Improving permafrost physics in the coupled Canadian Land Surface Scheme (v.3.6.2) and Canadian Terrestrial Ecosystem Model (v.2.1) (CLASS-CTEM)

Abstract

Abstract. The Canadian Land Surface Scheme and Canadian Terrestrial Ecosystem Model (CLASS-CTEM) together form the land surface component of the Canadian Earth System Model (CanESM). Here, we investigate the impact of changes to CLASS-CTEM that are designed to improve the simulation of permafrost physics. Overall, 18 tests were performed, including changing the model configuration (number and depth of ground layers, different soil permeable depth datasets, adding a surface moss layer), and investigating alternative parameterizations of soil hydrology, soil thermal conductivity, and snow properties. To evaluate these changes, CLASS-CTEM outputs were compared to 1570 active layer thickness (ALT) measurements from 97 observation sites that are part of the Global Terrestrial Network for Permafrost (GTN-P), 105 106 monthly ground temperature observations from 132 GTN-P borehole sites, a blend of five observation-based snow water equivalent (SWE) datasets (Blended-5), remotely sensed albedo, and seasonal discharge for major rivers draining permafrost regions. From the tests performed, the final revised model configuration has more ground layers (increased from 3 to 20) extending to greater depth (from 4.1 to 61.4 m) and uses a new soil permeable depths dataset with a surface layer of moss added. The most beneficial change to the model parameterizations was incorporation of unfrozen water in frozen soils. These changes to CLASS-CTEM cause a small improvement in simulated SWE with little change in surface albedo but greatly improve the model performance at the GTN-P ALT and borehole sites. Compared to the GTN-P observations, the revised CLASS-CTEM ALTs have a weighted mean absolute error (wMAE) of 0.41–0.47 m (depending on configuration), improved from >2.5 m for the original model, while the borehole sites see a consistent improvement in wMAE for most seasons and depths considered, with seasonal wMAE values for the shallow surface layers of the revised model simulation of at most 3.7 ∘C, which is 1.2 ∘C more than the wMAE of the screen-level air temperature used to drive the model as compared to site-level observations (2.5 ∘C). Subgrid heterogeneity estimates were derived from the standard deviation of ALT on the 1 km2 measurement grids at the GTN-P ALT sites, the spread in wMAE in grid cells with multiple GTN-P ALT sites, as well as from 35 boreholes measured within a 1200 km2 region as part of the Slave Province Surficial Materials and Permafrost Study. Given the size of the model grid cells (approximately 2.8∘), subgrid heterogeneity makes it likely difficult to appreciably reduce the wMAE of ALT or borehole temperatures much further.

Ground ice, organic carbon and soluble cations in tundra permafrost soils and sediments near a Laurentide ice divide in the Slave Geological Province, Northwest Territories, Canada

“…The some sediment, Fig. B2c), as previously reported for polygonal ground in organics (Mackay, 2000;Morse and Burn, 2013). In the valley and in organic terrain, the increase in water and excess-ice content with depth ( Figs.…”

The ERA5-Land soil temperature bias in permafrost regions

“…ERA5 is produced using four-dimensional variational data assimilation in ECMWF's Integrated Forecast System; it has a horizontal resolution of 0.25 • (31 km), a temporal resolution of 1 h, and a vertical resolution of 137 hybrid sigma model levels. The 37 pressure levels of ERA5 are identical to those of ERA-Interim (Noël et al, 2020). ERA5 assimilates improved input data that better reflect observed changes in climate forcing, as well as many new or reprocessed observations that were not available during the production of ERA-Interim.…”

Responses to RC1

“…l. 168: what do you mean by "although less permafrost processes are coupled"? Compared to CLASS-CTEM presented by Melton et al, (2019), HTESSEL includes less physical processes regarding permafrost. We'll change this part to "...although fewer permafrost specific processes are included in the HTESSEL..." to clarify.…”