Earlier large-scale Greenland ice sheet experiments e.g. those run during ice2sea and SeaRISE initiatives have shown that ice sheet initialisation can have a large effect on sea-level projections and gives rise to important uncertainties. Improving initialisation techniques is currently a field of active research, which makes it difficult to prescribe one technique as the method of choice for ISMIP6. Instead, we first propose a “Come as you are”- approach, which allows participants to contribute with their currently used model setup and initialisation technique for intercomparison (initMIP). This, we hope, allows getting modellers involved early in the ISMIP6 process and keeps the workload for participants as low as possible. Furthermore, the proposed schematic experiments may facilitate to document on-going model development. Starting early in the CMIP6 process implies relying on schematic forcing for the initiation experiments that is independent from CMIP6 AOGCM output, which will only become available later on. The initMIP-Greenland is the first in a series of ISMIP6 ice sheet model intercomparison activities and is led by Heiko Goelzer.
• Compare and evaluate the initialisation methods used in the ice sheet modelling community
• Estimate uncertainty associated with initialisation
• Get the ice sheet modelling community started with ISMIP6 activities
• Document on-going model development, as the simple experiments could be repeated with new model versions
Experiments are for the large scale Greenland ice sheet and are designed to allow intercomparison between models of 1) the initial state itself and 2) the response in two schematic forward experiments:
1. init: Initialisation to present day with method of choice
2. Schematic forward experiments
2a. ctrl: Unforced control run (100 years)
2b. asmb: Prescribed schematic surface mass balance anomaly (100 years)
The two forward experiments serve to evaluate the initialisation in terms of model drift (2a, ctrl) and response to a large perturbation (2b, asmb). For 2a, the models are run forward without any anomaly forcing, such that whatever surface mass balance (SMB) was used in the initialization technique would continue unchanged. The perturbation in 2b consists of a given surface mass balance anomaly, which has to be applied relative to the initial SMB inherent to the individual initialisation technique. The SMB anomaly in 2b (the same for each model) is schematic and should not be considered as a realistic projection. The core experiment duration is set to 100 years.
Requirements for the experiments:
• Participants can and are encouraged to contribute with different models and/or initialisation methods
• Models have to be able to prescribe a given SMB anomaly
• No adjustment of SMB due to geometric changes in forward experiments (i.e. no elevation – SMB feedback is allowed)
• No bedrock adjustment in forward experiment
• The choice of model input data is unconstrained to allow participants the use of their preferred model setup without modification. Modelers without preferred data set choice can have a look at the ISMIP6 wiki for possible options
• The specific year of initialization (between 1950 and 2014) is equally unconstrained to allow the use of different observational data sets that may be tied to certain time periods.
The SMB anomaly can be obtained via the ISMIP6 ftp server (email email@example.com to obtain the log in information), and was prepared by Heiko Goelzer. Modeling groups should use the 1km version to conservatively interpolate to their model native grid (see Appendix 1, below). Files of lower resolution (5km, 10km, and 20km) are provided for groups using the standard output grid (Bamber et al., 2001) as “native grid”. For 2b, the SMB anomaly is to be implemented as a time dependent function, that takes the form of a linear function which increases stepwise every full year (it is therefore independent of the time step in the model):
SMB(t) = SMB_initialization + SMB_anomaly * (floor (t) / 40); for 0 < t < 40 in years
SMB(t) = SMB_initialization + SMB_anomaly * 1.0; for t > 40 years
where SMB_anomaly is the anomaly provided by ISMIP6 and SMB_initialization is the SMB used for the initialization. The units of SMB_anomaly are (meter ice equivalent/year) with an assumed density of 910 kg/m^3 and 31556926 s/yr.