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Kevin J. Horsburgh and A. Edward Hill

Abstract

A semi-implicit version of the Princeton Ocean Model, ECOMsi, was used to simulate the cyclonic gyre that is found in the western Irish Sea during the spring and the summer. Mesoscale, seasonal, density-driven circulations such as this are an important component of the long-term flow in shelf seas, and they pose a challenge to coastal ocean models. Extensive comparisons are made here with observational data to assess model performance. The model successfully reproduced the development of the temperature field, and the associated density-driven currents, throughout seasonal simulations. The results demonstrate conclusively that the gyre is density-driven and reinforce the dynamical importance of strong nearbed horizontal density gradients. Maximum baroclinic currents of 0.14 m s−1 were obtained, and results showed that the regional kinetic energy due to the residual flow was 20%–25% of that due to tidal flow during periods in which density gradients were strongest. The model revealed important interactions between both wind and tide and the density structure; these interactions can direct and modulate density-driven flow.

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David A Williams, David M Schultz, Kevin J Horsburgh, and Chris W Hughes

Abstract

Meteotsunamis are shallow-water waves that, despite often being small (~ 0.3 m), can cause damage, injuries and fatalities due to relatively strong currents (> 1 m s−1). Previous case studies, modelling and localised climatologies have indicated that dangerous meteotsunamis can occur across northwest Europe. Using 71 tide gauges across northwest Europe between 2010–2017, a regional climatology was made to understand the typical sizes, times and atmospheric systems that generate meteotsunamis. A total of 349 meteotsunamis (54.0 meteotsunamis per year) were identified with 0.27–0.40 m median wave heights. The largest waves (~ 1 m high) were measured in France and the Republic of Ireland. Most meteotsunamis were identified in winter (43–59%), and the fewest identified meteotsunamis occurred in either spring or summer (0–15%). There was a weak diurnal signal, with most meteotsunami identifications between 1200–1859 UTC (30%) and fewest between 0000–0659 UTC (23%). Radar-derived precipitation was used to identify and classify the morphologies of mesoscale precipitating weather systems occurring within 6 h of each meteotsunami. Most mesoscale atmospheric systems were quasi-linear systems (46%) or open-cellular convection (33%), with some non-linear clusters (17%) and a few isolated cells (4%). These systems occurred under westerly geostrophic flow, with Proudman resonance possible in 43 out of 45 selected meteotsunamis. Because most meteotsunamis occur on cold winter days, with precipitation, and in large tides, wintertime meteotsunamis may be missed by eyewitnesses, helping to explain why previous observationally-based case studies of meteotsunamis are documented predominantly in summer.

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David A. Williams, Kevin J. Horsburgh, David M. Schultz, and Chris W. Hughes

Abstract

On the morning of 23 June 2016, a 0.70-m meteotsunami was observed in the English Channel between the United Kingdom and France. This wave was measured by several tide gauges and coincided with a heavily precipitating convective system producing 10 m s−1 wind speeds at the 10-m level and 1–2.5-hPa surface pressure anomalies. A combination of precipitation rate cross correlations and NCEP–NCAR Reanalysis 1 data showed that the convective system moved northeastward at 19 ± 2 m s−1. To model the meteotsunami, the finite element model Telemac was forced with an ensemble of prescribed pressure forcings, covering observational uncertainty. Ensembles simulated the observed wave period and arrival times within minutes and wave heights within tens of centimeters. A directly forced wave and a secondary coastal wave were simulated, and these amplified as they propagated. Proudman resonance was responsible for the wave amplification, and the coastal wave resulted from strong refraction of the primary wave. The main generating mechanism was the atmospheric pressure anomaly with wind stress playing a secondary role, increasing the first wave peak by 16% on average. Certain tidal conditions reduced modeled wave heights by up to 56%, by shifting the location where Proudman resonance occurred. This shift was mainly from tidal currents rather than tidal elevation directly affecting shallow-water wave speed. An improved understanding of meteotsunami return periods and generation mechanisms would be aided by tide gauge measurements sampled at less than 15-min intervals.

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