Evaluation of E3SM snowpack and firn initialization and early 20th century simulations
Abstract
Prediction of ice sheet surface mass balance (SMB) in future climate simulations requires realistic, spatially dependent initial conditions including snowpack density, thickness, and temperature without which Earth system models (ESMs) may spin-up to unrealistic equilibrium snow distributions. The Energy Exascale ESM (E3SM) simulates snow accumulation with snowpack densification determined as the sum of four physical compaction terms: the weight of overburden, temperature-dependent destructive metamorphism, wind packing, and water phase-change. We evaluate the results of the new processes and processes implemented to initialize snow and firn in the E3SM land model. We assess firn densification in dry snow zones using a steady-state empirical model well-suited for the Antarctic ice sheet interior where there is little-to-no melt. E3SM-simulated near-surface densities in dry snow zones compare well with firn core density measurements from Antarctica and Greenland, while simulated deep firn densities stabilize at approximately 600 kg m-3, a 30 percent underestimate of the firn-to-ice transition density at which pore close-off typically occurs. We hypothesize that slow compaction is due to an overly responsive inverse relationship between the densification rate and firn density itself, yielding an overestimated effective viscosity at densities greater than 550 kg m-3. Initializing ice sheets, glaciers, and northern high latitudes with a thin fresh snow layer results in a positive SMB over most of the Antarctic and Greenland ice sheets and a negative SMB over most mountain glaciers during a pre-industrial simulation forced by atmospheric reanalysis during 1901-1920. Accumulation areas with high precipitation rates develop perennial snow depths up to 100 m, a new capability not included in most ESMs. Resolving firn densities to depths of 100 m also enables the formation of thick ice lenses which can play an important role in the SMB. These findings will serve the development of global coupled atmosphere and ice sheet models that are used to predict ice sheet mass balance and sea level rise in future E3SM climate simulations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.C51C1321S
- Keywords:
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- 0726 Ice sheets;
- CRYOSPHERE;
- 0774 Dynamics;
- CRYOSPHERE;
- 0776 Glaciology;
- CRYOSPHERE;
- 0798 Modeling;
- CRYOSPHERE