The firn meltwater Retention Model Intercomparison Project (RetMIP): evaluation of nine firn models at four weather station sites on the Greenland ice sheet

Perennial snow, or firn, covers 80 % of the Greenland ice sheetand has the capacity to retain surface meltwater, influencing the ice sheetmass balance and contribution to sea-level rise. Multilayer firn models aretraditionally used to simulate firn processes and estimate meltwaterretention. We present, intercompare and evaluate outputs from nine firnmodels at four sites that represent the ice sheet's dry snow, percolation,ice slab and firn aquifer areas. The models are forced by mass and energyfluxes derived from automatic weather stations and compared to firn density,temperature and meltwater percolation depth observations. Models agreerelatively well at the dry-snow site while elsewhere their meltwaterinfiltration schemes lead to marked differences in simulated firncharacteristics. Models accounting for deep meltwater percolation overestimate percolation depth and firn temperature at the percolation andice slab sites but accurately simulate recharge of the firn aquifer. Modelsusing Darcy's law and bucket schemes compare favorably to observed firntemperature and meltwater percolation depth at the percolation site, butonly the Darcy models accurately simulate firn temperature and percolationat the ice slab site. Despite good performance at certain locations, nosingle model currently simulates meltwater infiltration adequately at allsites. The model spread in estimated meltwater retention and runoffincreases with increasing meltwater input. The highest runoff was calculatedat the KAN_U site in 2012, when average total runoff acrossmodels (±2σ) was 353±610 mm w.e. (water equivalent), about 27±48 % of the surface meltwater input. We identify potential causes for themodel spread and the mismatch with observations and provide recommendationsfor future model development and firn investigation.