Runoff from Greenland's firn area – why do MODIS, RCMs and a firn model disagree?

Abstract. Due to increasing air temperatures, surface melt and meltwater runoff expand to ever higher elevations on the Greenland ice sheet and reach far into its firn area. Here, we evaluate how two regional climate models (RCMs) simulate the expansion of the ice sheet runoff area: MAR, and RACMO with its offline firn model IMAU-FDM. For the purpose of this comparison we first improve an existing algorithm to detect daily visible runoff limits from MODIS satellite imagery. We then apply the improved algorithm to most of the Greenland ice sheet and compare MODIS to RCM runoff limits for the years 2000 to 2021. We find that RACMO/IMAU-FDM runoff limits are on average somewhat lower than MODIS and show little fluctuations from year to year. MAR runoff limits are substantially higher than MODIS, but their relative fluctuations are more similar to MODIS. Both models apply a bucket scheme where meltwater is routed vertically. On the example of the K-transect we demonstrate that differences in the implementation of the bucket scheme are responsible for the disparity in RCM simulated runoff limits. The formulation of the runoff condition is of large influence: in RACMO/IMAU-FDM meltwater is only considered runoff when it reaches the bottom of the simulated firn pack; in MAR runoff can also occur from within the firn pack, which largely causes its higher runoff limits. We show that total runoff along the K-transect, simulated by the two RCMs, diverges by up to 29 % in extraordinary melt years. Out of this, three quarters are caused by the differences in the simulated runoff limits, the remainder being mostly due to differences in simulated ablation area runoff. Consequently, accurate simulation of meltwater hydrology in a melting firn area is essential to assess Greenland's current and future mass changes.