DOI: 10.5281/zenodo.15830307
title: IMAU-FDM v1.2G variables over Greenland (1958-2020)

description: This dataset contains the main outputs from the firn model IMAU-FDM v1.2G over Greenland and its surrounding ice caps on a 5.5x5.5 km2 grid. The data covers the period 1 Jan 1958 - 31 Dec 2020. The model was driven by a combination of ERA40, ERA-Interim, and ERA5 reanalysis data dynamically downscaled over Antarctica by the regional climate model RACMO2.3p2 (Van Wessem et al., 2018). These data are used in several manuscripts, the basis of which is the model as described in Geophysical Model Development: https://gmd.copernicus.org/articles/15/7121/2022/ (Brils et al. 2022). 

IMAU-FDM is a semi-empirical 1D firn densification model that simulates the evolution of firn density, temperature, liquid water content and surface height changes due to firn and surface mass balance (SMB) processes. The model employs up to 3000 layers of 3 to 15 cm thickness, which represent the firn properties in a Lagrangian way. An initial firn layer was obtained by spinning up the model over the reference period (1960-1979) until the firn layer was fully refreshed. The time coordinate of the file is in fractional years. Firn compaction is calculated based on the semi-empirical dry-snow densification equations of Arthern et al. (2010). Meltwater percolation is simulated using the bucket method, whereby each firn layer has a maximum irreducible water content that decreases with increasing density (Coléou and Lesaffre et al., 1999). 

This dataset is provided on a rotated polar coordinate grid. In such a rotated pole projection the grid is defined over the equator and then rotated to the area of interest. One of the advantages is that the grid distance can be defined in fraction of degrees, which results in near equidistant grid cells as long as the domain is small enough, and provides the most accurate model calculations. However, re-projecting these data on other grids is often troublesome, as after rotation the grid is non-equidistant and most software packages cannot directly handle this. Stef Lhermitte provided a nice solution for reprojecting the RACMO data on his gitlab-page: https://gitlab.tudelft.nl/slhermitte/manuals/blob/master/RACMO_reproject.md.

The dataset includes the following variables at a 10-daily temporal resolution, or 30-daily temporal resolution for z550 and z830:

Firn properties:



FirnAir: firn air content (total pore space) [m]

ssd: surface snow density (top 0.5 m) [kg/3m]

Temp10m: 10-m firn temperature [K]

z550: depth of the critical density level of ρ=550 kg m−3 [m]

z830: depth of the critical density level of ρ=830 kg m−3 [m]

zs: surface elevation [m]


Vertical velocity of the firn top surface:



vacc: vertical velocity component due to snowfall [m]

vsnd: vertical velocity component due to snowdrift erosion [m]

vsub: vertical velocity component due to sublimation/riming [m]

vmelt: vertical velocity component due to surface snow or ice melt [m]

vice: assumed vertical velocity of the bottom of the firn column, taken as the climatologically mean accumulation rate [m]

vtotal: total vertical velocity [m]

vfc: vertical velocity component due to firn compaction [m]


Positive values of the vertical velocity component indicate an increasing effect on the elevation of the firn top surface, whereas negative values indicate a decreasing effect. Additional variables (e.g. depth profiles of density) are available on request.