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The treatment of surface melt, runoff, and the snow-firn-ice transition in ice-sheet models (ISMs) is becoming increasingly important, as mobile liquid on Greenland and Antarctic flanks increases due to climate warming in the next century and beyond. Simple Positive Degree Day (PDD)-based box models used in some ISMs crudely capture liquid storage and refreezing, but need to be extended to include vertical structure through the whole firn-ice column, as in some regional climate models (RCMs). This is a necessary prelude to modeling the flow of mobile meltwater in channel-river-moulin systems, and routing to the base and/or margins of the ice sheet. More detailed column models of snow and firn exist, that include compaction, grain size, and other processes. Some focus on dry-snow zones, and have fine vertical resolution spanning the entire firn column with Lagrangian tracking of annual snow layers (e.g., FirnMICE: Lundin et al., J. Glac., 2017). However, they are mostly too computationally expensive for ISM applications, and are not designed for ablation zones with meltwater and bare ice in summer. More general models are used in some RCMs that include similar physics but with fewer layers, and are applicable both to accumulation and ablation zones. Here we formulate a new snow-firn model, similar to those in RCMs, for use within an ice-sheet model. A limited number of vertical layers is used (∼10), with Lagrangian tracking of layers, grain size evolution, compaction, ice lenses, liquid melting, storage, percolation and runoff. Surface melting is computed from linearized net atmospheric energy fluxes, not from PDDs. The model is tested using the FirnMICE experiments, and using gridded RACMO2 modern climate input over Greenland, seeking to balance model performance with computational efficiency.
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A simple physics-based improvement to the positive degree day model
ABSTRACT Meltwater is important to understanding glacier health and dynamics. Since melt measurements are uncommon, ice ablation estimates are often based on models including the positive degree day (PDD) model. The PDD estimate is popular since it only requires air temperature as input, but suffers from the lack of physical motivation of an energy-balance model. We present a physics-based alternative to the PDD model that still only takes air/surface temperature as input. The model resembles the PDD model except accounting for time lags in ablation when cold ice needs to be warmed. The model is expressed as a differential equation with a single extra parameter related to the efficiency of heating a near-surface layer of ice. With zero thickness, the model reduces to the PDD model, providing a physical basis for the PDD model. Applying the model to data from Greenland, it improves modestly upon the PDD model, with the main improvement being better prediction of early season melting. This new model is a useful compromise, with some of the physics of more realistic models and the simplicity of a PDD model. The model should improve estimates of meltwater production and help constrain PDD parameters when empirical calibration is challenging.
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- Award ID(s):
- 1735715
- PAR ID:
- 10193598
- Date Published:
- Journal Name:
- Journal of Glaciology
- Volume:
- 64
- Issue:
- 246
- ISSN:
- 0022-1430
- Page Range / eLocation ID:
- 661 to 668
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jog.2018.55
