Data assmilation tests using NISE10 Storm Surge Model

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Observations and modelling of the western Irish Sea gyre.

Observations from 1995 and 1996 described the seasonal evolution of the threedimensional density field in the western Irish Sea. A cold, dense pool flanked by strong nearbed density gradients was present from May until October. Temperature had the dominant effect on density from June onwards. The trajectories of 55 satellite-tracked drifters defmed the full spatial extent of the cyclonic circulation that is the western Irish Sea gyre. Several distinct recirculation paths were observed and drifter speeds were in good agreement with geostrophic calculations based on the observed density field. The existence of such organised, baroclinic flows in shelf seas demands that coastal ocean models should reproduce their dynamics correctly, if the models are to be useful as environmental management tools. One such model, ECOMsi, was applied to the study area and results from seasonal simulations were compared with the observations. A new technique was developed to perform quantitative comparisons between modelled and observed flow fields. The model successfully reproduced the three-dimensional temperature structure throughout the seasonal simulations, and also predicted the cyclonic, near-surface residual circulation of the gyre. The model demonstrated conclusively that the gyre is density-driven and revealed the same recirculation paths that were visible in the drifter tracks. The vertical structure of the modelled density-driven flow confirmed the geostrophic nature of the currents and emphasised the important dynamical role of sharp density gradients near the bed (bottom fronts). A quantitative comparison of different model runs identified the critical parameterisations and forcing quantities for this application. An accurate specification of air temperature over the sea region was required for the model to achieve the correct timing of the stratification breakdown. During this phase, convective cooling at the surface was seen to be as important as the mixing by autumnal winds in eroding the density structure. The possibility of a seasonal reversal in density-driven flow along the east coast of Ireland was also identified. A new interaction between the wind and the density field, which could defme where the strongest currents in the gyre are to be found, is described. The model is now considered to be sufficiently well tested to use in a predictive capacity and for biological transport studies. This work highlights the benefits that can be obtained using high quality spatial and temporal field observations in the critical testing of numerical models, and furthermore suggests that shelf seas are the perfect location for such tests to be performed

Proudman resonance with tides, bathymetry and variable atmospheric forcings

Proudman resonance is a primary amplification mechanism for meteotsunamis, which are shallow-water waves generated by atmospheric forcings. The effect of tides, sloping bathymetry and the speed, amplitude and aspect ratio of the atmospheric forcing on Proudman resonant wave growth are investigated using analytical approximations and numerical models. With tides included, maximum wave growth through Proudman resonance occurred when the atmospheric-forcing speed matched the tidal-wave speed. Growth greater than Proudman resonance occurred with a positive tidal elevation together with a tidal current in the opposite direction to wave propagation, due to linear growth combined with further amplification from wave-flux conservation. Near-Proudman resonant growth occurred when the forced-wave speed or free-wave speed varied by either a small amount, or varied rapidly, around a speed appropriate for Proudman resonance. For a forcing moving at Proudman resonant speed, resultant wave growth was proportional to the total, time-integrated forcing amplitude. Finally, Proudman resonant wave growth was lower for forcings with lower aspect ratios (AP), partly because forced-wave heights are proportional to 1 + A 2P , but also because free waves could spread in two dimensions. Whilst the assumptions of strict Proudman resonance are never met, near-Proudman resonant growth may occur over hundreds of kilometres if the effective Froude number is near 1 and the resultant wave propagates predominantly in one dimension

Radiational tides: their double-counting in storm surge forecasts and contribution to the Highest Astronomical Tide

Tide predictions based on tide-gauge observations are not just the astronomical tides; they also contain radiational tides – periodic sea-level changes due to atmospheric conditions and solar forcing. This poses a problem of double-counting for operational forecasts of total water level during storm surges. In some surge forecasting, a regional model is run in two modes: tide only, with astronomic forcing alone; and tide and surge, forced additionally by surface winds and pressure. The surge residual is defined to be the difference between these configurations and is added to the local harmonic predictions from gauges. Here we use the Global Tide and Surge Model (GTSM) based on Delft-FM to investigate this in the UK and elsewhere, quantifying the weather-related tides that may be double-counted in operational forecasts. We show that the global S2 atmospheric tide is captured by the tide-and-surge model and observe changes in other major constituents, including M2. The Lowest and Highest Astronomical Tide levels, used in navigation datums and design heights, are derived from tide predictions based on observations. We use our findings on radiational tides to quantify the extent to which these levels may contain weather-related components

Spatial and temporal analysis of extreme sea level and storm surge events around the coastline of the UK

In this paper we analyse the spatial footprint and temporal clustering of extreme sea level and skew surge events around the UK coast over the last 100 years (1915-2014). The vast majority of the extreme sea level events are generated by moderate, rather than extreme skew surges, combined with spring astronomical high tides. We distinguish four broad categories of spatial footprints of events and the distinct storm tracks that generated them. There have been rare events when extreme levels have occurred along two unconnected coastal regions during the same storm. The events that occur in closest succession (< 4 days) typically impact different stretches of coastline. The spring/neap tidal cycle prevents successive extreme sea level events from happening within 4-8 days. Finally, the 2013/14 season was highly unusual in the context of the last 100 years from an extreme sea level perspective

A user-friendly database of coastal flooding in the United Kingdom from 1915–2014

Coastal flooding caused by extreme sea levels can be devastating, with long-lasting and diverse consequences. Historically, the UK has suffered major flooding events, and at present 2.5 million properties and £150 billion of assets are potentially exposed to coastal flooding. However, no formal system is in place to catalogue which storms and high sea level events progress to coastal flooding. Furthermore, information on the extent of flooding and associated damages is not systematically documented nationwide. Here we present a database and online tool called ‘SurgeWatch’, which provides a systematic UK-wide record of high sea level and coastal flood events over the last 100 years (1915-2014). Using records from the National Tide Gauge Network, with a dataset of exceedance probabilities and meteorological fields, SurgeWatch captures information of 96 storms during this period, the highest sea levels they produced, and the occurrence and severity of coastal flooding. The data are presented to be easily assessable and understandable to a range of users including, scientists, coastal engineers, managers and planners and concerned citizens

Examination of generation mechanisms for an English Channel meteotsunami: combining observations and modeling

On the morning of 23 June 2016, a 0.70 m meteotsunami was observed in the English Channel between the UK and France. This wave was measured by several tide gages 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 crosscorrelations 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 gage measurements sampled at less than 15-minute intervals

Tide and skew surge independence: new insights for flood risk

Storm surges are a significant hazard to coastal communities around the world, putting lives at risk and costing billions of dollars in damage. Understanding how storm surges and high tides interact is crucial for estimating extreme water levels so that we can protect coastal communities. We demonstrate that in a tidal regime the best measure of a storm surge is the skew surge, the difference between the observed and predicted high water within a tidal cycle. Based on tide gauge records spanning decades from the UK, US, Netherlands and Ireland we show that the magnitude of high water exerts no influence on the size of the most extreme skew surges. This is the first systematic proof that any storm surge can occur on any tide, which is essential for understanding worst case scenarios. The lack of surge generation dependency on water depth emphasises the dominant natural variability of weather systems in an observations-based analysis. Weak seasonal relationships between skew surges and high waters were identified at a minority of locations where long period changes to the tidal cycle interact with the storm season. Our results allow advances to be made in methods for estimating the joint probabilities of storm surges and tides