The ERA5-Land Soil-Temperature Bias in Permafrost Regions

• Main Authors: Cao, Bin, Gruber, Stephan, Zheng, Donghai, Li, Xin
• Format: Text
• Language: English
• Published: 2020
• Subjects: Canada; Active layer thickness; permafrost; Alaska
• Online Access: https://doi.org/10.5194/tc-2020-76; https://tc.copernicus.org/preprints/tc-2020-76/

ERA5-Land (ERA5L) is a reanalysis product derived by running the land component of ERA5 at increased resolution. This study evaluates its soil temperature in permafrost regions based on observations and published permafrost products. Soil in ERA5L is predicted too warm in northern Canada and Alaska, but too cold in mid-low latitudes, leading to an average bias of −0.08 °C. The warm bias of ERA5L soil is stronger in winter than in other seasons. Diagnosed from its soil temperature, ERA5L overestimates active-layer thickness and underestimates near-surface (< 1.89 m) permafrost area. This is, in part, due to the shallow soil column and coarse vertical discretization in the ERA5 land-surface model and to warmer simulated soil. The soil-temperature bias in permafrost regions correlates with the bias in air temperature and with maximum snow height. Review of the ERA5L snow scheme and a simulation example point to low-bias in ERA5L snow density as a possible cause for warm-biased soil. The apparent disagreement of station-based and spatial evaluation of ERA5L highlights challenges in our ability to test permafrost simulation models. While global reanalyses are important drivers for permafrost simulation, ERA5L soil data is not well suited for directly informing permafrost research decision making. To alleviate this, future soil-temperature products in reanalyses would require permafrost-specific alterations to the land-surface models used.