particular thanks also go to data providers (ESA, CNES, CTOH, ISPRA and ICPSM, etc.)

Global Storm Surge User Forum - Venice – November 2013 1

particular thanks also go to data providers (ESA, CNES, CTOH, ISPRA and ICPSM, etc.)

with invaluable help (and material) from

S. Zecchetto, F. De Biasio (Consiglio Nazionale delle Ricerche, Italy)

Papa, L. Boato (Municipality of Venice, Italy)

H. Snaith, G Quartly (National Oceanography Centre, Southampton, UK)

Comparison and validation of multi-mission coastal altimetry in the Adriatic sea

Stefano Vignudelli1, Paolo Cipollini2 and Marcello Passaro3,

1 Consiglio Nazionale delle Ricerche, Italy

2 National Oceanography Centre, Southampton, UK

3 University of Southampton, UK


Storm surges in the Adriatic SeaStorm surges in the Adriatic Sea

a regional phenomenon due to the Sirocco wind, directed along the main

axis of the basin The stronger signal is in the northwestern part,

especially in the city of Venice where the well-known “Acqua Alta” (High Water) takes place

The challenge is to forecast the height of water at time of a surge event and the extent to which it will inundate the city


Altimetry on the ground: relative Altimetry on the ground: relative

The vertical reference in Venice area is

the Mean Sea Level of the years 1884-

1909 (conventionally referred to 1897

and called Zero Sea Level at Punta

della Salute (Italian acronym: ZMPS)

Every altimetric level in town is referred to this Mean Sea Level (ZMPS)

By using the same reference for height of water and land elevations allow people to know if their house will be flooded or not when a surge is predicted

The height of water is usually measured by tide gauges

It is referred to a vertical reference (called tidal datum) computed averaging heights over an epoch Punta Salute


Altimetry from space: absoluteAltimetry from space: absolute

Can be done using GPS, radars and gradiometry

GPS provides heights of any arbitrary points of land. Unfortunately not easily done at sea, except in few dedicated sites (usually oil platforms or buoys)

Radar altimetry is designed to measure heights at sea (globally so even where there are no tide gauges)

Gradiometry technique used to measure geoid heights (one global levelling system).

Integrating height data sets from different data sources is not as simple as it seems! e.g., the various satellite height data may use different vertical references depending on the original purpose of

the measurements The great advantage is that each spaceborne technique can be referenced to the same ellipsoid everywhere

This is not immediate with a tidal datum The Venice area has its own tidal datum The other areas of the Adriatic Sea refer to the official Italian reference i.e. the Mean Sea Level in Genoa between 1936-

1940!

Global Storm Surge User Forum - Venice – November 2013

Satellite radar altimetry provides the ‘true’ water level measured by an observer at the coast (i.e. what the storm surge community calls “total water level envelope”, TWLE )

5

How satellite radar altimetry can How satellite radar altimetry can contribute ?contribute ?

Facts in the Adriatic Sea Satellite radar altimetry not yet

exploited for storm surge research eSurge is filling this gap by providing

a consistent high resolution multi-mission data set, reprocessed with optimized techniques

Along-track only, but everywhere, i.e. also in zones with no tide gauges around

Low temporal sampling (at the moment) but we can densify it by merging multiple missions

With a better processing we can extend the measurements to the coastal strip, and at higher resolution (18/20 Hz) than usual (1 Hz)

ENVISAT JASON

CRYOSAT


Objectives, datasets and methodsObjectives, datasets and methods

to assess of how closely the altimeter-derived TWLE estimates from various altimetric products correspond to the in situ values measured at tide gauges

Two case-studies: Tide gauges at “Acqua Alta” platform and Trieste Envisat , Jason-1, Jason-2 and CryoSat-2

Altimeter data sources CTOH - post-processed operational coastal product at 1 Hz ( 7 km) SGDR - operational standard product at 18 Hz (0.37 km along track) eSurge - pre-processed experimental product 18 Hz (0.37 km along track)

Altimeter-derived quantities Sea surface height referred to WGS84 with no correction for tidal and atmospheric (wind &

pressure) effects Sea surface anomaly (i.e. a sea surface height anomaly with respect to a mean sea surface)

TG-derived quantity is the sea surface height referred to the local tidal vertical reference (Mean Sea Level called ZMPS)

Relative and absolute validation tests according to proposal “Validation of Coastal ALtimetry from Sentinel-3 (VOCALS3)” approved by ESA under call “Sentinel-3 validation team”

Zero-lag correlation Rms error of differences


What in situ measurements tell usWhat in situ measurements tell us

The richness of the spectrum Tidal constituents relevant in the

Adriatic Sea are well identified Also the seiche signal (having

slightly broader bandwidth than tides) can be captured, with fine-tuning of spectral analysis

Acqua Alta Platformabout 14 km offshore of the Venice Lido Island, one of the two sandbars separating Venice lagoon from the open Adriatic Sea

High variability in time

Storm surge events are well captured (see events where the height of water is more than 1 meter)


HHow altimetry captures the variability ow altimetry captures the variability of the sea levelof the sea level

Envisat pass 0543

Distance:13 km

TWLE here is the SSH ‘anomaly’

18 Hz

1 Hz

Bias due to the use of different MSS Vertical references can differ from one dataset to the other

due to different averaging periods, different data sources, different merging techniques.

Need to use a common vertical reference to ensure that TWLE from each mission/product comparable to each other

MSS used in SGDR reproduces better the local MSL

Good agreement although still some missing data and discrepancies

Note that during this example year Envisat never passed near TG during a storm surge!


Envisat pass 0543 – Along track distribution Envisat pass 0543 – Along track distribution of outliers of outliers

We compare the percentage of outliers between the standard SGDR product and the new eSurge data from the ALES retracker

The percentage of outliers rises approaching coastline, as expected ALES is able to capture more valid data in some cases In particular ALES is better than SGDR very close to the coastline

Land


Envisat pass 0543 – Along track correlation Envisat pass 0543 – Along track correlation with TG at “Acqua Alta” platformwith TG at “Acqua Alta” platform

Along track slowly changing correlation eSurge high resolution data allows much shorter scales to be

resolved Of course the 18Hz data are noisier ( lower correlations)

eSurge data

1 Hz18 Hz


Correlation coefficient between “Acqua Alta” TG and Env Correlation coefficient between “Acqua Alta” TG and Env 543 for the various altimetric datasets 543 for the various altimetric datasets

ALES improvement in correlation compared to SGDR is widespread. ALES maintains correlation above 0.8 over most of the 18-Hz locations

except for some drops. The correlation for the CTOH 1-Hz series is only marginally higher. This

is expected being ALES a high-rate non-filtered product.

“Acqua Alta” TG location

Land


Relative vs absoluteRelative vs absolute

Often altimeter/tide gauge intercomparison is RELATIVE, that is the common bias is removed and anomalies are computed and comparedWE aim at performing an ABSOLUTE calibration, i.e. closing the altimetric height budget using absolute references (WGS84 ellipsoid), without removing any biases by hand, and working with absolute levels.

Land

This difference is corrected artificially by computing anomalies

Both referred to absolute level (ellipsoid or geoid)

Altimeter TWLE at closest TG point’


Absolute RMS difference between “Acqua Alta” TG and Absolute RMS difference between “Acqua Alta” TG and Env 543 for the various altimetric datasets Env 543 for the various altimetric datasets

RMS error decreases in points in close proximity to the TG RMS error below acceptable threshold (< 15 cm near TG,

with average this can get below 10 cm)

“Acqua Alta” TG locationLand


Absolute RMS difference between TG at Trieste and Absolute RMS difference between TG at Trieste and Envisat, Jason-1 and Jason-2 for the various altimetric Envisat, Jason-1 and Jason-2 for the various altimetric datasets datasets

Land

RMS error decreases until 2.5 km along track from the Istrian coast for Env 416 and 1.8 km for J-1 161.

RMS error more oscillating in the gulf, in particular for Env 416, but the comparison with the SGDR output for the same waveforms highlights the improvements

J-2 196 at greater distance from tide gauge. To be noted the geoid height difference between the altimetry track and the TG location.


Comparison Envisat SGDR vs Envisat reprocessed vs Comparison Envisat SGDR vs Envisat reprocessed vs Cryosat-2Cryosat-2

CryoSat consistent with Envisat Envisat SGDR: up to 5km from the coast Envisat (retracked with OceanCS - subwaveform): up to 3 Km from coast; CryoSat-2 SAMOSA3: all the way to the coast

Land

Env 543 19 cycles (OceanCS)Cryosat 26/07/2012 (SAMOSA3)

(bias removed)

~5km

~3km


Cryosat-2- relative to Cryosat-2- relative to “Acqua Alta” TG“Acqua Alta” TG

The RMS difference with the tide gauge, computed with anomalies, is of the order of 8 cm.

Unresolved bias problems prevent an absolute RMS calculation so far


Summary and next stepsSummary and next steps

Altimeter data available ...yet are not applied in the Adriatic Sea for storm surge research

A new opportunity thanks to eSurge Optimized reprocessing (with the new ALES retracker) of nearly 20 years of a

consistent high resolution multi-mission data set Data analysis and comparisons with tide gauges tell us:

More data can be recovered in open ocean and closer to the coast The quality is better than SGDR 18-Hz Differences with CTOH are due post-processing Cryosat-2 data show an even better performance very close to the coast, with

noise levels compared to the offshore ones up to less than 1 km from the coast Recommendation:

The estimated error at <10-15 cm for 18Hz data is promising for assimilation in models, but this will have to be fully tested by the modellers

Work in progress: Extending comparisons to other tide gauges around coasts (e.g. Ancona) More “downstream” work on exploitation is needed


Thanks for your attention!Thanks for your attention!

Documents

particular thanks also go to data providers (ESA, CNES, CTOH, ISPRA and ICPSM, etc.)