Fleury, Sara

Guerreiro, K.

Heinemann, Günther

Kouraev, A.

Rémy, F.

Willmes, Sascha

Zakharova, E. A.

OceanRep

Leads detection using SARAL/AltiKa - Comparison with Modis segmented data S. Fleury1, Zakharova E.A.1,2, K.Guerreiro1, F.Rémy1, S.Willmes4, A.Kouraev 3,2, G. Heinemann 4 Objectives Results and Discussion 1) CNRS; LEGOS, F-31400 Toulouse, France 2) State Oceanography Institute, St. Petersburg branch, Russia 3) Universite de Toulouse; UPS (OMP-PCA), LEGOS, F-31400 Toulouse, France 4) University of Trier, Germany Contact: sara.fleury@legos.obs-mip.fr Data and Method MODIS/TIR SARAL/AltiKa Physics T° Wvf (backscatter)  + height Spatial resolution  1km 170m (footprint 4km) Spatial  coverage 90°N 82°N Limitations Clouds Melting-pounds  Superficially  freezed water Melting-pounds  Period daily monthly Improvement of two fundamental parameters for global climate studies •  Arctic sea mean level •  Seaice volums   Climatology of these parameters over the 15 years of the altimetry •  ERS1 & 2 (ku band) :     1991 - 2002 •  Envisat  (ku band) :      2002 - 2010 •  Cryosat2  (ku band, SAR-altimetry) :  2010-… •  Saral/Atika (ka band) :     2013-… •  Sentinel 3 (ku band, SAR altimetry) :  2015-… Methodology: 1.  Leads localisation 2.  Mesurement of sealevel in leads and polynyas and of the seaice height 3.  Computation of freeboards  4.  Deduction of volums using knowledges relative to: •  ice density (through ice age) •  snow depth,  •  seaice surface spreads Conclusion The difficulties to obtain reliable and large reference dataset on seaice claims for inter comparision and validation of the measurments and observations. This study has demonstrated the contribution of such analysis.  The key role of seaice on the climate should make such collaborations even more precious. Need for collaborations for: •  Snow depth •  Type/age of ice (density) •  Spatial Multi-sensors (passive/active) synergy is need to detect and to measure the geometry of the leads and polynyas (freeboard, length, distributions). distribution of the ice field structure Good agreements between Saral and Modis for the leads  Concentration and dynamics at regional level: MODIS (T°) AltiKa (PM) Laptev sea Kara sea Beaufort sea Possible causes of discrepancy •  From spatial resolution:  •  MODIS: can miss small leads (<1km) •  MODIS: can miss leads/polynyas along coast •  From measurement technics:  •  MODIS: can miss re-frozen (thermic isolation of free-water) •  SARAL: can mixed-up free-water of large polynyas with seaice •  SARAL is more sensitive to the edges of the leads  •  (still water with strong backscatter) while  •  MODIS is more sensitive to the centers of the leads with  •  Higher temperature contrasts with the seaice Impact of the resolution and/or target favours: Both methods observe the leads but not at the same time. -> instantaneous observation is not always coherent -> the coherency is maintained “in average” ‘Leads’ MODIS ‘Leads’  ALTIKA Along-track discrepencies seaice Probable leads leads land A one day map of MODIS/TIR segmented data,  with about 30 SARAL tracks colocalized on the same day Colocalized Saral tracks. The color codes the value of the backscatter along-track.  We have good agreements between the strong backscatters of Saral/Altika altimeter and the leads segmented within MODIS/TIR images. zoom seaice all  leads « sure »  leads 50 60 70 80 18% Total population 78 … but along-track discrepancies: P max de la FO Formes d’onde Latitudes le long de la trace Image Landsat 8 Puissance max de la FO Along-track waveforms Along-track Max Power Max Power values Landsat 8 image (30m res.) Some MODIS/TIR & Saral/AltiKa caracteristics Principle of leads detection with Saral/AltiKA MODIS Saral Some visual checkings on Modis VNIR images  http://earthdata.nasa.gov/data/near-real-time-data/visualization/worldview Some times Saral/AltiKA failed: on left image, the polynyas above is detected as a ‘sure’ lead or a probable lead (green circle), wheras Saral (on the right image) does not see it, according to the Maximum Power criteria. It is well known limitation of the approch. Indeed MP over free-water is between 48 to 55. On going work: •  Improvement of leads detection using  altimetry for the various technologies : -  ka/ku bands -  nadir SAR (cryosat2) •  Measurement of sea level and seaice heights using adapted retracker. •  Multi-missions mesurement to improve      the coverage The value of the threshold for the Maximum Power (MP)  that we have choosen to detect leads is 78.  This histogram shows that the percentage of leads gets higher than the percentage of seaice for this value (second plot). However, with this criteria, only 18% of data identified as ‘leads’ by Modis are also detected as leads by Saral (bottom plot). MODIS Saral Saral: 2 weeks  orbit coverage Saral + Cryosat2:  2 weeks orbit coverage … and some times the failures impact MODIS: in the image above Modis on top)  does not perceveive the free water polynyas (red circle), nor the field of ice (green circle).  

Leads detection using SARAL/AltiKa - comparison with segmented MODIS thermal images

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