Presented by Stefano Vignudelli

Transcription

1 Satellite radar altimetry for monitoring Caspian sea level changes Lessons learned from the EU/INTAS ALTICORE project Presented by Stefano Vignudelli Consiglio Nazionale delle Ricerche, Italy Contributors: Lyard F., Roblou L., Cretaux F., Testut L., Calzas F. (LEGOS, France), Cipollini P., Snaith H., Venuti F. (National Oceanography Centre Southampton, UK) Kostianoy A., Sheremet N., Ginzburg A., Kuzmina E. (Shirshov Institute of Oceanology, Russia) Lebedev S., Sirota A., Medvedev D., Khlebnikova S. (Geophysical Institute, Russia) Mamedov R., Ismatova K., Alyev A., Nabiyev T. (Institute of Geography, Azerbaijan) Satellite ALTImetry for COastal REgions NATO ARW Ferrara December 2-4 1

2 Outline of my talk Satellite Altimetry in few words Open ocean Coastal zone ALTICORE Project in a nutshell Who, what, where, how Lesson learned in the Caspian Sea Challenges with sea level data Some examples Preliminary results from a pilot sea level station Beyond ALTICORE Initiatives Summary a personal view NATO ARW Ferrara December 2-4 2

3 How does satellite altimetry work? Satellite What are we measuring? The Altimeter is nadir-poiting radar Not Images but points along a track! Orbit S R = ½ ct Radar Return Signal Open Ocean SSH Reference Ellipsoid η G Geoid Individual return signals are averaged on board (e.g. by 100 in Envisat) then trasmitted on ground at a rate, e.g. for Envisat of 18 hz (1/18 second of flight) which means measurements every 350 m along track but usually available (in open ocean) as averages over 1 second of flight, i.e. 7 km along track The Altimeter has low revisit (10 days and more) Satellite altimetry is success story in open ocean and next step is NATO ARW Ferrara December 2-4 3

4 Meeting challenges in other areas Coastal Zone, Inland Seas, Rivers, Sea Ice Areas 17 yrs multi-mission archive Not fully exploited Normally flagged as bad in the official products Common technical issues Contamination of radar footprints Inadequacy of even failure of some auxiliary corrections Re-thinking the quality control strategy Source: COASTALT Envisat data Coastal Zone There is much interest in bring altimetry to the coastline Not only for using in synergy with modelling tools and other data sources, but also to understand the error budget in global sea level rise when altimeters are tied to coastal tide gauges for calibration. A hope at horizon: progresses in technology promise better resolution capability New techniques (Delay-Doppler, Interferometry, Reflectometry) New concepts (Constellations) A good thing is that future sensors (AltiKa, WSOA, SIRAL on Cryosat, Sentinel-3 altimeter ) are being designed with an eye to coastal zone NATO ARW Ferrara December 2-4 4

5 Coastal Altimetry a bit of story Some seminal papers Crout could recover data when coastal topography is flat Anzenhofer et al retracking waveforms Vignudelli et al Signal recovered consistent with in situ data Started first program in 2001 ALBICOCCA - ALtimeter-Based Investigations in COrsica, Capraia and Contiguous Area NW Mediterranean proposed as the operational lab for coastal altimetry Backward reanalysis of standard products (1HZ i.e. 7 km) A revised post-processing strategy Precursor of other projects ALTICORE ( ) More here today to replicate in Caspian, Black, White, Barents, India and Africa COASTALT (2008 now) Focus on Envisat mission at 18 Hz (350 m) NW Med, Iberian and West Britain are the pilot sites PISTACH ( ) Focus on Jason mission Global coasts & inland waters NASA OST ST call (2008-now) 3 projects on Coastal Ocean and other three ones coastal-related & at centre of the community Regular workshops Silver Spring 2007, Pisa 2008, Frascati 2009, suggested Porto 2009) see at Springer Book 20 chapters, 70 people involved see at One of the chapters is about Caspian Sea NATO ARW Ferrara December 2-4 5

6 ALTICORE - Cooperation Funded by EU under INTAS Program Started on December 2006 for for a period of two years Coordinating partner: Consiglio Nazionale delle Ricerche (Italy) EU partners involved: National Oceanography Centre Southampton (UK) Laboratoire d'etudes en Géophysique et Océanographie Spatiales (France) Three Eastern partners took part: P. P. Shirshov Institute of Oceanology (Russia) Geophysical Center (Russia) Institute of Geography (Azerbaijan) Aiming at providing altimeter data more, better, closer coasts enhancing the capacity of Eastern partners to exploit data This is done in a number of regional seas Mediterranean, Black, Caspian, White, Barents Three priority topics for action in the Caspian Sea: Getting the local information (e.g., in situ data, knowledge of metocean conditions, etc.) Improving the processing (editing, corrections, etc.) Making access to altimeter data easier NATO ARW Ferrara December 2-4 6

7 ALTICORE - running Teams met in two occasions Progress Meeting Sept 2007 hosted at Institute of Geography (Baku, Azerbaijan) Final Meeting Sept 2008 held at Castiglioncello (Livorno, Italy) Project supported exchange visits of team members to cooperate on specific topics, e.g.: White and Barents seas (NOCS & GC) Development of a new local tidal correction Caspian Sea (CNR & LEGOS & IG) Validation of the altimetric products NATO ARW Ferrara December 2-4 7

8 Caspian Sea in situ sea level infrastructure Concerns: Where are data located? In what quantity? Of what quality? Who owns data? How to access? What we have seen: Different sampling Manual recording Often gappy or collection stopped Just tables (no metadata) Language differences Jiloy, Zhiloy, Chilov same station!!! Source: Caspian Env Programme 1999 Not surprised!!! We liaised with scientists, who play with numbers. They are not data producers! BUT.the quality and usability of the altimeter-derived observations is dependent upon good calibration/validation of the satellite sensors with in situ observations. NATO ARW Ferrara December 2-4 8

9 Caspian Sea Water level variability from in situ data Baku Longest record at Baku (since 1837) Shows a rising of more than 2 metres between 1977 and 1995 Now stabilized near the 27 m level Understanding variability at all scales a complicated puzzle Combination of factors: climatic (atmospheric variations), anthropogenic (e.g. river drainage and water use, especially Volga), geologic (e.g. subsidence) The question - Will the Caspian sea level rise again? NATO ARW Ferrara December 2-4 9

10 Makhachkala Caspian Sea Comparison at different stations Makhachkala - Krasnovodsk Kulaly Island Krasnovodsk Ogurchinsky Island Assumed Makhachkala as reference Good agreement with observations at Krasnvodsk and Ogurchinsky Some discrepances at Kulaly island To be noted: The Northern Caspian presents peculiar metocean dynamics with strong storm surges over shallow waters (averaging just 4 meters) difference (m) difference (m) difference (m) Date (year) Makhachkala - Ogurchinsky date (year) Makhachkala-Kulaly Date (year) NATO ARW Ferrara December

11 Caspian Sea Multi-Mission Altimetry Ground Track Coverage Fixed issues: Radar Altimeter land contamination of footprint (0-10 km) Wet Tropo correction (radiometer) land contamination of footprint (0-50 km) Dry and Wet Tropo corrections (from ECMWF/NCEP fields) erroneous altitude in the algorithm (e.g. T/P) Mean Sea Surface (CLS01) not usable to compute SLAs Aliasing of high-frequency variability (weather) - mis-modeled or un-modeled Varying features (Northern Caspian): Uncertainties in the retrieval (surge or spike) Sea waters frozen for around 4-5 months (from November to March) Unknown features: Land/sea transition (high res DEM and Bathymetry) NATO ARW Ferrara December

12 47 Caspian Sea Ice in altimeter and radiometer footprints Wet Troposphere from satellite radiometer 46 Chistaya Banka Banka Is. Island North latitude 45 Tyuleniy Is. 44 p Jason-1 pass 92 East longitude Backscatter Coefficient White band - Island effects? Black dots Ice Edges from AMSR-E/Aqua data (available since 2004) NATO ARW Ferrara December

13 Caspian Sea Mean Sea Surface (MSS) Sea Level Anomaly computed from standard MSS is good for deep basins but not for shelves and slopes Where & Why? Sea level (m) Time intervals used to construct different MSS MSS Model: CSR95 OSU95 GSFC00 CLS_SHOM98 CLS Time (year) Starting computing local MSS (GCRAS06) NATO ARW Ferrara December

14 Caspian Sea Water level variability from multi-mission altimetry Based on 1 Hz data Referred to Baltic Sea level reference frame Shows seasonal variations probably due to climatological (evaporation & precipitation) and hydrological (river runoff, discharge to Kara Bogaz Gol) cycle Inter-annual tendencies might be interpreted in the light of decadal climate variability and amplification of irrigation in the Volga river basin NATO ARW Ferrara December

15 Caspian Sea Comparison of TOPEX/Poseidon altimetry (red) and in situ stations (black) Sea level (m) -26,5-26,6-26,7-26,8-26, ,1-27,2? Time (year) Agree well with ground truth at basin scale, but still some local «mysteries» (e.g. 70 mm in 1995) Errors in altimetry ~15 mm/yr Possible uncertainty in the global lake level deduced from in situ stations Need of a permanent Cal/Val site for satellite altimetry NATO ARW Ferrara December

16 Sea level pilot station operating at Absheron Port (Baku) Satellite altimetry ground tracks Jason (red), Envisat (white), GFO (green), T/P (yellow) NATO ARW Ferrara December

17 Some pictures about field work in June 2008 Started to collect data in June 2008 and still ongoing Processed the first 8 months Work in progress.. NATO ARW Ferrara December

18 Sea level variability at Absheron Port During the observational period, the sea level at Port Absheron exhibits a large high frequency variability apparently more pronounced during summer time There are also other oscillations superimposed that need careful investigation The spectrum analysis shows the existence of a little tidal contribution to the sea level The large contribution is observed in the 3-30 day band, possibly linked to the meteorological forcing NATO ARW Ferrara December

19 Caspian Sea - Beyond ALTICORE New EU/FP7 Coordination Action CASPINFO - (started on Sep 2008) Fostering improved co-operation between research institutes, oil & gas industries, and international bodies in the Caspian Sea MARIS (Netherlands) is the coordinator Brings together partners from riparian countries (Russia, Azerbaijan and Kazakhstan) + EU ( Greece, Italy) + IOC-IODE, CEP and BSCS More at Raising funding (jointly with LEGOS) to make the pilot station GLOSS-compliant, including near real time transmission, permanent GPS Station to control the vertical motion of the sea level sensor and regular offshore leveling along the altimetry ground tracks Recommendation to ESA for Sentinel-3 Caspian Sea is currently the best natural target for calibration over continental water bodies being a big lake with favorable location of satellite tracks and cross-over points Caspian Sea be used as testbed for developing a cal/val site for lakes. No similar optimality of any existing cal/val site in Europe would satisfy these requirements. NATO ARW Ferrara December

20 Summary Sea level in the Caspian Sea Topic of growing concern to all five surrounding countries (Azerbaijan, Republic of Iran, Kazakhstan, Russian Federation and Turkmenistan) Several low-lying and densed-populated coastal areas Satellite altimetry in the Caspian Sea Conceptually simple, but challenged by specific processing Measuring sea level changes beyond political barriers In situ sea level measurements in the Caspian Sea Critical for the generation of accurate altimeter-derived estimates Not only numbers but also knowing how they were generated Additional in situ resources in the Caspian Sea Key role to support analysis of the various met-ocean-hydro contributions Oil & Gas sector interested - it would be an important player Data are only one piece of the puzzle We know the sea level story We don t know What-if Remote Sensing in the Caspian Sea would be an important factor of integration and a valuable opportunity for data sharing NATO ARW Ferrara December