Assessing firn water storage from a multi-data firn-model inversion

The ESA 4DGreenland project has the objective of performing an integrated assessment of Greenland’s hydrology through maximizing the use of Earth observation (EO) data. However, as meltwater is activated on the surface of the Greenland ice sheet and percolates into the firn, the firn water storage and delayed runoff are unobservable from EO, and we must resort to modeling to quantify this component. 

The DTU-firn model has previously been used to quantify the change in firn air volume as a leading component of altimetric mass balance estimates and was initially tuned using firn core density observations. Here, we revisit the fundamentals of the model and perform model inversion using as many observational datasets as possible. This observational data ranges from direct measurements of firn compaction derived from “coffee-can” experiments, over traditional observation of densities and temperature from firn cores to regional observation of firn stratigraphy from airborne radar surveys. This large diversity in the data sources ensures the best possible constraints for capturing as many aspects as possible of the firn dynamics.  

One important model feature of the DTU-firn model is its ability to resolve individual precipitation events as model layers. This feature promotes the capability of retaining water and shows promising results in line with the in-situ observation. The updated DTU-firn model is therefore used within the 4DGreenland project to provide updated estimates of meltwater retention and delayed runoff as a function of the available water at the surface of the firn. Combined with results obtained by the HIRHAM- and MAR-firn models, it also enables better quantification of firn model uncertainties. Having built confidence in the model we can treat the retention and delayed runoff as a function of the available water at the surface of the firn. This then gives the possibility of using the observed surface melt, also acquired within 4DGreenland from satellite microwave data, in the assessments of Greenland’s hydrology and thereby increasing the useability of EO data.