Coastal Altimetry: past, present an future - A Review Presented by Stefano Vignudelli Consiglio Nazionale delle Ricerche, Italy vignudelli@pi.ibf.cnr.it With invaluable help (and material) from COASTALT and eSurge Projects S. Vignudelli, A. Scozzari (CNR, Italy) P. Cipollini, C. Gommenginger, H. Snaith, S. Gleason, G. Quartly, L. West (NOCS, UK) Henrique Coelho (Hidromod, Portugal) J. Fernandes , L. Bastos, C. Lázaro, A. Nunes. N. Pires, I. Araujo (U Porto, Portugal) M. Bos (CIIMAR, Portugal) S. Barbosa (U Lisbona, Portugal) Jesus G óm ez-Enri (U C ádiz , Spain) C. Martin-Puig, M. Caparrini, L. Moreno (Starlab, Spain) P. Woodworth, J. Wolf (POL, UK) S. Dinardo, B. M. Lucas (SERCo/ESRIN, Italy) J. Benveniste (ESA/ESRIN, Italy) Stefano Zecchetto, Francesco De Biasio (CNR-ISMAR, Italy) Georg Umgiesser, Marco Bajo (CNR-ISAC, Italy) Alvise Papa (Centro Maree, Venice, Italy) particular thanks also go to Ron Abileah (jomegaK) and to the Coastal Altimetry Community 1 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Outline of my talk Introducing Coastal Altimetry Closer to coasts – why A look back – key milestones A lively community Coastal Altimetry in action A little on fundamentals of satellite radar altimetry What the re-analysis tells us More, better and closer to coasts – how Error budget Moving from research to routine use Coastal altimetry products Showcase of emerging applications A summary What we have learned Recommendations from Coastal Altimetry Workshop series 2 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
The “Google Earth” effect: from global to local 3 3 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Why “Coastal Altimetry” now? Coastal Zone Coastal Zone uncharted domain ∼ 20 yrs multi-mission archive Coastal Zone of strategic There is much interest in bringing importance altimetry to the coastline most people live there Not only for using in synergy with source of food and raw materials modelling tools and other data sources, vital link to transport and trade but also to understand the error budget in global sea level rise when altimeters host valuable habitats and landscape are tied to coastal tide gauges for favored destination for leisure calibration. A hope at horizon: progresses in Key impacts are where policy decisions are usually made technology New techniques (Delay-Doppler, Interferometry, Reflectometry) New concepts (Constellations) New missions (AltiKa, CryoSat-2, Sentinel-3, SWOT) will have much greater coastal capabilities 4 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
What do we mean by “Coastal Altimetry” ? Coastal domain based on Jason-1 tracks We define coastal (courtesy: PISTACH) altimetry as altimetry over that ocean domain close to land where standard processing is problematic (information is somehow hidden) These data are normally flagged as ‘bad’ in official products for a number of reasons non-standard waveforms, inaccurate corrections, etc. -250 -5000 Bathymetry (m) These data can – and should - be recovered! and users are actually asking for them! 5 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Coastal Altimetry – a bit of history Some seminal papers Manzella et al. 1997 - custom wet tropospheric correction Crout 1998 - could recover data when coastal topography is flat Anzenhofer et al. 1999 – retracking waveforms Vignudelli et al. 2000 - Signal recovered consistent with in situ data ALBICOCCA France-Italy-UK 2001/04 ALTICORE-EU Feasibility EU/INTAS 2006/08 Capacity building DATA MAP/XTRACK/MARINA ALTICORE-India CNES/LEGOS/CTOH available ALTICORE-Africa Integrated approach PRODUCT DEVELOPMENT STUDIES INCLUDING RETRACKING now following with eSurge PISTACH COASTALT ( multimission, 2011-2014 ) CNES 2007-present ESA 2008-present For Jason-2 For Envisat …plus several OSTST Projects funded by NASA and CNES 6 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Coastal Altimetry – at centre of the community Regular workshops (Silver Spring 2008, Pisa 2008, Frascati 2009, Porto 2010, Pisa 2008 San Diego 2011, Riva del Garda, 2012, Boulder 2013, Lake Constance 2014) – see at www.coastalt.eu ) Springer book published San Diego 2011 (see TOC at http://www.springer.com ) 7 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Recap – how satellite altimetry works Satellite SSH = S - R corr Range (R) must be corrected for various Orbit (S) effects, with equation of form: R corr = R altimeter + R instr + R atmos + R surface R instr Instrument dependent (e.g. USO, Doppler) Range (R) R atmos Atmospheric path corrections given by: R atmos = R iono + R wet tropospheric + R dry tropospheric R surface Surface dependent given by: R surface = R SSB Sea Surface Bias SSH = S - R = G + h Some applications require correction of high-frequency signals (tides, wind and air pressure) h SSH R surface = R SSB + R tides + R wind-pressure G Reference Geoid Ellipsoid 8 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
But the Sea Surface Height (SSH) contains the geoid signal!! SSH is composed of a variable oceanic part, Satellite the Absolute Dynamic Topography (ADT), and of a geophysical constant the Geoid. These latter deals with the position of the ocean at rest. Orbit (S) Its small scales are not known with enough accuracy to permit the separation of the two components of the SSH. Consequently, SSH is decomposed into a Range (R) Mean Sea Surface (MSS) and a Sea Level Anomaly (SLA) SLA which takes into account the variation of height around the MSS due to the variability of the ocean currents: SSH = S - R = G + ADT SSH =MSS + SLA =G+ADT The MSS contains then both the Geoid and the permanent part of the ADT called the Absolute Mean Dynamic Topography (MDT) which DynamicTopography deals with the stationary part of the ocean SSH currents. Its knowledge permits to bypass the ADT Geoid to study the ADT of the ocean: G ADT = MDT + SLA Reference Geoid Ellipsoid 9 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
History of satellite altimetry accuracy in open ocean Courtesy by L.L. Fu, NASA 10 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
TOPEX Latest Error Budget 11 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
One picture is worth 1000 words - Starting point … really no data ? From this… To this… 12 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Beyond flags: new editing strategy Circles : uncorrected sea level anomalies (SLA) and original corrections from the AVISO Geophysical Data Records (GDR). Brown line : SLA after application of the standard corrections from the GDR. Purple line : the new SLA profile computed Screening profiles rather than single values Reconstructing /extrapolating profiles where possible Much more data on average than using standard editing 13 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Lesson learned I - Reprocessing standard along-track products A significant part of data is recoverable just de-flagging, filtering, editing, re-interpolation of corrections 14 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Recap - What are we measuring? In situ The altimeter system is just a more complicated radar tide gauge mounted on satellite Need of additional data (e.g. orbits and corrections) But more uses (waves, winds, currents, bathymetry in addition to sea level) Averages over footprints vs Point point-wise Sampling of order of days vs min/hour It sends a microwave pulse towards the ocean surface, f = 13.5 Ghz Each individual return signal or echo (known as waveforms ) is very noisy This is a result of random distribution of the ocean wave facets at any instant 15 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Averaging many successive pulses can reduce noise Envisat is averaging 100 echoes on board then trasmitted on ground over 1/18 second of flight This means measurements every 350 m along track However, data are furtherly averaged on ground over 1 second of flight This means measurements every 7 km along track 7 km is the standard resolution for use in open ocean Source: ESA 16 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
Real radar return signals (waveforms) in open ocean RA- 2 altimeter on ESA’s Envisat 17 Joint COSPAR and WMO Capacity Building Workshop – 20 July – 1 August
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