C12A-01 INVITED
Optimal estimate of basal properties from surface data on ice streams
An optimal estimation method for simultaneously determining both basal slipperiness and basal topography from variations in surface flow velocity and topography along a flow line on ice streams and ice sheets is presented. Bayesian inference is used to update prior statistical estimates for basal topography and slipperiness using surface measurements along a flow line. The methods requires no smoothing of surface data. The main focus is on how errors and spacing of surface data affects estimates of basal quantities and on possibly aliasing/mixing between basal slipperiness and basal topography. Effects of spatial variations in basal topography and basal slipperiness on surface data can be accurately separated from each other, and mixing in retrieval does not pose a serious problem. For realistic surface data errors and density, small-amplitude perturbations in basal slipperiness can only be resolved for wavelengths larger than about 50 times the mean ice thickness. Bedrock topography is well resolved down to horizontal scale equal to about one ice thickness. Estimates of basal slipperiness are not significantly improved by accurate prior estimates of basal topography. Retrieval of basal slipperiness is found to be highly sensitive to unmodelled errors in basal topography.
C12A-02 INVITED
Equilibrium sensitivities of the Greenland ice sheet inferred from the adjoint of the three- dimensional thermo-mechanical model SICOPOLIS
We present a new and original approach to understanding the sensitivity of the Greenland ice sheet to key model parameters and environmental conditions. At the heart of this approach is the use of an adjoint ice sheet model. MacAyeal (1992) introduced adjoints in the context of applying control theory to estimate basal sliding parameters (basal shear stress, basal friction) of an ice stream model which minimize a least-squares model vs. observation misfit. Since then, this method has become widespread to fit ice stream models to the increasing number and diversity of satellite observations, and to estimate uncertain model parameters. However, no attempt has been made to extend this method to comprehensive ice sheet models. Here, we present a first step toward moving beyond limiting the use of control theory to ice stream models. We have generated an adjoint of the three-dimensional thermo-mechanical ice sheet model SICOPOLIS of Greve (1997). The adjoint was generated using the automatic differentiation (AD) tool TAF. TAF generates exact source code representing the tangent linear and adjoint model of the parent model provided. Model sensitivities are given by the partial derivatives of a scalar-valued model diagnostic or "cost function" with respect to the controls, and can be efficiently calculated via the adjoint. An effort to generate an efficient adjoint with the newly developed open-source AD tool OpenAD is also under way. To gain insight into the adjoint solutions, we explore various cost functions, such as local and domain-integrated ice temperature, total ice volume or the velocity of ice at the margins of the ice sheet. Elements of our control space include initial cold ice temperatures, surface mass balance, as well as parameters such as appear in Glen's flow law, or in the surface degree-day or basal sliding parameterizations. Sensitivity maps provide a comprehensive view, and allow a quantification of where and to which variables the ice sheet model is most sensitive to. The model used in the present study includes simplifications in the model physics, parameterizations which rely on uncertain empirical constants, and is unable to capture fast ice streams. Nevertheless, as a proof-of-concept, this method can readily be extended to incorporate higher-order physics or parameterizations (or be applied to other models). It also opens the door to ice sheet state estimation: using the model's physics jointly with field and satellite observations to estimate a best estimate of the state of the ice sheets.
C12A-03 INVITED
Reconstruction of Past Climatic Changes in Polar Regions using Borehole Paleothermometry
In what is becoming a classic climate-reconstruction method, the temperature-history on the upper boundary of an ice-sheet can be inferred from the present-day temperature field observed within the ice; temperature measurements from one or more boreholes are generally used to ascertain the present-day temperature field. This process is known as borehole paleothermometry. The method can be applied equally well to terrestrial areas where groundwater flow is quite limited or does not occur at all, such as areas dominated by thick permafrost. A number of factors influencing the temperature field in the vicinity of the borehole(s) must be taken into account during the analysis, including: the environmental setting of the hole, the thermal disturbance caused by drilling, possible convection of the borehole fluid, thermal conductivity variations, ice advection, and heat sources due to shear strain and other affects. Once these factors are taken into account, the subsurface temperature measurements can be used to reconstruct the temperature history on the surface using any number of geophysical inverse methods (e.g. Monte Carlo, spectral expansion). With all these methods, the amount of detail available in the solutions (climate histories) is determined by the number and location of the temperature measurements, the measurement uncertainties, and characteristically diminishes for older times. Different inner-product spaces can be used when formulating the inverse problem. An investigation of these spaces using the spectral expansion (SE) and true-norm-minimizing (TNM) inverse methods show that for borehole paleothermometry: 1) Solution errors are greatly reduced for inner-product spaces based on log(τ) instead of τ, where τ is time-before-present. This is partly because the basis functions are better balanced. 2) Inner-product spaces that include a temporal weighting function can be effectively used to reduce the magnitude of false oscillations ("ringing") in the solutions, particularly for recent times. The investigation also revealed deficiencies in the SE method when applied to borehole paleothermometry that are associated with the limited number of basis functions that can be effectively used with SE for this inverse problem. The TNM solutions are superior in the context of borehole paleothermometry.
C12A-04 INVITED
Ice Stream Basal Conditions From Block-wise Surface-Data Inversion: Application toBindschadler Ice Stream
Widespread basal conditions controlling ice stream flows are stillbeyond the scope of direct observation, thus knowledge of their magnitudes and variabilities comes from inversion of surfacemeasurements - ice velocities, surface elevations and thicknesses.We present a new approach to implement a widely accepted inversemethod on regular (10 × 10 km) blocks, smaller than the wholedomain, to enhance the spatial resolution of calculated basalconditions. Inverted basal friction coefficients and calculatedshear stress have sharp transitions and large variations in smallareas. Overall, the obtained basal shear stress is very small inregions of fast flowing ice.
C12A-05 INVITED
Monitoring Tidewater Terminus Dynamics Using Laser Ranging and Fixed Photography: Hubbard Glacier, Southeast Alaska
Fixed photography is a well-documented tool used throughout the cryospheric science community for
monitoring local and regional activity in glacier and ice sheet behavior. Significant technological advances in
digital hardware and software, alongside reductions in cost, have made high-resolution/temporal time-lapse
photography an accessible and standard tool in monitoring activities. In this paper, we describe current
research and monitoring activities at Hubbard Glacier which seek to quantify near realtime glacier terminus
activity. A key tool is an automated high-resolution laser ranging system coupled with fixed photography and
meteorologic data. These data provide unique, high-resolution (4 times daily) measurements of ice margin
motion along an active section of the terminus that are coincident with hourly photographs. A meteorological
station, operating in tandem with the laser system, collects 15-minute data that are transmitted hourly via
satellite to a web-accessible database for near real time access. In continuous operation since 2006, our
results indicate highly variable daily rates in ice motion set within larger seasonal periods of advance and
retreat. Daily advance rates have exceeded 10 m/day while retreat varied between 1 and 3 m/day. Current
and future work is building on the rich datasets already acquired and methods to combine the high-resolution
photography and range measurements to quantify ice marginal change using automated frame alignment,
feature detection and surface generation. Ultimately this will lead to a better understanding of the controls
that affect the extreme seasonal advance and retreat within the tidewater glacier cycle and a toolset that will
benefit the greater scientific community.
http://www.glacierresearch.com
C12A-06
Front Dynamics of Hans Glacier in Svalbard Derived From Repeated Photos of Different Type
Importance of climate signal and local topography for dynamics of Hans Glacier in Spitsbergen has been studied. Repeated photos of different time and spatial resolution together with meteorological data, record of ice melting and subglacial water pressure have been used. Hans Glacier (56 sq. km) is a polythermal grounded tidewater glacier emptying into sea close to the Polish Polar Station. The first photogrammetric record of it was done in 1936 (oblique air photos). Terrestrial photogrammetry has been used later in some years. More systematic monitoring has started in 1982. Classical terrestrial stereoscopic photos have been taken from permanent metal tripods cemented into solid rock at intervals of 2-6 weeks, with breaks in some years. Digital time lapse cameras were mounted on the same tripods in July 2007. They take stereoscopic pictures at one hour interval. Comparison of photos from 1936 and 2007 shows the mean recession on the centerline by c. 25 m per year. Seasonal fluctuations of the front position were significantly higher up to hundreds of meters. Advance has been observed in winter and retreat in summer and fall. Determination of glacier surface velocity has been done by time parallax method. Distinct seasonal inter annual fluctuations were noted. Short period fluctuations of velocity has been registered by time lapse pictures and the GPS station. Higher glacier flow accompany periods of intense melting. Maximum speed of 0.92 m per day was recorded in periods 1-2 August, 9-10 August and 13-14 August 2007, while it drop down close to zero immediately after. The speed up events were preceded by high sublacier water pressure and followed by appearance of turbid water outflows on the sea surface near the ice cliff. Pictures of Spitsbergen glaciers repeated in intervals of dozen of years document retreat and thinning their tongues, thus a general trend of climate warming. Photos taken with high time resolution detect glacier velocity fluctuations related to changeable meteorological conditions during summer. Seasonal and inter annual glacier front dynamics is driven by superimposed climatic factors, bedrock topography at the glacier front and hydro-glaciological processes of longer time scale.
C12A-07
Time-lapse Photogrammetric Observations of Columbia Glacier, Alaska, in Continued Retreat
After approximately seven years of slowed retreat rate, the terminus of Columbia Glacier (Prince William Sound) has backed up out of the major constriction in the mid-point of its fjord and has started retreating rapidly once again. In 2 months during the summer of 2006, the flow speed at the terminus dropped to approximately 1 m/day, and the lowermost 2 km thinned by 80 m, or roughly 10% of its thickness. Subsequent surveying and time-lapse photography shows that the glacier terminus is now afloat, contradicting the conventional assumption that temperate tidewater glaciers must always be grounded. Between September 2006 and June 2008 the glacier terminus has retreated approximately 2 km, and the confluence of the glacier's two lowest tributaries has been eliminated, leaving two separate glaciers (West Branch and Main Branch) flowing into Columbia Bay. Flow velocity in the Main Branch was measured in June 2008 at 16 m/day. Observations at Columbia Glacier now includes time-lapse photography at 3 sites (established June 2007), broad-band seismic monitoring on bedrock at one site near the terminus (established June 2008), and continued annual aerial photogrammetric surveys. In this presentation the changes since 2006 and the current state of Columbia Glacier is reviewed. We also describe recent developments in time-lapse photographic hardware, outreach efforts using the time lapse photography, and analysis of time-lapse photography for high time-resolution measurements of speed and thickness changes.
C12A-08
Terrestrial photogrammetry of Greenland glacier discharge variability: comparison with surface climate anomalies
Repeat photography provides a powerful tool to quantify glacier speed variability without the need to set foot
on the hazardous glacier front. Surface displacements are derived from daily digital images from several west
Greenland ice sheet outlet glaciers. Displacements are compared with surface melt intensity data to test the
hypothesis that a significant inter-daily correlation exists between outlet glacier discharge and melt intensity.
Neighboring and regional glacier speed co-variability on inter-daily time scales is evaluated. The effect of
winter sea ice and its breakup is evaluated. Numerous time lapse sequences are presented.
http://extremeicesurvey.org/