Summer Antarctic sea ice as seen by ASAR and AMSR-E and observed during two IPY field cruises: a case study Ahmet E. TEKELI, 1 Stefan KERN, 2 Stephen F. ACKLEY, 1 Burcu OZSOY-CICEK, 1 Hongjie XIE 1 1 Department of Geological Sciences, University of Texas at San Antonio, 1 UTSA Circle, San Antonio, TX 78249, USA E-mail: ahmet.tekeli@utsa.edu 2 Center for Marine and Atmospheric Science, CliSAP/KlimaCampus, University of Hamburg, Grindelberg 5, D-20144 Hamburg, Germany ABSTRACT. Envisat Advanced Synthetic Aperture Radar (ASAR) Wide Swath Mode (WSM) images are used to derive C-band HH-polarization normalized radar cross sections (NRCS). These are compared with ice-core analysis and visual ship-based observations of snow and ice properties observed according to the Antarctic Sea Ice Processes and Climate (ASPeCt) protocol during two International Polar Year summer cruises (Oden 2008 and Palmer 2009) in West Antarctica. Thick first-year (TFY) and multi-year (MY) ice were the dominant ice types. The NRCS value ranges between –16.3  1.1 and –7.6  1.0 dB for TFY ice, and is –12.6  1.3 dB for MY ice; for TFY ice, NRCS values increase from –15 dB to –9 dB from December/January to mid-February. In situ and ASPeCt observations arenot, however, detailed enough to interpret the observed NRCS change over time. Co-located Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) vertically polarized 37 GHz brightness temperatures (TB37V), 7 day and 1 day averages as well as the TB37V difference between ascending and descending AMSR-E overpasses suggest the low NRCS values (–15 dB) are associated with snowmelt being still in progress, while the change towards higher NRCS values (–9dB) is caused by commencement of melt– refreeze cycles after about mid-January. INTRODUCTION Sea ice is a sensitive indicator of climate change (Lubin and Massom, 2006). With its high albedo relative to the ocean and low thermal conductivity, it regulates the ocean–atmos- phere heat, moisture, and energy exchange (Smith and others, 1990; Meier and Stroeve, 2008). Variations in sea-ice albedo and these exchanges are difficult to monitor on a basin-wide scale. Satellite data can support the determin- ation of the sea-ice albedo and the mentioned fluxes by providing detailed ice-property distribution information by, for example, classification of different ice types, mapping of the sea-ice roughness distribution, or identification of leads and polynyas (Jeffries, 1998; Lubin and Massom, 2006). Detailed ice information at fine spatial resolution can be provided by satellite synthetic aperture radar (SAR) operating at a frequency of 5.3 GHz (wavelength 5.6 cm), i.e. in C-band (Onstott and Shuchmann, 2004; Rees, 2006; Johannessen and others, 2007). Ice types and radar backscatter values obtained with C-band SAR or satellite scatterometer were correlated in various studies for the Arctic and Antarctic (Carsey and others, 1992; Kwok and others, 1992; Jeffries and others, 1995; Drinkwater and Lytle, 1997; Tsatsoulis and Kwok, 1998; Drinkwater and Liu, 2000; Kwok and others, 2003). During (cold) winter conditions ice-type discrimina- tion is relatively straightforward because the typically shallow and dry snow cover has only a small influence on the radar backscatter at C-band. However, during summer or summer-like conditions this discrimination is hampered by an increasing influence of the snow cover. Haas (2001) investigated C-band scatterometer data obtained over perennial sea-ice regions around Antarctica and found a marked increase in C-band radar backscatter values by, on average, 5.6 dB between spring (–16.3dB) and summer (–10.7 dB); sudden drops in C-band radar back- scatter values during spring/summer were also observed. According to Haas (2001), the increase is most likely caused by snow meltwater. A layer of superimposed ice is formed from refreezing snow meltwater percolating through the snow layer. This ice layer contains air bubbles and is rough at the centimeter scale. The above-mentioned sudden drops, on the other hand, can be associated with periods of a meltwater-saturated surface snow layer. These observations are confirmed by Kawamura and others (2006) who investi- gated C-band SAR data in relation to the snow property changes on landfast sea ice in Lu ¨tzow-Holm Bay, Antarctica, and who reported minimum C-band normalized radar cross- section (NRCS) values during mid-summer and a remarkable increase in the NRCS values at the end of summer. Willmes and others (2006, 2009) investigated snow properties on second-year ice in the Weddell Sea during mid-summer. They confirmed the initial decrease in C-band NRCS values once snowmelt commences and observed an increase in the NRCS values as summer progresses. This increase was caused by an increase in snow-grain size due to snow metamorphism triggered by diurnal melt–refreeze cycles. In addition to an increase in C-band NRCS values, brightness temperatures (TB) measured by the Special Sensor Microwave/Imager (SSM/I) at a frequency of 37 GHz, vertical polarization, start to vary diurnally once melt– refreeze cycles have set in. Willmes and others (2009) used this diurnal variation, together with the fact that an increased snow wetness also causes a substantial increase in TB(37 GHz) values (see Garrity, 1992), to characterize the nature and map the progress of snowmelt on Antarctic sea ice. On Antarctic sea ice, surface flooding and formation of a slush layer on top of the sea ice under the submergence of Annals of Glaciology 52(57) 2011 327