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The basal mass balance comes from the basal conservation of energy problem described in https://doi.org/10.3189/2012JoG11J088. See Fig. 5 and surrounding text. It is a bit of a mess because of various inequalities constraining the quantities. For example, temperature of an ice/water mixture does not exceed melting temperature even as you change its enthalpy. But also refreeze stops once all liquid in the transportable and till subglacial water system is used up. Etc. To answer your question, yes geothermal flux is an input. Frictional heating is computed and contributes to the energy balance computation, thus the basal mass balance. [Edit: Not a trustworthy paragraph. See comments below.] No, I don't think there is any surface runoff to the bed. (There was not such back when I was expert on PISM. I am not expert anymore, however. I may have lost track.) I think you are right that there is currently no attempt to keep track of moulins. Note that, because PISM uses enthalpy, some of the energy other models ascribe to "basal friction" is modeled as viscous dissipation within temperate ice. Some of the resulting liquid water within the temperate ice is "instantly" transported to the bed according to a "drainage model" (invented by Greve for his polythermal ice work). Specifically for PISM, from the cited enthalpy paper above, there is drainage to the bed of a portion of the liquid water in the temperate ice. We have a highly simplified drainage model. First there is an assumed amount of liquid water within temperate ice below which there is no drainage ( |
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On Dec 20, 2023, at 09:35, Ed Bueler ***@***.***> wrote:
Two more things.
Your title has "subglacial conduit volume". I am not sure what you mean by this. Within PISM there is no quantity that precisely corresponds to this phrase. See https://www.pism.io/docs/manual/modeling-choices/subglacier/hydrology.html. The closest to "conduit volume" is the transportable water thickness bwat in the -hydrology routing mass conserving model.
I see that I am out of date regarding surface runoff being routed to the bed. From the above manual page, I see that by setting hydrology.surface_input_from_runoff you can make the top-of-the-ice surface runoff get added to the basal hydrology water amount. This mechanism seems to exist both for surface runoff provided in a file, and as computed by a temperature index scheme at the surface. I have limited understanding of -hydrology steady. I should read the docs before answering questions based on what I remember!
That’s correct. I have been using this mechanism to allow surface meltwater runoff to be routed to the base and then transported to marine termini where it is used to calculate frontal melt.
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What terms go into this basal mass balance? I assume geothermal heat flux and frictional heating from sliding? What about viscous heat dissipation of surface-runoff routed to the bed?. This is about the same order of magnitude (in Greenland) as frictional heating mass loss or geothermal heat flux (see http://doi.org/10.1038/s41467-021-23739-z).
Is surface runoff routed to the bed? I assume not, because that implies treatment of moulins and a level of detail I did not think existed in PISM.
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