Comparison of CryoSat-2 and ENVISAT radar freeboard over Arctic sea ice: toward an improved Envisat freeboard retrieval

Abstract. Over the past decade, sea-ice freeboard has been monitored with various satellite altimetric missions with the aim of producing long-term time series of ice thickness. While recent studies have demonstrated the capacity of the CryoSat-2 mission (2010–present) to provide accurate freeboard measurements, the current estimates obtained with the Envisat mission (2002–2012) still require some large improvements. In this study, we first estimate Envisat and CryoSat-2 radar freeboard by using the exact same processing algorithms. We then analyse the freeboard difference between the two estimates over the common winter periods (November 2010–April 2011 and November 2011–March 2012). The analysis of along-track data and gridded radar freeboard in conjunction with Envisat pulse-peakiness (PP) maps suggests that the discrepancy between the two sensors is related to the surface properties of sea-ice floes and to the use of a threshold retracker. Based on the relation between the Envisat pulse peakiness and the radar freeboard difference between Envisat and CryoSat-2, we produce a monthly CryoSat-2-like version of Envisat freeboard. The improved Envisat data set freeboard displays a similar spatial distribution to CryoSat-2 (RMSD  =  1.5 cm) during the two ice growth seasons and for all months of the period of study. The comparison of the altimetric data sets with in situ ice draught measurements during the common flight period shows that the improved Envisat data set (RMSE  =  12–28 cm) is as accurate as CryoSat-2 (RMSE  =  15–21 cm) and much more accurate than the uncorrected Envisat data set (RMSE  =  178–179 cm). The comparison of the improved Envisat radar freeboard data set is then extended to the rest of the Envisat mission to demonstrate the validity of PP correction from the calibration period. The good agreement between the improved Envisat data set and the in situ ice draught data set (RMSE  =  13–32 cm) demonstrates the potential of the PP correction to produce accurate freeboard estimates over the entire Envisat mission lifetime.

[1]  D. Rothrock,et al.  Thinning of the Arctic sea‐ice cover , 1999 .

[2]  Robert Ricker,et al.  About the consistency between Envisat and CryoSat-2 radar freeboard retrieval over Antarctic sea ice , 2015 .

[3]  Michael Studinger,et al.  An Improved CryoSat-2 Sea Ice Freeboard Retrieval Algorithm Through the Use of Waveform Fitting , 2014 .

[4]  R. Keith Raney,et al.  The delay/Doppler radar altimeter , 1998, IEEE Trans. Geosci. Remote. Sens..

[5]  Seymour W. Laxon,et al.  Sea ice altimeter processing scheme at the EODC , 1994 .

[6]  Angelika Humbert,et al.  Elevation and elevation change of Greenland and Antarctica derived from CryoSat-2 , 2014 .

[7]  Kevin Guerreiro,et al.  Potential for estimation of snow depth on Arctic sea ice from CryoSat-2 and SARAL/AltiKa missions , 2016 .

[8]  Humfrey Melling,et al.  Measurements of the Underside Topography of Sea Ice by Moored Subsea Sonar , 1995 .

[9]  Vladimir F. Radionov,et al.  Snow Depth on Arctic Sea Ice , 1999 .

[10]  Robert Ricker,et al.  Sensitivity of CryoSat-2 Arctic sea-ice freeboard and thickness on radar-waveform interpretation , 2014 .

[11]  John C. Ries,et al.  Chapter 1 Satellite Altimetry , 2001 .

[12]  Ola M. Johannessen,et al.  The relation between sea ice thickness and freeboard in the Arctic , 2010 .

[13]  L. Phalippou,et al.  CryoSat: A mission to determine the fluctuations in Earth’s land and marine ice fields ☆ , 2006 .

[14]  Ron Kwok,et al.  Annual cycles of multiyear sea ice coverage of the Arctic Ocean: 1999–2003 , 2004 .

[15]  Duncan J. Wingham,et al.  Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre , 2012 .

[16]  Natalia Galin,et al.  Sea ice thickness, freeboard, and snow depth products from Operation IceBridge airborne data , 2013 .

[17]  Malcolm Davidson,et al.  CryoSat‐2 estimates of Arctic sea ice thickness and volume , 2013 .

[18]  Neil Peacock,et al.  High interannual variability of sea ice thickness in the Arctic region , 2003, Nature.

[19]  Seymour W. Laxon,et al.  Sea surface height determination in the Arctic Ocean from ERS altimetry , 2004 .

[20]  Greg Holloway,et al.  Has Arctic Sea Ice Rapidly Thinned , 2002 .

[21]  Stefan Kern,et al.  The impact of snow depth, snow density and ice density on sea ice thickness retrieval from satellite radar altimetry: results from the ESA-CCI Sea Ice ECV Project Round Robin Exercise , 2015 .

[22]  R. Raney.  A delay/Doppler radar altimeter for ice sheet monitoring , 1995, 1995 International Geoscience and Remote Sensing Symposium, IGARSS '95. Quantitative Remote Sensing for Science and Applications.

[23]  Duncan J. Wingham,et al.  Increased Arctic sea ice volume after anomalously low melting in 2013 , 2015 .

[24]  Marta Zygmuntowska,et al.  Waveform classification of airborne synthetic aperture radar altimeter over Arctic sea ice , 2013 .

[25]  Seymour W. Laxon,et al.  Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum , 2008 .

[26]  L. N. Connor,et al.  Comparison of Envisat radar and airborne laser altimeter measurements over Arctic sea ice , 2009 .

[27]  R. Kwok,et al.  Variability of Arctic sea ice thickness and volume from CryoSat-2 , 2015, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[28]  Josefino C. Comiso,et al.  Large Decadal Decline of the Arctic Multiyear Ice Cover , 2012 .

[29]  F. Lyard,et al.  FES 2014, a new tidal model on the global ocean with enhanced accuracy in shallow seas and in the Arctic region , 2015 .

[30]  Ron Kwok,et al.  Decline in Arctic sea ice thickness from submarine and ICESat records: 1958–2008 , 2009 .