The Third EISMINT Workshop on Model Intercomparison was held in the SunStar Hotel in Grindelwald from Thursday September 27 th to Saturday September 29 th 1997. The principal objective of this meeting was to discuss the results of the final phase of the model intercomparison venture, where existing ice-flow models were compared under real-world situations and under much more challenging conditions than had been the case during previous workshops held in Brussels in 1993 and Bremerhaven in 1994.
A first planning meeting to prepare the Grindelwald workshop was organized i n Brussels on 29-30 March 1996. This planning meeting was attended by Drs. Richard Hindmarsh, Doug MacAyeal, Catherine Ritz, Vincent Rommelaere, Tony Payne, Mikhail Verbitsky, and Philippe Huybrechts. We discussed which tests we considered most useful for the third phase of the venture, what suitable data would be available, and how the work could be practically organised. It was decided to define 5 intercomparison topics (Greenland models; Antarctic models; Ice-shelf models; Thermomechanical effects; and Grounding-line treatments), each of which would be taken care of by a coordinator. The respective coordinators (C. Ritz, T. Payne, and P. Huybrechts) first performed the experiments they designed themselves, and then produced a written description. All was put on the web in September 1996 (http://homepages.vub.ac.be/~phuybrec/eismint.html), and widely advertised, both directly to previous participants, as on the E-mail forum of the International Glaciological Society. These documents form part of this report.
Participants could directly load the experimental descriptions and the required datasets from the webpages set up in Brussels, with further links to sites in Grenoble and Southampton. Results of the tests were sollicited before May 1 st 1997. During the summer of 1997, the respective topic coordinators collated and plotted with some student help all results for presentation in Grindelwald.
In total, 13 scientists participated in the various experiments, most of them in more than one topic. Almost all major ice-sheet and ice-shelf modeling groups were represented at the meeting, with a dominance of European groups, but also including several groups and individuals from Japan, the USA and Canada. The workshop was attended by 21 persons from 8 different countries, including 3 non-European. The workshop itself took three days. The first day was entirely devoted to a plenary presentation and discussion of the model intercomparison results by topic coordinators. This was followed during the second morning by group discussions of model results per intercomparison topic, and a plenary presentation of conclusions by designated rapporteurs. Highlights of the intercomparison results and the minutes of the group discussions form the major part of this report, and summarise the main achievements of the workshop. It was agreed to write up the main results in three separate papers prepared by the respective topic coordinators and co-authored by the participants. These papers are to be submitted to the international literature within the next few months.
From the intercomparison results, it turned out that most of the Greenland models produced very similar results. As these models only dealt with grounded ice dynamics, this clearly indicated that the previous intercomparison tests concentrating on numerics and detecting errors in the various codes had borne fruit (topic 1). Two more or less complete Antarctic models dealing with the entire ice-sheet/ ice-shelf system were presented at the meeting. An interesting difference between these two models concerned the way the grounding zone is treated, which resulted in a different chronology of, in particular, the history of the last deglaciation in West Antarctica (topic 2). The two ice-shelf models participating in the ice-shelf tests behaved nearly identical, thereby confirming previous results from the Ross Ice Shelf benchmark (topic 3). Perhaps most interestingly, all thermomechanically coupled models were shown to exhibit the radial instabilities in basal temperature and ice flux first documented in papers by Tony Payne, confirming that these instabilities are probably not a mere product of the particular numerics employed by Tony in his prior work (topic 4). A concensus on how to deal with the marine ice-sheet problem and grounding-line dynamics, on the other hand, could not be established. All models behaved very differently, raising fundamental questions about the state of equilibrium and stability of the ice-sheet/ ice-shelf junction, and the reversibility of the process of grounding-line migration, which subjects deserve to be investigated more thoroughly in future work (topic 5).
The afternoon of the second day was devoted to an open talks session on recent developments in the field of ice-flow modeling. Talks were presented on topics like dealing with small-scale features such as outlet glaciers and ice streams, the grounding-line problem, numerical techniques, basal boundary conditions, and model applications to West Antarctica and the northern hemisphere ice sheets. Taking advantage of the splendidly clear weather on the third day, the 'informal discussions' were continued on Jungfraujoch, where we hiked to the Mnchsjochhtte. The excellent views on Aletschgletscher and the surrounding mountains were very much appreciated.
In all, the general feeling was that the series of intercomparison workshops held between 1993 and 1997 have definitely pushed the art of ice sheet modelling a decisive step forward. It has enabled groups and individuals worldwide to further develop and improve their dynamic ice-flow codes. Several European groups are now in possession of upgraded models that have been thoroughly tested under a wide variety of boundary conditions. These models can therefore be considered as sufficiently reliable tools to better investigate ice sheets and their interaction with the climate system.