absence of other constituents improves the accuracy of the bottom current, although the thickness of the turbulent boundary layer is under- predicted with a resulting overprediction of the surface current that cannot be corrected for by changing the bottom friction coef®cient. 1. Introduction surements exist in the region, although they are outside The North Channel connects the Irish Sea to the Heb- the area of the HF radar, and they have been used to rides Malin Shelf off the west coast of Scotland and is validate the model. an area of signi®cant exchange between the two regions The HF radar system deployed here is the Ocean Sur- during major storm events. Water depths in the area are face Current Radar (OSCR) details of which are given typically of the order of 100 m increasing to over 250 in Prandle et al. (1993) and Prandle (1991). Surface m in a deep area between Portpatrick and Orlock Point current vectors can be measured to a theoretical accu- racy of about 4 cm s21, and bins of the order of 1.2- (Fig. 1). Tidal currents are strong, with M2 velocities of order 100 cm s21, and hence the area remains well km radial length. The range of the system is such that mixed. The variation in water depth gives rise to a sig- it gives an adequate coverage over the North Channel, ni®cant spatial variability in the magnitude of the tidal with a resolution comparable with the 1-km model grid. currents. To examine the variability of the tidal current The OSCR system was deployed for 418 days from involves a high-resolution three-dimensional (3D) nu- Portpatrick (the master site) and Crammag Head (the merical model (described here) and a detailed set of slave site) (Fig. 2). A 150-KHz broadband ADCP and tidal measurements, made using an HF radar system a pressure recorder were deployed in a seabed frame in deployed for 14 months with the location of the master the center of the channel (at 548469N, 58259W, water and slave as shown in Fig. 2. By this means the radar depth 142 m) (position A2) from 13 July 1993 to 28 could cover the full width of the North Channel at ap- October 1994. Currents were recorded in the vertical in proximately 548489N. An acoustic Doppler current pro- 23 5-m bins, from 12.5 m to 122.5 m above the seabed. ®ler (ADCP) was deployed to the north of the region The nominal accuracy of the current measurements ob- covered by the HF radar for 1 month (position A1 in tained from the ADCP was 0.75 cm s21 (Howarth et al. Fig. 2), and a second for 15 months in the center of the 1995). A 150-KHz narrowband ADCP was deployed channel (position A2 in Fig. 2). Other tidal current mea- for 1 month (20 September±31 October 1993) at A1 (558009N, 058309W in 139 m of water) currents were recorded in thirteen 8-m bins from 14 to 110 m above Corresponding author address: Dr. Alan M. Davies, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead L43 the bed. 7RA, United Kingdom. The three-dimensional model is based on Davies and E-mail: [email protected] Hall (1998, hereafter referred to as DH), which was used q 2001 American Meteorological Society Unauthenticated | Downloaded 09/29/21 09:10 AM UTC J ULY 2001 DAVIES ET AL. Unauthenticated |Downloaded 09/29/21 09:10 AMUTC FIG. 1. Bottom topography of the region and place names. FIG. 2. Location of measurement sites, with x denoting current, and 1 tide gauge measurements. Also shown is the location of the HF radar Master and Slave, and the cross section X±Y. ADCPs are denoted A1 and A2. 1765 1766 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 FIG. 3. Finite difference grid of the model. to compute the M 2 and M4 tides in the region, extended The objective of this paper is to use the HF radar to include the S 2, N 2, K1, and O1 tides and for com- measurements together with the detailed current pro®le parison with the HF radar and ADCP measurements. derived from the bottom-mounted ADCPs and any other Although these tidal constituents have been examined current measurements in the region to determine the over the whole shelf (Davies et al. 1997), the grid res- accuracy of the three-dimensional tidal ¯ow ®eld de- olution of that model (of order 12 km) was insuf®cient rived from the model, and to use the model and mea- to resolve the North Channel. Recent tidal calculations surements to examine the detailed spatial variability of in the eastern Irish Sea (Jones and Davies 1996) have the tidal currents in the area. To date, there have been shown that a 1-km grid is required to examine spatial no detailed comparisons of high-resolution three-di- variations in tidal currents (although elevations show mensional model computations against HF radar mea- signi®cantly less variability), suggesting that a model surements and ADCP current pro®les. These compari- with this resolution is required in the North Channel. sons show signi®cant spatial variability related to chang- Although such a model, due to limitations in computing, es in bottom topography and the limitation of coarse will be restricted in its geographical extent (Fig. 3), the grid models and comparison with point measurements. open boundaries are signi®cantly well removed from Subsequently calculations are performed with only the the area covered by the HF radar. Also, the form of the M2 tidal input but increased bottom friction to try to open boundary condition is such that the tidal current compensate for the frictional effects of other constitu- is uniform from sea surface to seabed and hence current ents. These calculations show that this approach is very structure, which is the most signi®cant test of the model limited and that the other constituents must be included is determined by the physics within it. in order to have the correct turbulence and hence vis- Unauthenticated | Downloaded 09/29/21 09:10 AM UTC JULY 2001 DAVIES ET AL. 1767 TABLE 1. Summary of parameters used in the calculations. cosity in the water column to reproduce the M 2 current pro®le. Calc Constituents k value In the next section we brie¯y outline the basics of 1 All 0.01 the model and the turbulence energy submodel used to 2 All 0.005 compute the coef®cient of momentum transfer (eddy 3 M2 only 0.01 viscosity). In subsequent sections comparisons with the 4 M2 only 0.005 data are presented. 2. The hydrodynamic model coef®cients, but only including the M2 tide in order to determine the in¯uence of frictional effects due to the Since details of the model are given in DH, they will other tidal constituents upon the M 2 tide. not be presented here. A radiation condition (Davies Besides comparing the amplitude and phase of the u 1986) involving only the depth mean current (hence no and y components of velocity it is also useful to compare pro®le of the current was speci®ed) was applied at open their rotary components, namely with R1 a velocity vec- boundaries. On closed boundaries the normal compo- tor that rotates anticlockwise when viewed from above, nent of ¯ow was zero, and in shallow water a drying and R2 the clockwise component. The phase of the an- condition was applied. At the sea surface the stress was ticlockwise and clockwise are denoted by f1 and f2 zero, and a quadratic friction law was applied at the (Soulsby 1983). seabed. The boundary layer thickness of the anticlockwise Discretization in the vertical involved 25 irregularly (d1) and clockwise (d2) components can also be com- spaced levels on a sigma coordinate, with enhanced res- puted, using (Soulsby 1983) olution in the near-bed region. A uniform ®nite differ- CU* CU* ence grid with a resolution of 1 km was used in the d 5 and d 5 (1) horizontal (Fig. 3). A time splitting approach was used 12v 1 f v 2 f to integrate the equations (see DH for details).