Gedankenexperimente: Assessing Field-Program Effectiveness by Numerical Simulations (Abstract)

Abstract

Glaciological field programs may be regarded as imperfect sampling schemes designed to provide fundamental physical information on the dynamics and climatic sensitivity of the Antarctic ice sheet. Uncertainty arises as a result of technical and human factors such as: (i) logistic and financial constraints, (ii) measurement errors, (iii) low spatial resolution (see (i)), and (iv) (possibly!) misconceptions on the part of glaciologists who plan and execute field work. Regardless of such uncertainty, we depend on field data as the fundamental intellectual driving force of glaciology. Introspective evaluation of our field methods and program designs is thus reasonable, and perhaps necessary, to insure that our field programs are indeed satisfying their intended purpose. In our study, we conduct a variety of Gedankenexperimente (imaginary field programs), which sample an arbitrary, idealized ice shelf, subject to fluctuations and climatic changes on a variety of time and space scales. The “actual ” behavior of this ice shelf is produced by a time-dependent numerical simulation of ice-shelf evolution under specified forcings, using a model based on that of Lange and MacAyeal (1986). Each Gedankenexperiment consists of a spatially incomplete sampling of the model grid data at a particular moment in the evolution of the ice shelf (just as a real field program presently would sample the current state of an Antarctic ice shelf). The spatial sampling patterns are based on particular techniques commonly used in field programs (Kohnen 1985, Bindschadler and others 1987, Doake and others 1987, Shabtaie and Bentley 1987). Such sampling is designed to simulate field techniques such as airborne radio echo-sounding, surface geodetic measurements, aerial photography, and satellite altimetry (Fig. 1). We also add “random noise” to the sampled data, to simulate instrumental and navigational uncertainties. Having sampled the idealized ice shelf by using an imaginary field program, we “process” the supposed field data in order to test how well it reveals certain aspects of ice-shelf flow and evolution. This test is conducted by comparing the field-program results with the “known” behavior (by definition) of the numerical simulation. A variety of field-program design schemes are compared on the basis of their ability to predict: (i) the long-term growth or decay of the ice shelf, (ii) the “current” state of mass balance, (iii) the “current” partitioning of ice-stream input, and (iv) the balance of forces acting on the grounding line, and the tendency of the balance to change with time. A major aim of our study will be to point out how seriously the understanding of current ice-shelf dynamics and the ability to measure initial effects of global climatic changes (due to CO2 warming) are hampered by: (i) inability to map accurately all the regions of ice-shelf grounding, and (ii) inability to distinguish the effects of short-term variability from long-term, large-scale trends. To simulate the effects of ice-shelf grounding and ice-stream -temporal fluctuations, we specify in our idealized simulations that: (i) several ice rumples occasionally appear or disappear, and (ii) ice-stream fluxes, which feed the imaginary ice shelf, fluctuate (arbitrarily) with periods of 300 years. Since we assess the Gedankenexperimente in terms of their ability to detect long-term climatic trends, we run the ideal ice-shelf simulation forward in time until a statistically steady state is achieved (that is, all thickness and velocity patterns are stationary when averaged over the time-scale of fluctuation). At this point, we conduct the imaginary field programs in our study. Our main intention is to determine which Gedankenexperiment can best “see through” the short-term transient “noise” of the ideal ice-shelf evolution to detect the long-term condition of steady state.