The following abstracts are taken from the proceedings of the following IGSOC sponsored symposia, and they all relate remote sensing of information needed for fundamental glaciology in Antarctica:
International Symposium on Changes in Glaciers and Ice Sheets: observations, modelling and environmental interactions; 28 July–2 August; Beijing, China; Contact: Secretary General, International Glaciological Society
http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htmMass balance of Antarctic ice sheet 1992–2008 from ERS and ICESat: gains exceed losses H. Jay ZWALLY, Jun LI, John ROBBINS, Jack L. SABA, Donghui YI, Anita BRENNER
Corresponding author: H. Jay ZWALLY
Corresponding author e-mail: zwally@icesat2.gsfc.nasa.gov
"During 2003–2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the loss from ice discharge by 73±23 Gt a–1 (3.7% of input), as derived from ICESat laser altimetry. The 131 Gt a–1 gain in East Antarctica (EA) and the 70 Gt a–1 gain in four drainage systems (DS) of West Antarctic (WA2) exceeded combined losses of 98 Gt a–1 from three coastal DS of West Antarctic (WA1) and 28 Gt a–1 from the Antarctic Peninsula (AP). Re-analysis of ERS radar-altimeter data, including a new post-glacial-rebound correction, indicates an even larger overall gain of 120 ± 51 Gt a–1 during 1992–2001. In WA2 and EA, persistent dynamic thickening (deficiency of ice flow relative to long-term accumulation) contributed more than 200 Gt a–1 to the net positive balance in both periods. Consistent with observed outlet-glacier accelerations, loss increases of 38 Gt a–1 in WA1 and 21 Gt a–1 in AP from increased dynamic thinning dominated a gain increase of 9 Gt a–1 from positive accumulation anomalies in WA1 and AP. These decadal-scale changes are small relative to the long-term dynamic thickening in EA and WA2, which may buffer additional dynamic thinning for several decades."
Modeling dynamic thickening in East Antarctica as observed from ICESat Weili WANG, H. Jay ZWALLY, Jun LI
Corresponding author: Weili WANG
Corresponding author e-mail: weili.wang@nasa.gov
"Mass changes of the Antarctic ice sheet derived from ICESat laser altimetry show that during 2003–08 mass gains from snow accumulation exceeded losses from ice discharge by 73 Gt a–1 (0.20 mm a–1 sea level depletion). Results from ERS radar altimetry give a similar net gain of 120 Gt a–1 for 1992–2001. In East Antarctica and four West Antarctic drainage systems, most of the net mass gain is caused by persistent dynamic thickening (excess of long-term accumulation relative to ice flow) at a rate of 207 Gt a–1, and not by contemporaneous increases in snowfall. To investigate the dynamic thickening rate, we apply a 3-D ice-sheet model to the Antarctic ice sheet for the sensitivity experiments with climate change. The model results indicate that the East Antarctic ice sheet has been growing due to increased snowfall after the last ice age. The modeled thickening rate near Vostok is 2.5 cm a–1 for the present time, which is consistent with the observations from ICESat and ERS data. Overall, the model and observations indicate a long-term mass gain for East Antarctica and the interior of West Antarctic, which has been offsetting dynamic losses that have increased in the Antarctic Peninsula and West Antarctica during the last two decades."
Improved Antarctic surface mass-balance remote sensing using ASCAT Alexander D. FRASER, Simon WOTHERSPOON, Hiroyuki ENOMOTO, Neal W. YOUNG
Corresponding author: Alexander D. Fraser
Corresponding author e-mail: adfraser@utas.edu.au
"Large-scale distribution of Antarctic surface mass balance (SMB) is currently poorly understood. High-quality in situ measurements of SMB are sparse, particularly in the interior of the continent. Remote sensing can be used to guide interpolation between in situ measurements. Previously, passive microwave polarization ratio, which is sensitive to the density of horizons of different dielectric properties in the upper snowpack (a proxy for SMB), has been used to guide interpolation of SMB points in Antarctica. We present evidence that maps of alternative parameters may be more suitable maps upon which to base interpolated fields. These maps come from the EUMETSAT Advanced Scatterometer (ASCAT) C-band scatterometer, which was launched in 2007. In particular, we use the ‘A’ (isotropic component of backscatter, sensitive to grain size within the C-band penetration depth of ~20 m) and ‘B’ (linear component of backscatter dependence on incidence angle, sensitive to grain-size profile). Importantly, these maps are sensitive to recently mapped extensive areas of surface wind glaze, which are areas of near-zero net accumulation and thus are less prone to overestimation of SMB compared with earlier large-scale SMB maps. A further focus of this work is a comparison of several statistical interpolation methods, including a careful consideration of the statistical treatment of negative SMB values. A primary output of this work is a new SMB map of the Antarctic continent based on these improved fields."
Synoptic-timescale observations of Antarctic snowfall/wind redistribution events from scatterometer data Alexander D. FRASER, Melissa A. NIGRO, John CASSANO, Neal W. YOUNG, Benoit LEGRESY, Hiroyuki ENOMOTO
Corresponding author: Alexander D. Fraser
Corresponding author e-mail: adfraser@utas.edu.au
"The orbit and swath configuration of the EUMETSAT Advanced Scatterometer (ASCAT) instrument gives C-band backscatter measurements from a wide range of azimuth and incidence angles over most of the Antarctic continent. A 5 day orbital subcycle combined with this excellent observation angle diversity means that complete maps of accumulation-sensitive parameters can be produced on a 5 day basis. Analysis of time series of these parameters reveals several abrupt changes in localized regions, particularly in the ‘A’ parameter (isotropic component of backscatter, which is sensitive to snow grain size) and the ‘M2’ parameter (the magnitude of the second-order Fourier term describing the near-bi-sinusoidal azimuthal response, which is an indicator of the presence/magnitude of sastrugi/other surface microrelief). Using 15 km grid spacing Antarctic Mesoscale Prediction System (AMPS) numerical weather prediction model data, we show these abrupt changes in the ‘A’ and ‘M2’ parameters are associated with snowfall events arising from incursions of air from lower latitudes. Both the ‘A’ and ‘M2’ parameters show a complex response to precipitation events, with both the sign and magnitude of the response depending on wind reworking/redistribution. This observation of changes in near-surface snowpack conditions complements recent results from other authors using GRACE-derived gravity and CloudSat-derived snowfall observations to detect similar snowfall events in East Antarctica."
How accurately can radar altimetry contribute to estimate the Antarctic ice sheet volume and mass balance? B. LEGRÉSY, M. HORWATH, S.R.M. LIGTENBERG, M.R. VAN DEN BROEKE, F. BLAREL
Corresponding author: B. Legresy
Corresponding author e-mail: benoit.legresy@legos.obs-mip.fr
"Knowing the interannual variations of the Antarctic ice sheet net snow accumulation, or surface mass balance (SMB), is essential for analyzing and interpreting present-day observations. For example, accumulation events like the one in East Antarctica in 2009 challenge our ability to interpret observed decadal-scale trends in terms of long-term changes versus natural fluctuations. We developed a higher accuracy time series of radar altimetry with ERS2 and Envisat data from 1995 to 2010. We will present the surface topography variations, the internal error levels for both altimeters and the radar echo and ground miss-repeat corrections made. We show that a different echo correction has to be applied to ERS2 and Envisat as the firn changed in between the two periods of observation. Therefore the possibility to correct the radar altimetry data for echo shape changes from the echo shape is limited, limiting the attainable accuracy of volume change estimates. We illustrate the great potential and limitations of radar altimetry by internal assessment and by comparing with other changes estimates as temporal gravity variations and atmospheric modeling of firn densification. We evaluate the limits of techniques depending on the temporal and spatial scales of interest. SMB variations cause changes in the firn density structure, which need to be accounted for when converting volume trends from satellite altimetry into mass trends. Recent assessments of SMB and firn volume variations mainly rely on atmospheric modeling and firn densification modeling. The modeling results need observational validation, which has been limited until now. Geodetic observations by satellite altimetry and satellite gravimetry reflect interannual firn volume and mass changes, among other signals like changes in ice-flow dynamics. Therefore, these observations provide a means of validating modeling results over the observational period. We present comprehensive comparisons between seasonal and interannual volume variations from radar altimetry and firn densification modeling, and between interannual mass variations from SMB."
Estimated ICESat inter-campaign bias and its impact on the determination of ice-sheet mass balance Donghui YI, H. Jay ZWALLY, John W. ROBBINS, Jun LI, Jack L. SABA, Jinlun ZHANG
Corresponding author: Donghui Yi
Corresponding author e-mail: donghui.yi@nasa.gov
"ICESat operated for 18 campaign periods from March 2003 to October 2009. Most of the operational periods were between 34 and 38 days long. Because of laser failure and orbit transition from 8 day to 91 day orbit, there were four periods lasting 57, 16, 23 and 12 days. Owing to laser characteristic changes (three different lasers, laser energy decreasing with time, the changes in laser pulse shape and beam pattern, etc.), there are range biases (D) between ICESat campaign periods. The long-term trend of the inter-campaign biases (dD/dt) directly affects the derived ice-sheet mass-balance results. In this study, we used the ICESat measured mean sea level over the sea-ice-covered Arctic Ocean to estimate ICESat inter-campaign biases and evaluate the impact of the inter-campaign biases on ice-sheet mass balance. The mean sea level was calculated by averaging the elevation of the leads (open water and thin ice) within the Arctic Ocean sea-ice pack, with waveform saturation correction, inverse barometer correction, dry and wet troposphere corrections, and tidal corrections applied. The ocean dynamic topography effect was also evaluated. We adjusted the derived D by a trend of 0.31 ± 0.07 cm a–1 to account for the current rate of sea-level rise. The resulting mean inter-campaign bias trend (dDsl/dt) from September 2003 to November 2008 (the four full year period of ICESat’s 91 day orbit operation) is –1.60 ± 0.77 cm a–1. Converting this to a volume change rate (dV/dt), we get about 28 km3 a–1 for Greenland and about 198 km3 a–1 for Antarctica. Comparing ICESat elevation profiles over Lake Vostok, Antarctica, with ERS elevation profile over the same region, the bias-corrected ICESat profiles show more consistency than the profiles with no bias correction. The ICESat data used in this study are release version 633 with transmitted pulse Gaussian/Centroid peak location correction (G-C) applied."