INTER-COMPARISON OF COASTAL MODELS: CASE STUDY OF STORM SURGE AT NEMURO IN JAPAN

The numerical coastal circulation models play an essential role in predicting storm surges. Several models (e.g. ADCIRC: Dietrich et al., 2004, FVCOM: Chen et al., 2003) have been previously inter-compared (Kerr et al., 2013; Chen et al., 2013). In these studies, storm surges were reproduced in locations where the bathymetry has a gradual increase from offshore to coast, within a closed gulf. On the other hand, there are few studies in regards to modelling storm surge where the near coast bathymetry is steep and connected to open ocean. Considering the storm surge dependence on local bathymetry, it can be important to conduct an inter-comparison of ocean circulation models in such a region. In this study, numerical coastal circulation models (2D-ADCIRC and 3D-FVCOM) are compared by using a 2014 Dec. storm surge event at Nemuro city in Hokkaido (Japan), which was caused by a rapidly intensified extra-tropical cyclone approaching the area. In this region, local bathymetry is steep due to Japan Trench. The cyclone caused a storm surge of nearly up to 1.8 m within the Nemuro city between 00:00 UTC 16th and 17th Dec. 2014. The aim of this study is to evaluate the performance of ocean circulation models using several air-sea drag coefficients and contribute to inter-comparison studies using ADCIRC and FVCOM.

A New Drag Relation for Aerodynamically Rough Flow over the Ocean

From almost 7000 near-surface eddy-covariance flux measurements over the sea, the authors deduce a new air–sea drag relation for aerodynamically rough flow:Here u* is the measured friction velocity, and UN10 is the neutral-stability wind speed at a reference height of 10 m. This relation is fitted to UN10 values between 9 and 24 m s−1. A drag relation formulated as u* versus UN10 has several advantages over one formulated in terms of . First, the multiplicative coefficient on UN10 has smaller experimental uncertainty than do determinations of CDN10. Second, scatterplots of u* versus UN10 are not ill posed when UN10 is small, as plots of CDN10 are; u*–UN10 plots presented here suggest aerodynamically smooth scaling for small UN10. Third, this relation depends only weakly on Monin–Obukhov similarity theory and, consequently, reduces the confounding effects of artificial correlation. Finally, with its negative intercept, the linear relation produces a CDN10 function that naturally rolls off at high wind speed and asymptotically approaches a constant value of 3.40 × 10−3. Hurricane modelers and the air–sea interaction community have been trying to rationalize such behavior in the drag coefficient for at least 15 years. This paper suggests that this rolloff in CDN10 results simply from known processes that influence wind–wave coupling.