Post on 14-Dec-2015
Application of coastal altimetry to storm surge studies
Paolo Cipollini
National Oceanography Centre, UK
Global Storm Surge Networking Forum, Venice, 18-20 November 2013
Satellite AltimetryNine missions since early 90s, huge success•measures sea surface height (precision ~ few cm)
• synoptic, sustained view of surface ocean dynamics (currents, eddies, planetary waves)
• accurate, long-term global and regional sea level monitoring
•measures wave field (precision ~10s of cm)•measures wind
Remko Scharroo, Altimetrics LLC
Sea Level component on dedicated ESA programme, the “Climate Change Initiative”
Why do coastal altimetry? - 1Traditionally, data in the coastal zone are not used:
problems with wavefoms
problems with some corrections
(coastal zone: in practice, any place where standard altimetry gets into trouble as radar waveforms are non-standard and/or corrections become inaccurate)
20+ years of data in the coastal stripcan be recovered!
corruptedwaveform
We believe that most of those coastal data can be recovered!
(and users are actually asking for them!)
Why do coastal altimetry? - 2• coastal dynamics• coastal management• link oceanic sea level to tide gauges• coastal wave field• global dataset, 20+years of data• new missions (SAR altimeter, Ka-band
altimeter) have good coastal capabilities– better measurements at the coast are a requiremen
of any future mission
• storm surges now emerging as crucial application
Coastal altimetry: one simple example
We believe so, with dedicated reprocessing and corrections.
For instance - can we recover these measurements?
How we recover more dataA. Specialized retracking
– Use new waveform models, accounting for change of shape in coastal environment
– Use specialized retracking techniques (for instance retrack groups of adjacent waveforms in one go)
B. Improved Corrections– Most crucial is the
correction of path delay due to water vapour (“wet tropospheric” correction)
– Some applications (NOT storm surges!) require correction of tidal and high-frequency signals, which are also difficult to model in the coastal zone
0-10 km0-10 km 0-50 km0-50 km
Both validation and applications require exploitation of coastal
models & in situ measurements
“Retracking” of the waveforms
= fitting the waveforms with a waveform model, therefore estimating the parameters
Figure from J Gomez-Enri et al. (2009)
Maximum amplitude: related to wind speedMaximum amplitude: related to wind speed
“Epoch”: gives range (therefore height)“Epoch”: gives range (therefore height)
Slope of leading edge: related to significant wave height
Slope of leading edge: related to significant wave height
Retracking – Coastal waveforms
• waveforms in coastal zone and island passes have been extensively studied
• we often see disturbances in the ‘tail ‘ of the waveform
• Our suggested workaround: a retracker that only fits the ‘good’ portion of the waveform (the leading edge), neglects the rest– “OceanCS” retracker by Lee et al 2012, further developed at
NOC by Passaro et al., 2013 ALES retracker
This recovers more data over open ocean too!
Jason-2 ExampleJason-2 Example
Jason-2 pass 096 over the coast of South Africa and example of coastal waveform for cycle 83 and its retracking with various models: BGP (Halimi et al.,
2013), Brown (1977), OceanCS (Yang et al., 2012), ALES
(Passaro et al., 2013)
Retracking – an example
M. Passaro, NOC
QUALITY HIGH RATE DATA UP TO 2.5 KM !
IMPROVEM
ENT IN COASTAL SWH
RETRIEVAL PRECISION!
With specialized retrackers we get much closer to the coast!
Envisat example, 20 cycles of pass 0543 over Northern Adriatic
SSH SWHVenice
New ALES retracker now implemented in NOC’s eSurge coastal altimetry processor see next talk
Note: works well everywhere, not just at the coast!
New ALES retracker now implemented in NOC’s eSurge coastal altimetry processor see next talk
Note: works well everywhere, not just at the coast!
/ALES/ALES
The role of altimetry• Key quantity: Total Water Level Envelope (TWLE)
– That’s the level you get – inclusive of tide, pressure, HF atmospheric effects, wave setup, etc…
– altimetry measures TWLE– coastal altimetry extends TWLE measurements to the coastal
strip, i.e. where they are most relevant to storm surge research, applications and services
• The wave field in the coastal strip is also relevant– helps development of more realistic wave models
• Of course there is a sampling issue – but altimetry is still useful, in combination with Tide Gauges, to ascertain the modes of variability of the coastal ocean
• Let’s see some data!
Katrina surge in Altimetry - Scharroo et al 2005
Han 2011 – Hurricane IgorJason-2: before, during, after
Coast
Hurricane Isaac
• eSurge AOI-120: Gulf of Mexico
• 28/29 Aug 2012
Jason-1 p16828Aug 5 UTC
Jason-2 p02628Aug 5 UTC
`̀
1 – Old Port Tampa
2 – Cedar Key
3 – Panama City
4 – Apalachicola
5 – Pensacola
Predicted tide Observed WL Obs–Pred J1/J2
Isaac: Jason-1/-2 TWLE (RADS)Jason-1Jason-2
Coast Coast
How best to use these observations? validation in hindcasts? assimilation in forecasts?How best to use the fine-scale information at the coast?
Jason-1 p16828Aug 5 UTC
Jason-2 p02628Aug 5 UTC
`̀
Cryosat-2 p064528Aug 16:27 UTC
Cryosat-2 p070330Aug 16:24 UTC
Cryosat-2 p066029Aug 04:34 UTC
1 – Old Port Tampa
2 – Cedar Key
3 – Panama City
4 – Apalachicola
5 – Pensacola
Predicted tide Observed WL Obs–Pred J1/J2C2
Cryosat LRM TWLE (from RADS)
0.7 m
Note: not reprocessed with coastal altimetry techniques as yet!
Cryosat LRM TWLE (from RADS)0.4 m
Note: not reprocessed with coastal altimetry techniques as yet!
Cryosat LRM TWLE (from RADS)0.55 m
Note: not reprocessed with coastal altimetry techniques as yet!
Hurricane Sandy storm surge flooding the World Trade Center construction site
Sandy29 Oct 2012
PredictedTides
MeasuredTides
Surge:Measured - Predicted
Lillibridge et al -Hurricane Sandy storm surge
HY-2A only altimeter to capture surge < 12 hr duration
Montauk
HY-2A
Cryosat in SAR mode over Indian coast
In acquisition Mask 3.4Since Oct 1, 2012
highly vulnerable coastline, surges are one of the deadliest phenomena in the areasatellites can provide much needed information to improve the modelling+forecasting
highly vulnerable coastline, surges are one of the deadliest phenomena in the areasatellites can provide much needed information to improve the modelling+forecasting
SAR mode: increased accuracy and along-track resolution
Cyclone Mahasen
How to use the altimetric data• Blended with Tide Gauges : most naturally used for
assimilation into models, hindcasts/forecasts• Single altimetric profiles:
– verification– Ensemble pruning
• NRT:– challenging! (error on NRT orbit)– there may be cases where the relative height profile is still
useful in an assimilation exercise (Example: Adriatic, to get the seiche phase right)
– remains useful for verification in real time– is being pioneered over Indian Coast and North Sea in
eSurge Live, from Summer 2013
TG1
TG2
Blending Altimetry with Tide Gauges
Technique developed by DMI
1.combine (‘blend’) long time series of altimeter(delayed) and TG to extract modes of variability along altimeter track
2.then use Real-Time TG data to estimate ‘blended’ Real-Time level along altimeter track
see Jacob Høyer’s talk
Conclusions• Storm surges natural application for coastal
altimetry– in synergy with in situ measurements an model
• ESA supporting further development of processor via eSurge
• Specialized retrackers improve TWLE retrieval near coastSAR Altimetry (Cryosat-2) also very promising
• Data need to be exploited in synergy with in situ & models to make up for inadequate sampling