page 1© crown copyright 2004 simulated future changes in extreme water levels jason lowe 1, katja...

© Crown copyright 2004

Simulated Future Changes in Extreme Water Levels

Jason Lowe1, Katja Woth2, Kathy McInnes3

June 2006

1 The Hadley Centre, Met Office, UK.

2 GKSS, Geesthacht, Germany.

3 CSIRO, Aspendale, Australia.


The problem and the tools Surge case studies:

1 – Europe 2 – Australia

Conclusions and key recommendations Don’t forget about waves


We require credible predictions of future changes in extreme water levels caused by storm surges.

1. Are we able to simulate real surge events with existing surge models driven by numerical weather predictions or climate model simulations of the present day?

2. Can we estimate the future (century scale) time average local sea level?

3. Can we estimate future (century scale) Meteorology?


Two main approaches for surges and waves

Dynamic approach Physically-based models used to

simulate storm surge levels and waves in past/present day and future periods. Driven by tidal and meteorological (wind stress and air pressure) forcings across the model domain.

Driving winds and pressure are taken directly from atmospheric climate models for both past/present and future periods or large-scale climate model predictions used to perturb reanalysis winds and pressure.

Do not rely on the past or present

relationship between Meteorological drivers and surges being the same in the future.

May be bias even in present day.

Statistical approach1. Relationships between large-scale

driving meteorology and extreme water levels are developed for present day.

2. Projection made of future large-scale meteorology using a climate model.

3. Future extreme water levels estimated from 1 and 2.

Don’t need to run dynamic storm surge or wave model for future.

Assumes that the relationship between the large-scale variables and the extreme sea level remain unchanged in a future perturbed climate.


Surge Model

•h: Surface elevation H: The total depth•q: Depth mean current Ts: Wind stress on sea-surface•tb: Stress on the sea bottom A: Coefficient of horizontal diffusion: Water density Cd: Drag coefficientsair: Air density : Friction coefficient

Barotropic shelf seas models: e.g. CSX/CS3, TRIMGEO, barotropic version of POM, GCOM2D

h

tHq ( ) 0

q

tq q fk q g h P

HA qa s b

1 1 2( )

s D airC uu

s q qb


Are surge models adequate?

Comparison of 40+ years hindcast with observations from Woth et al. studies at Cuxhaven.

RMS on surge forecast: all stations- no threshold -

0

0.05

0.1

0.15

0.2

0.25

0-6h 6-12 h 12-24 h 24 -48 hrm

s on

su

rge

[m]

bsh_oper dmi_oper dnmi_oper knmi_noos ukmo_oper

RMS errors on storm surge forecasts from 5 operational European surge models along North sea coasts.

Results courtesy of Martin Verlann RIKZ and Martin Holt, NCOF

+simulations of Bernier and Thompsonfor Canadian region (see poster)


Surge results are region specific

EuropeanVon Storch and Reichardt (1997)Langenberg et al. (1999)Flather and Smith (1998)WASA and STOWASUSLowe et al. (2001)Debernard et al. (2002)Lowe and Gregory (2005)Woth et al. (2005) and Woth (2005)

Australia (North and South)McInnes et al. (2003)McInnes and Hubbert (2003)McInnes et al. (2005)

Bay of BengalFlather and Khandker (1993)Flather (1994)As-Selek and Yasuda (1995)Unnikrishnan et al. (2006)Mitchell, Lowe, Wood and Vellinga (2006) + CLASIC (ongoing)

Note: surges do occur in other regions. The regions highlighted in the position paper are only a sample. They do include NH and SH plus tropical and mid-latitude regimes.


Overview of modelling system

Global coupled model

Higher resolutionatmospheric model

Regional climate model

Barotropic stormsurge model

Historic scenario

SRESfuture

scenario

Tide only Surge plus tide

Results &Statistics

Estimate ofmean SLR

Generate 2x30 year regional time slices


Changes in 50-year storm surge height (m) due to changes in storminess.

A2 Scenario B2 Scenario

2080s minus present day.


Changes in 50-year water level (m) due to changes in storminess, mean sea-level rise and vertical land movement.

A2 Scenario B2 Scenario

2080s minus present day.


Simulated extreme water levels (m) for Immingham. 2080s and present day. SRES A2 scenario for 2080s

2080s includes changes in storminess, mean sea-level rise and vertical land movements.


All modelsall SRES

Include uncertainty in ice parameters

IPCC TAR range of global sea-level rise

© Crown copyright 2004 0 0.1 0.2 0.3 0.4 0.5 0.6 m

Sea level rise regional variationsdue to thermal expansion and ocean circulation changes only

Source:IPCC


Comparison of storm surge predictions (50-year surge height [m]). Changes are due to future changes in storminess.

ECHAM42*CO2GEV

HadCM2/HadRM2IS92aGumbel

HadCM3/HadAM3H/HadRM3SRES A2GEV

Lowe, Gregory and Flather, 2001

STOWASUS (from R Flather, POL)

Lowe and Gregory, 2005


An alternative examination of uncertainty by Woth et al.

Domain

Spread due to choice of downscaling RCM was less important

Spread due to driving GCM and scenario

See poster by Katja Woth

99.5th


Methodology + see poster by Kathy McInnes

Historic Identify population of sea level events in historical record Model under current conditions using reanalysis plus surge model

Future 2070 Since storm surges are driven by mid latitude westerlies, analyse

changes in surface winds in climate models Range of change in wind speed determined from analysis of 13

climate models using pattern scaling technique which regresses wind against model’s global warming signal, then scales to temperature uncertainty range

Changes applied as a perturbation to current climate winds and surge model was rerun

Mean Wind Speed 95th Percentile Wind Speed

2070 2070 Low Mid High Low Mid High -5 3 10 -6 3 11 -4 5 14 -6 7 19


Future (2070) extreme levels (m)

R eturn period (years)

Re

turn

leve

l (m

)

1 10 100

0.4

0.6

0.8

1.0

1.2

F it to P res en t C lim a te R e s idu a lsF it to Lo w S ce na rio R e s idu a lsF it to M id S c e na rio R e s id u a lsF it to H igh S ce n ar io R es id ua lsF it to H igh J JA S ce n ar io R es id ua ls


Combining storm surge and tide using Monte-Carlo sampling. 100 yr event.

storm surge + astronomical tide = storm tide

Land subsidence could add a further 1 m

At Surge level (m) Storm tide level (m)

Storm tide level (m) with wind speed (High) increase

Storm tide level (m) with wind speed increase and SLR (49cm)

Lakes Entrance 0.71 0.98 1.07 (adds 0.09) 1.56 (adds 0.49)

Further downscaled with nested higher resolution model

100 year events


Modelling conclusions and recommendations

European examples show importance of both mean sea level change and changes in storminess for projections of future extreme water levels. In present studies both uncertainties are probably underestimated.

Australian example shows dominance of mean sea level uncertainty in projections of future extreme water levels. This is probably underestimated – e.g. no MSL pattern information.

The current studies do not provide information on the shape of the uncertainty distribution. This would be useful for risk calculations.

The results need to be linked to credible inundation models with knowledge of defences (where appropriate) at more sites.


Waves

Damage coastal defences plus lead to additional overtopping

WASA and STOWASUS → future increases in high waves were found in the north eastern part of the North Atlantic but decreases occurred further southwest.

Correlation with the NAO (e.g. Woolf et al., 2002). Wang et al. (2004) assumed the relationship will continue to hold for predictive purposes.

Caires et al. (2006); Wang and Swail (2006) → significant changes. Most significant changes under the more severe emission scenarios.

Wolf and Woolf (2006) used a dynamic wave model approach to show how different climate change effects (e.g. increase in wind speed or change in wind direction) are likely to alter wave conditions around the United Kingdom.

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