Resolving wave effects on storm surges with an unstructured high resolution 2-way coupled surge-wave model for Australia
Ivica Janekovic, Yasha Hetzel, Charitha Pattiaratchi, E.M.S. Wijeratne, Aron Roland
School of Civil, Environmental and Mining Engineering The UWA Oceans Institute The University of Western Australia Extreme sea levels
Cardwell after Yasi (source: DSITIA) Extreme sea levels
Essential for coastal planning
A range of processes influence extreme water levels
Previous work suggested the model underestimated extreme sea levels in some areas
The present model attempts to improve these estimates through (Haigh et al. 2013 a,b) improved forcing, resolution, and coupling with a wave model Motivation
Better prediction of storm surge around Australia
Increased computational power (Pawsey) allows
better resolutions, representation of the system
better numerical models with complex dynamics
coupling btw w-o-a models
Unstructured formulation 3D – efficient, MPI scaling
horizontal (triangle / quads)
vertical s-z (LSC2- uneven number of levels) AUS model grid
Coastal ~100 m Simulations
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a M TROPICAL CYCLONE CYCLONE YASI GEORGE FEBRUARY 2011 APRIL 2007 HJRA maximum wind speed v15
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CYCLONE * Thevenard 0.9 EAST COAST 0.8 Port * Pirie ALBY APRIL 0.6
0.6 M Port Lincoln Port a x LOW
Adelaide 0.5 s * u * r Victor g 1978 e 0.4 Harbour
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0 EXTRATROPICAL STORMS Models SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) unstructured (triangles, quads, in vertical LSC2 scheme “s-z”) semi-implicit integration, opposite CFL>0.6 (!)
• Wave-dependent wind stress • Wave momentum stress (wave setup)
WWM SCHISM wave hydrodynamic model model
• Sea level • Currents WWM III (Wind Wave spectral model, ~ WW3) hybrid fractional splitting method, 3rd Ultimate Quickest Wind@10m when waves ~15m? advanced source and dissipation formulations (Ardhuin), wave breaking (Battjes and Janssen,1978) Coupled Wave Effects
Effect on wind stress
TC and 10m wind concept? Classical stress using Cd Charnock coef. is not constant, used wind-wave dependent α Wave model can compute stress and U*
Schism (SL, U, V) 2-way coupled @ 300s WWM (WL, Wind stress, wave Sxx) Coupled Wave Effects
During TC Hs ~10-15m Wave energy --> sea level
Old way empirical relations (Hb 0.7- 1.3)
However breaking depends on many wave steepness, bottom slope ...
WWM <-> SCHISM Numerical experiments: a) Fully 2-way coupled case b) As (a) but WWM does not send Sxx, Sxy, Syy into SCHISM Cyclone Alby, April 1978
Started as a category 4 cyclone that interacted with an approaching cold front with devastating results
Flooding and erosion in SW
‘Worst case’ scenario
Modeling challenges - lack of data & complex storm dynamics – extratropical transition (no longer a tropical cyclone)
Source: BOM Cyclone Alby, April 1978
Alby accelerated to 80 km/h as it passed the SW of the state.
Wind gusts of 150 km/h were recorded with little or no rain along the coast Strong, dry northerly winds 10 m waves reported offshore
JRA55 reanalysis winds Cyclone Alby, April 1978
Source: BOM Perth Bunbury Wave-surge model setup
Coupled & Uncoupled simulations
100m-100km horizontal resolution
61 vertical sigma-Z levels
JRA55 Reanalysis atmospheric forcing (wind and pressure)
Validation with tide gauge observations at Perth, Bunbury, Busselton
Output – hourly water levels, currents, wave height, period Wind forcing JRA55
30-Mar-1978 06:00 Z (a) 02-Apr-1978 06:00 Z (b) 04 April 1978 12 Z
03-Apr-1978 06:00 Z (c) 04-Apr-1978 06:00 Z (d) 30
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04-Apr-1978 12:00 Z (e) 04-Apr-1978 18:00 Z (f) Simulated waves & surge
Waves >10m matched well with ship reports Simulated waves & surge
Waves >10m matched well with ship reports Simulated waves & surge Wave contributions
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04/02 04/03 04/04 04/05 Surge difference Surge difference %
10-35% surge height difference due to waves Tropical cyclone Yasi
Dunk Island, QLD
Source: News Ltd. BEFORE AFTER
One of the most intense and largest tropical cyclones to cross the coast in Australia
Major damage (~$800 M) caused by inundation and 02 February 2011 erosion from extreme waves (>5m) and storm surge (up to 5.3 m!) Model forcing
Weather models typically underestimate Simulated maximumHJRA maximum wind speed windv15 speed the intensity of tropical cyclones
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Parametric wind models are used to simulate the vortex of tropical cyclones (e.g. Holland 2010)
To force our surge-wave model we merged a parametric wind model with JRA55 model data Extreme waves
Yasi waves > 12 m offshore and 6 m nearshore
Simulated waves matched well with satellite observations
Great Barrier Reef dissipated much of the wave energy
Waves improved model accuracy Storm surge
Storm surge in Cardwell >5 m!
(source: DSITIA) Storm surge
Storm surge in Cardwell >5 m!
(source: DSITIA) Storm surge
Storm surge in Cardwell >5 m!
(source: DSITIA)
Wave setup
Surge Surge (m) Wave setup (m) Wave
Wave height
Cross shelf depths determined contribution of wave setup Australia-wide simulation Australia-wide simulation
2011
TC Anthony, QLD (23,28-30 Jan) TC Bianca, WA (27-30 Jan) TC Yasi, QLD (2-4 Feb) Cold fronts, SA & Tas (1-4 Feb) Summary
An advanced coupled surge-wave model has been set up for the entire Australian coastline and allows for estimation of wave setup over large areas
Multiple test cases, shown only for Cyclone Alby (1978) in W.A. and Cyclone Yasi (2011) in Queensland
15-35% of simulated surge heights during an extreme event were due to wave setup
The amplitude of wave setup is closely linked with wave height and bathymetry
Wave setup at the coast ranged between 6-10% of significant wave height measured at 10 m depth
Accurate high resolution bathymetry is the major limiting factor for simulations of wave setup
The capability to simulate waves and hydrodynamics over all Australia is made possible by supercomputer Thank you