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

摘要

pstrongAbstract./strong 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 (2010a??present) to provide accurate freeboard measurements, the current estimates obtained with the Envisat mission (2002a??2012) still require some large improvements. brbr 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 2010a??April 2011 and November 2011a??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. brbr 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 (RMSDspan class="thinspace"/spana??=a??span class="thinspace"/span1.5span class="thinspace"/spancm) during the two ice growth seasons and for all months of the period of study. brbr 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 (RMSEspan class="thinspace"/spana??=a??span class="thinspace"/span12a??28span class="thinspace"/spancm) is as accurate as CryoSat-2 (RMSEspan class="thinspace"/spana??=a??span class="thinspace"/span15a??21span class="thinspace"/spancm) and much more accurate than the uncorrected Envisat data set (RMSEspan class="thinspace"/spana??=a??span class="thinspace"/span178a??179span class="thinspace"/spancm). brbr 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 (RMSEspan class="thinspace"/spana??=a??span class="thinspace"/span13a??32span class="thinspace"/spancm) demonstrates the potential of the PP correction to produce accurate freeboard estimates over the entire Envisat mission lifetime./p.