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. This study introduces and evaluates a comprehensive,model-generated dataset of Northern Hemisphere permafrost conditions at81 km2 resolution. Surface meteorological forcing fields from theModern-Era Retrospective Analysis for Research and Applications 2 (MERRA-2)reanalysis were used to drive an improved version of the land component ofMERRA-2 in middle-to-high northern latitudes from 1980 to 2017. Theresulting simulated permafrost distribution across the Northern Hemispheremostly captures the observed extent of continuous and discontinuouspermafrost but misses the ecosystem-protected permafrost zones in westernSiberia. Noticeable discrepancies also appear along the southern edge of thepermafrost regions where sporadic and isolated permafrost types dominate.The evaluation of the simulated active layer thickness (ALT) against remotesensing retrievals and in situ measurements demonstrates reasonable skillexcept in Mongolia. The RMSE (bias) of climatological ALT is 1.22 m (−0.48 m) across all sites and 0.33 m (−0.04 m) without the Mongolia sites. Innorthern Alaska, both ALT retrievals from airborne remote sensing for 2015and the corresponding simulated ALT exhibit limited skill versus in situmeasurements at the model scale. In addition, the simulated ALT has larger spatial variability than the remotely sensed ALT, although it agrees well with the retrievals when considering measurement uncertainty. Controls onthe spatial variability of ALT are examined with idealized numericalexperiments focusing on northern Alaska; meteorological forcing and soiltypes are found to have dominant impacts on the spatial variability of ALT,with vegetation also playing a role through its modulation of snowaccumulation. A correlation analysis further reveals that accumulatedabove-freezing air temperature and maximum snow water equivalent explainmost of the year-to-year variability of ALT nearly everywhere over themodel-simulated permafrost regions.

Permafrost variability over the Northern Hemisphere based on the MERRA-2 reanalysis