Comment on tc-2021-379

Le résumé fourni par la source

The future of the Greenland ice-sheet largely depends on the changing climate. When ice-sheet models are run for time periods that extend far beyond the observational record they are often forced by climatology instead of a transient climate. We investigate how this simplification impacts the surface mass balance using the Bergen Snow Simulator. The model was run for up to 500 years using the same atmospheric climatology, but different variability, as forcing. We achieve this by re-arranging the years in the ERA-interim reanalysis while leaving the intra-annual variations unchanged. This changes the surface mass balance by less than 5 % over the entire Greenland ice sheet. However, using daily averages as forcing introduces large changes in intra-annual variability and thereby overestimates the Greenland-wide surface mass balance by 40 %. The biggest contributor is precipitation followed by temperature. The most important process is that small amounts of snowfall from the daily climatology overestimate the albedo, leading to an increased SMB. We propose a correction that distributes the monthly precipitation over a realistic intra-monthly variability. This approach reduces the SMB overestimation to 15–25 %. We conclude that simulations of the Greenland surface mass and energy balance should be forced with a transient climate. Particular care must be taken if only climatological data is available for simulations with a model that was calibrated with transient data. If daily transient data cannot be used, at least the precipitation should follow a natural daily distribution.

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