A detailed comparison of measured and modeled wind-driven currents in the North Channel of the Irish Sea.

The wind-induced flow in the North Channel of the Irish Sea is examined usi ng an acoustic doppler current profiler (ADCP), to determine current profil e, together with ocean surface current radar (OSCR) measurements of surface current and numerical models. The period considered is a major outflow eve nt which took place during February 3-5, 1994. Although OSCR has been used previously to examine the spatial variability of tidal currents and long-te rm wind-driven flows, no data sets exist, and comparisons with models have not been performed before during a major storm event. Such an event is cons idered here. A three-dimensional shelf-wide coarse grid (resolution similar to 12 km) model is used to take account of large-scale wind events. A more limited area model of the North Channel (of higher resolution, similar to1 km) with boundary forcing from the large-area model is used to determine c urrent profiles in the vertical for comparison with ADCP measurements and s urface currents in the area for comparison with OSCR measurements. The grid size of this model is the same as the bin size over which the OSCR system measures the current. This combination of models and measurements gives sig nificant insight into the temporal and spatial variability of the shelf-wid e winds, which produced the major outflow event, and the spatial variabilit y of the flow in the North Channel. A comparison between along-channel flow s computed with the shelf-wide model and ADCP measurements shows that this model overestimates the flow. However, a similar comparison with the higher -resolution model shows that this model slightly underestimates the flow. S ince both models contain the same physics, namely, solve the same three-dim ensional equations with identical vertical resolution, these differences ca n be attributed to the differences in horizontal resolution. These give ris e to differences in the magnitudes of elevation gradients and local wind-fo rced currents computed with the models. Although there are errors in the fl ow fields computed in both models, they do reproduce the major features of the currents recorded by the ADCP within the North Channel and explain the major features, namely, the time variability of the shelf-wide winds influe ncing the flow in the region. A detailed comparison of surface currents mea sured with the HF Radar and model results interpolated to the radar bins sh ows a reasonable agreement (to within 0.15 in s(-1)), (taking into account errors in HF Radar measurements) in the computed and observed spatial and t emporal variability of the surface current. However, the variability (in bo th space and time) of the computed current is significantly smoother than t hat determined by the radar. Also, away from the transmitter sites the sign al to noise ratio in the radar signal is so large that accurate measurement s (to within 0.3 m s(-1)) could not be made during this storm event. Calcul ations showed that the value of the computed surface current was sensitive to the assumed roughness length, although below the surface layer (of the o rder of 3 m) the current was not affected by the surface roughness value. C omparison between computed currents determined with the high-resolution mod el and those measured by the radar showed a similar bias to that found in t he ADCP comparison.