The effect of density stratification on the predic

Authors: Tsubasa Kodaira Keith R Thompson Natacha B Bernier

Publish Date: 2016/10/06

Volume: 66, Issue: 12, Pages: 1733-1743

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Abstract

With the longterm goal of developing an operational forecast system for total water level we conduct a hindcast study of global storm surges for Fall 2014 using a baroclinic ocean model based on the NEMO framework The model has 19 vertical levels a horizontal resolution of 1/12° and is forced by hourly forecasts of atmospheric wind and air pressure Our first objective is to evaluate the model’s ability to predict hourly sea levels recorded by a global array of 257 tide gauges It is shown that the model can provide reasonable predictions of surges for the whole test period at tide gauges with relatively large tidal residuals ie gauges where the standard deviation of observed sea level after removal of the tide exceeds 5 cm Our second objective is to quantify the effect of density stratification on the prediction of global surges It is found that the inclusion of density stratification increases the overall predictive skill at almost all tide gauges The increase in skill for the instantaneous peak surge is smaller The location for which the increase in overall skill is largest east coast of South Africa is discussed in detail and physical reasons for the improvement are givenStorm surges are variations of sea level about the tide due to storms Positive surges can cause severe coastal flooding and negative surges can be navigation hazards for ships operating in shallow water Needham et al 2015 recently provided an overview of the global data sources observations and impacts of historically extraordinary storm surges To reduce possible casualties and economic losses caused by storm surges many countries have developed regional forecast systems to provide warnings to coastal communities and marine related industriesThe early development of numerical storm surge prediction systems was reviewed by Heaps 1983 and Bode and Hardy 1997 Twodimensional shallow water equations have been used extensively in operational regional forecast systems eg SLOSH for the US Jelesnianski et al 1992 and CS3 for the UK Flather 2000 Statistical models are sometimes used in conjunction with a dynamical model eg SLOSH to compensate for errors in the model or its atmospheric forcing To take account of uncertainty in the atmospheric forcing there is now a move towards the development of ensemblebased systems For example Flowerdew et al 2010 have developed an ensemble surge forecast system for the UK and more recently Bernier and Thompson 2015 have evaluated an ensemble prediction system for Atlantic CanadaIn contrast to the regional systems mentioned above the present study focuses on the development of a global baroclinic storm surge prediction system The global system is not designed to compete with regional models it only has a resolution of 1/12° Its purpose is to improve the predictions of the regional forecast systems For example the global model can be used to identify regions with relatively low predictability due for example to low predictability in the atmospheric forcing and also to provide lateral boundary conditions for regional systemsThe first objective of the present study is to quantify the skill of the global model in the prediction of water levels caused by variations in wind and air pressure In this initial development of the global system we focus on Fall of 2014 and compare the model’s predictions to observations made by a global array of 257 tide gauges The existing model most relevant to the present study is MOG2DG Carrère and Lyard 2003 see also Appendix This is an excellent global surge prediction model that is used widely to provide a Dynamic Atmospheric Correction DAC for dealiasing altimeter data The DAC data for Fall 2014 are also evaluated against the tide gauge dataset to compare its predictive skill with our modelThe second objective is to quantify how density stratification may influence the predictions of water level and storm surges One way in which density stratification can influence the surges is by changing the vertical structure of the currents and thus bottom stress eg Gordon 1982 Although some studies eg Davies 1988 Hearn and Holloway 1990 Weisberg and Zheng 2008 Zheng et al 2013 have examined the effect of vertical structure in barotropic models the effect of baroclinicity has received less attention One exception is the modelling study of Minato 1998 of the storm surge in Tosa Bay produced by typhoon Anita It was shown that the predicted storm surge increased slightly by including density stratification because bottom stress was reduced Another way in which density stratification can influence surges is by changing the dispersion relation of coastal trapped waves eg Clarke 1977 Gill 1982 and also reducing the amount of backscattering by freely propagating coastal trapped waves as they encounter changes in the width of the shelf eg Wilkin and Chapman 1990 In the present study we compare model runs with and without realistic density stratification

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