Representation of Arctic hydrology in a global land surface model

Earth System Models (ESMs) are the best available tools to project the coupled dynamics of the climate and biogeochemistry under future emission scenarios. However, the future trajectories simulated by individual ESMs vary substantially with most pronounced differences in the high northern latitudes. As recently demonstrated (de Vrese et al., 2022), a significant part of this uncertainty might result from the different approaches and parametrizations of surface and soil hydrology in the permafrost regions. However, this study did not account for sub-grid lateral fluxes.

To make a step forward, we further develop ICON-Land/JSBACH4, the land surface model (LSM) used within the ICON-ESM. Our recent efforts are focused on improving the simulation of Arctic hydrology by accounting for lateral water flows on small spatial scales, i.e. within the grid cells of the LSM. For this, we apply a tiling structure in which we define the spatial relation between parts of the grid cell in terms of water exchange. In this way, the model gets information about the source and sink tiles of surface runoff (based on topography) but also of lateral soil water exchange (based on proximity and soil moisture gradient).

This approach results in a redistribution of surface and soil water within the grid cells with drier upland and wetter lowland regions and the correspondent changes in evapotranspiration. Comparing coupled land-atmosphere simulations with different prescribed fractions of upland and lowland areas, we see strong impacts of the tiling structure. Setups with a larger lowland-to-upland ratios lead to higher cloud cover and by up to 2K lower summer surface temperature over larger parts of the boreal regions. This result emphasizes the importance of representing the complex processes of Arctic hydrology, but also the need for detailed information about Arctic land surface properties.

References:

de Vrese, P., Georgievski, G., Gonzalez Rouco, J. F., Notz, D., Stacke, T., Steinert, N. J., Wilkenskjeld, S., and Brovkin, V.: Representation of soil hydrology in permafrost regions may explain large part of inter-model spread in simulated Arctic and subarctic climate, The Cryosphere Discuss., https://doi.org/10.5194/tc-2022-150, in review, 2022.