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IERS Message No. 514                                   November 21, 2024
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IAG WG 1.2.3: Recommendations for GIA and mass transport models


IAG WG 1.2.3 has a set of recommendations for quantities that we
recommend are computed both for GIA and present-day mass transport
models.
Draft 1, October 21, 2024
Contact: Jeff Freymueller, freymuel@msu.edu  

General Recommendations:

* Provide also intermediate results, like the gravitationally consistent
  redistribution of mass to the ocean in the case of GIA and present-day
  mass transport models 
  - These have implications for the loading and the geocenter.
  - Some present-day melting models do include gravitational
    redistribution.
  - Hydrological models do not include this.

* Model computations in the CM frame, with degree 1 coefficients 
  provided (with documented units/normalization) to allow models to be
  expressed in CF or other frames. 
  - This is needed for both GIA, hydrological, and present-day melting
    models (for GIA, we especially need this for the present day).
  - Provide degree 1 coefficients explicitly so that all users do not
    have to try to infer them from the model.
  - We might find important differences between models that will aid in
    understanding.
  - Uncertainties need to be estimated, even if that is difficult.

* Geocenter time series or trend (CF minus CM).
  - The trend is significant relative to long-term measurement
    precision, but not fully understood.
  - Hydrology and present-day ice mass change require a time-series
    estimate of present changes.
  - GIA will be a trend (at least for LGM ice).
  - Requires gravitationally consistent redistribution of ocean mass,
    but otherwise it is not complicated. Some have computed this, but
    some have not.

* Compute deformation using the full degree and order (spatial
  resolution) of the underlying mass model, and document this.
  - The land-sea mask for ocean-continent classification also limits the
    effective spatial resolution. Document what is used. Document
    whether the degree and order of the gravitational redistribution are
    different from that of the main load model.
 
Specific to GIA:
Sharing some intermediate results would be very useful!

* Make models freely available!
* Compute 3D deformation at least for the present day; many models
  provide only vertical.
* Gravitationally consistent redistribution of mass to the ocean
  (present-day rates, or time series if variable over recent decades).
* Shoreline position as a function of time.

Specific to Present-Day Mass Transport:
(Hydrology and Present-day ice melting)

* Combine these two processes (ice melt and continental hydrology). 
  - Addition of glacier mass change models with TWS models is not
    possible without double-counting of some mass.
* Groundwater! In many models groundwater behaves much like soil 
  moisture (does not move laterally). Document how groundwater is
  included or not in the model.
* Models should be mass-conserving, or should include an accounting for
  the mass of water going into the ocean. Given this information, and
  the spatial distribution of mass change, it would be possible to
  compute the gravitationally consistent redistribution of mass to the
  ocean.
* Gravitationally consistent redistribution of mass to the ocean.
  - Ocean mass distribution gives the total mass. We need to know what
    part is gravitationally driven so that we can add continental and
    oceanic models without double-counting.


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