Difference between revisions of "Experiments"

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(Initial Experiment - E1 - Increased Basal Lubrication: more, and refactored, comments on E1 Greenland)
(Category 1: Whole Ice Sheet Model Experiments)
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Once control runs are complete, experiments seek to quantify the ice sheet changes (specifically ice mass loss) resulting from imposed conditions on the ice sheet or its boundaries.  Useful and feasible experiments are currently being fleshed out by the SeaRISE group.  Agreed upon experiments are described below in sufficient detail for modelers to run the experiments and supply results to Sophie.  The wiki format allows discussion of the experiments is they are either insufficiently clear or not feasible.  Broad categories of model types, along with brief descriptions of experiments from the initial white paper can still be found in these links, [[Category 1: Whole Ice Sheet]], [[Category 2: Ice-Stream/Ice-Shelf]],[[Category 3: Ice-Shelf/Ocean]].  
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Once control runs are complete, experiments seek to quantify the ice sheet changes (specifically ice mass loss) resulting from imposed conditions on the ice sheet or its boundaries.  Useful and feasible experiments are currently being fleshed out by the SeaRISE group.  Agreed upon experiments are described below in sufficient detail for modelers to run the experiments and supply results to Sophie.  The wiki format allows discussion of the experiments is they are either insufficiently clear or not feasible.  Broad categories of model types, along with brief descriptions of experiments from the initial white paper can still be found in these links, [[Category 1: Whole Ice Sheet]], [[Category 2: Ice-Stream/Ice-Shelf]],[[Category 3: Ice-Shelf/Ocean]].
 
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==Category 1: Whole Ice Sheet Model Experiments==
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Jed Brown pointed the following out at the Summer 2009 CCSM/seaRISE meeting and it seemed too sensible an idea to leave off the wiki. At some level, all models solve the continuity equation
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:<math>\frac{\partial H}{\partial t} = -\nabla \cdot \left(\mathbf{u} H\right) + a</math>
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to determine the thickness evolution. The details of how this equation is solved vary, but it is a starting point for reasoning about a class of experiments that appear to be straight forward to implement.
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In SeaRISE experiments, arbitrarily increasing the flux at the grounding line is not useful because a primary goal of SeaRISE is to quantify the ice mass loss (i.e., the increase in grounding line flux) in a deterministic way using a dynamic ice sheet model forced by a specified environmental forcing condition.  In SIA models, it is likely that the mass loss is attributed to either increased flow rates caused by a decrease in basal, lateral or longitudinal stresses) or increased melt.  In SSA/SIA hybrid models, the additional flux of ice onto the shelf may be more problematic. Dave Pollard pointed out that it is probably better to impose the change by increasing the basal melt rate.
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The difficulty arises in reconciling the various participating models. SIA models provide SeaRISE with a baseline, and are an important part of the exercise. However, prescription of experiments that are comparable across participating models can be very challenging.
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===Antarctica===
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====Initial Experiment - '''E1''' - Increased Ice Shelf Melting====
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The initial experiment in Antarctica deals with increasing flux across the grounding line by manipulating the ice shelves.  In experiments '''E1a, E1b, and E1c''', a uniform sub-ice-shelf melting rate of 2, 20 and 200 meters per year (of ice equivalent) is applied, respectively.  Ice shelves are treated differently in different models.  If a whole ice sheet model lacks ice shelves, stopping at the grounding line, the above melt rates can be imposed at the grounding line.  It is not certain that this prescription works for all models.  Further feedback is appreciated.
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===Greenland===
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====Initial Experiment - E1 - Increased Basal Lubrication====
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Again the purpose of the initial experiment is to get a feel for the results of increased flux across the grounding line.  Because most of Greenland's outlet glaciers lack an ice shelf large enough to be resolved in whole ice sheet models, the approach is to cause an increase in modeled outlet glacier speed.  A ''doubling of sliding speed'' is selected as the new outlet glacier condition for this experiment.  This condition has subtleties (next) but there are enough "adjustable knobs" on the basal relations in ice sheet models so as to make a version of this condition implementable.
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Models vary in how sliding speed is calculated, and it is left up to the modeler to determine how to impose the ''doubling of sliding speed'' condition.  The sliding speed and the basal shear stress are, generally, model results, not model inputs.  The intent of the experiment is, therefore, to adjust a lubrication or friction factor.  An example mechanism is to halve the friction coefficient <math>C</math> if sliding follows a linear relation: <math>\tau_b = C u</math>.
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This is straightforward to understand and implement if the sliding velocity is a pointwise (local) function of basal shear stress, and vice-versa, ''and'' the basal shear stress is determined directly by the geometry (e.g. is the basal value of the driving stress).  Such is true in the SIA.  In the SIA case, therefore, a halving of <math>C</math> for some patch of the ice base becomes a doubling of sliding speed in the same region directly.  Dynamically, this doubling is sustained until the geometry (thus driving stress) undergoes significant change.
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In models with longitudinal (membrane) stresses, however, sliding speed is not such a local function of ice sheet geometry.  In most models we can still make an approximately-local assumption about the nature of sliding, and still replace a ''doubling of sliding speed'' condition by a ''halving of coefficient'' condition.  With such a coefficient change there must be the understanding that the sliding speed will only actually double in the interior of large-ish patches where the basal shear stress was already fairly high.  (At the extreme case we could replace <math>\tau_b = C u</math> by <math>\tau_b = 0</math>, which multiplies <math>C</math> by zero instead of 1/2, giving no basal resistance as in an ice shelf.  But we would ''not'' expect the sliding velocity to be infinite because, as in ice shelves, membrane stresses connect to distant ice with high basal resistance.)  If <math>C</math> goes down by half in ''small'' patches, or even to zero, the velocity might not increase much at all because the ice flow is held steady by higher-basal-resistance neighboring columns.
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A further condition is to require the basal ice to be at the pressure-melting point in the outlet glaciers.  This may already be a condition for a non-zero sliding speed.  Thermodynamic feedbacks (i.e. not just geometry changes) may lead to the sliding speed initially doubling in response to a sliding coefficient change and then dropping again as basal strength increases; witness the century-scale response of Kamb Ice Stream (Kamb = C).
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Thus an unresolved issue is whether this condition is dynamic, changing as the ice sheet evolves, or it is static, determined by only the initial state.  Bindschadler favors a dynamic treatment, but it may be more difficult for some models.  (Bueler also favors dynamic.)  This point needs discussion.
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An alternative thought (from Parizek?) on where to specify “super-lubricated” points inland is to take all points at or below (seaward of) the snow line.  A better representation of this would be to use the summer 0-deg isotherm as the map-plane boundary rather than the snow line.  Above this isotherm, no surface water is produced, inferring a physical source of the super-lubrication switch.  Maybe there is a more “model-friendly” manner to specify this boundary as a spatial condition for super-lubrication. Perhaps a surface-temperature-above-melting condition (combined with a warm base) to trigger the super-lubrication after which super-lubrication is sustained as long as the base stays warm?
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With these two conditions (wet bed and source of water), the effects of super-lubrication can turn
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on (and off) and propagate in a (hopefully) stable manner.  It also provides a means to start the simulation.  A wintertime start would forestall the first impact of super-lubrication until spring temperatures rise and trigger only the points of warm basal ice whose surface begins to melt.  Alternatively, the start could be set in summer but the transition might be more sudden. 
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''(Bindschadler) I think a winter start is best, but the outlet glaciers should be super-lubricated right from the first time step because they make their own basal water.  Is this simulation limiting the super-lubrication to delivery of surface meltwater to the base?  I can live with that, but we want to be clear.'' (Bueler) The super-lubrication idea can conceptually be the delivery of surface meltwater to the base, but what is essential is that it go with a rise in basal water pressure, a drop in effective stress on till, or etc., so that the ice flow equations actually see a reduction in resistance at the base.  The reduction in basal resistance is obligatory in this experiment, while the "delivery of surface meltwater to the base" concept is necessarily optional unless we all add a lot of minimally-constrained hydrology.  Also, as noted above, we also want to alias the effect of calving front changes onto bed strength changes.
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Feedback please!
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Revision as of 10:27, 21 April 2010

Once control runs are complete, experiments seek to quantify the ice sheet changes (specifically ice mass loss) resulting from imposed conditions on the ice sheet or its boundaries. Useful and feasible experiments are currently being fleshed out by the SeaRISE group. Agreed upon experiments are described below in sufficient detail for modelers to run the experiments and supply results to Sophie. The wiki format allows discussion of the experiments is they are either insufficiently clear or not feasible. Broad categories of model types, along with brief descriptions of experiments from the initial white paper can still be found in these links, Category 1: Whole Ice Sheet, Category 2: Ice-Stream/Ice-Shelf,Category 3: Ice-Shelf/Ocean.