Transcript The ESA eSurge-Venice project: how satellite data can

The ESA eSurge-Venice project: how satellite data can improve the
storm surge forecasting in the Gulf of Venice
S.Zecchetto, F. De Biasio, A. della Valle
CNR-ISAC, Padova, Italy
S. Vignudelli
CNR-IBF, Pisa, Italy
G. Umgiesser, M. Bajo
CNR-ISMAR, Venezia, Italy
A. Papa
Istituzione Centro Previsioni e Segnalazioni
Maree, Venice Municipality, Venezia, Italy
C. Donlon
ESA ESTEC, Noordwijk, The Nederlands
Contacts: [email protected]
http://www.esurge-venice.eu
Earth Observation for Ocean-Atmosphere Interactions Science 2014 – Frascati, 28-31 October 2014
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The ESA eSurge and eSurge-Venice projects

The Data User Element (DUE) program of the European Space Agency (ESA) is funding
two projects (eSurge and eSurge-Venice) aimed to demonstrate the improvement of the
storm surge forecasting through the use of Earth Observation (EO) data.
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eSurge-Venice is specifically focused on the Gulf of Venice.
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Project objectives:
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Select a number of Storm Surge Events occurred in the Venice lagoon;
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Provide the available EO, in situ and model data related to the Storm Surge Events;
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Provide a demonstration NRT service of EO data products and services in support
of operational and experimental forecasting and warning services;
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Run a number of re-analysis cases to demonstrate the usefulness of EO data.
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The eSurge-Venice project: team and website homepage
http://www.esurge-venice.eu

CNR-ISAC, Padova, Italy
Scatterometer wind
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CNR-ISMAR, Venice, Italy
Storm surge modelling
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CNR-IBF, Pisa, Italy
Altimeter data
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Istituzione Centro Previsioni e
Segnalazioni Maree, Venice
Municipality, Venezia, Italy
In situ data
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Storm surges in the Gulf of Venice
Storm surges in the Gulf of Venice are induced by:
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Strong southeasterly winds (sirocco) along the
Adriatic Sea main axis, resulting from the passage of
low atmospheric pressure and the basin orography
(duration: 3 days; level up to 90 cm)

Inverse barometer effect resulting from the MSL air
pressure pattern (roughly 1 cm /1 mb; duration 10
days; level up to 20 cm)
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Basin seiches caused by meteorological forcing
(duration: 5 days; level up to 40 cm)
Mediterranean Sea
Other phenomena (wave set-up, non-linear interactions
between storm surge, astronomical tide and waves,
baroclinic forces and river discharges) are negligible
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High water
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The surge added to the astronomical tide (~ 6 hour; level up to 60 cm above MSL) may cause flooding
in the venice Lagoon.
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The final level depends on the amplitude and mutual phase of meteorological surge, seiches and
astronomical tide: flooding begins when sea level > 60 cm on mean sea level

In the last century, natural and anthropogenic subsidence and eustatism have modified the local datum,
which at present is 28 centimetres below the mean sea level, causing the increment of the frequency of
high waters in Venice
adapted from: Carbognin, L., Gatto P., and Mozzi G., 1981
number of high tides (>110 cm) in the Venice city since 1872
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Earth Observation for Ocean-Atmosphere Interactions Science 2014 – Frascati, 28-31 October 2014
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The role of Earth Observation in the eSurge-Venice project
The main purpose of the eSurge-Venice project is to improve the sea level forecast in
the Adriatic Sea by using Earth Observation (EO) data.
In an operative context, the sea surface wind field needed to force storm surge
models (SSM) is supplied by numerical weather prediction (NWP) models, whose
accuracy may be reduced in semi-enclosed basins surrounded by complex
orography as the Adriatic Sea.
The initial sea surface elevation is generally supplied by previous simulations of the
SSM itself.
Our objective is to use satellite data to improve the NWP sea surface wind
forcing accuracy and the description of the SSM initial sea surface elevation,
in order to supply more accurate initial conditions and forcing to SSMs.
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How scatterometer wind can be used?
What we Know:
What we want to do:
 Atmospheric models in general
underestimate the wind speed with respect
to satellite winds
 Modify the NWP model wind
fields of today according to the
scatterometer-model biases
found in the previous 3 days
 The differences between models and
satellite data are variable in space and time
Satellite wind datasets:
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QuikSCAT L2B 12.5 km (1999-2009)
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OCEANSAT-2 L2 12.5 km (2010-2014)
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ASCAT A L2 12.5 km (2009-present)
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ASCAT B L2 12.5 km (2012-present)
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Earth Observation for Ocean-Atmosphere Interactions Science 2014 – Frascati, 28-31 October 2014
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NWP Wind Bias Mitigation (WBM)
1.
2.
3.
The mean scatt-model wind speed relative bias
Δws and direction Δθ bias, are computed over
a 3-day window before the day of forecast.
The two biases are used to modify the NWP
forecast wind field the day after the 3-day
window, according to these formulae:
ws(i, j)scatt  ws(i, j)model
Δws(i, j) =<
>
ws(i, j)scatt
Δθ(i, j) =< θ(i, j)scatt  θ i, j model >
'
ws(i, j)model
= ws(i, j)model  ( 1+ Δws(i, j))
The storm surge model is run with the modified
NWP field as forcing. The process is repeated
day by day, shifting the 3-day window ahead.
θ i, j model = θ(i, j)model + Δθ(i, j)
'
(i,j) indicates the position of the node on
the grid
The average <…> is made on any grid
node over the available
observations during the 3 day
window
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Earth Observation for Ocean-Atmosphere Interactions Science 2014 – Frascati, 28-31 October 2014
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NWP Wind Mitigation Bias (WBM)
Graphical representation of the mean wind
speed relative bias (left) and wind direction
bias (right) between ECMWF wind forecast
and scatterometer OVW during 3 days before
the surge event of 02/02/2009 23:00 GMT
The mean duration of the siroccotype wind in the Adriatic Sea
during the SEVs (asterisks) is
around 2,5 days. This justifies the
use of a 2-3 day window for the
calculation of the biases.
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Results of hydrodinamical modelling (S.HY.FEM model) with respect to
WBM
S.HY.F.E.M. (CNR-ISMAR, http://www.ismar.cnr.it/shyfem/): 3D shallow water hydrodynamic model
results
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the peak error (obs-mod surge difference on the maximum peak) drops to 6 cm from 10 cm;
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the RMSE between observed and modeled surge decreases by 8%;
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the correlation between observed and modeled surge rises by 1%;
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Use of satellite data to improve the SSM model initial conditions
through Altimeter data Assimilation (AA)
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Satellite altimeters measure the sea level along the satellite
subtrack over the ellipsoid
SSMs (SHYFEM) compute the sea level over the geoid
The lack of a common reference frame prevents the direct
comparison of the two absolute quantities
Their profiles should have comparable shapes, up to an
additive constant
Satellite altimeter datasets:
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Topex/Poseidon: 1992 to 2005
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Geosat Follow On: 2000 to 2008
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ENVISAT: 2002 to 2010
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Jason 1: 2002 to present
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Jason 2: 2008 to present
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AltiKa, Jason 3, Cryosat-2...
Objective:
Assimilation into SHYFEM of the differences between (zeromean) modelled and measured sea surface profiles along the
altimeter tracks as model errors with a dual 4d-VAR (4dPSAS) assimilation technique
snapshot of the satellite sea level anomaly
tracks in an eight day window around the
storm surge event of 31 October 2004
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Results of hydrodinamical modelling with AA
Results:
Assimilation of long altimeter tracks (track along the Adriatic Sea major
axis) has more impact on the SSM results (experiment n. 1, observedmodeled surge = 5 cm without assimilation, 3 cm with assimilation),
while in the opposite case (satellite track crossing the Adriatic Sea
along its minor dimension) the impact is less evident.
Limitations:
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Poor spatial and temporal coverage in the Adriatic Sea
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Altimeter data only along the satellite track
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Long latency for high quality altimeter data
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Results (in terms of skew surge: difference between the max. observed
and the max. predicted sea level during a tidal cycle)
Reanalysis experiments (22 SEVs in 2004-2012) performed
with respect to the standard forecast:
surge forecast with wind bias mitigation (WBM) – Exp.1

surge forecast with altimeter assimilation (AA) – Exp. 2

surge forecast with WBM+AA – Exp. 3
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Exp. 1 resulted in lower skew surges in 18 out of 22 SEVs
Exp. 2 resulted in lower skew surges in 8 out of 18 SEVs.
Together, (Exp. 3) WBM and AA grant lower skew surge in 14
out of 18 SEVs.
Overall results for the skew surge RMSD (cm):
Standard forecast (22 SEVs):
14.9
WBM alone (22 SEVs):
9.7
AA alone (18 SEVs):
13.8
WBM+AA (18 SEVs):
9.3
The observed-model skew surge for the 22 reanalysis experiments.
Green: skew surge of standard forecast
Red: with wind forecast mitigation alone (Exp. 1)
Blue: with altimeter assimilation alone (Exp. 2)
Black: with both WBM and AA (Exp. 3).
No data for AA for SEV #12, #14, #17 and #20.
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Earth Observation for Ocean-Atmosphere Interactions Science 2014 – Frascati, 28-31 October 2014
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Conclusions
Three reanalysis experiments conducted on ~ 20 storm surge events:
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modified wind forcing: the RMSD passed from 14.9 cm to 9.7 cm
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direct altimeter data assimilation: RMSD passed from 14.9 cm to 13.8 cm
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modified wind+altimeter assimilation: RMSD passed from 14.9 cm to 9.3 cm
Open issues:
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there is an optimal form of the factor (now it is: 1+Δws) to modify the NWP wind
speed?
Future directions:

assimilation of altimeter into SSMs will be extended to the Mediterranean Sea
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Conclusions
Thank you for your attention!
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