The ERA5-Land soil temperature bias in permafrost regions

Bin Cao,Xin Li,Donghai Zheng,Stephan Gruber

doi:10.5194/tc-14-2581-2020

Abstract

Abstract. ERA5-Land (ERA5L) is a reanalysis product derived by running the land component of ERA5 at increased resolution. This study evaluates ERA5L soil temperature in permafrost regions based on observations and published permafrost products. We find that ERA5L overestimates soil temperature in northern Canada and Alaska but underestimates it 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. As calculated from its soil temperature, ERA5L overestimates active-layer thickness and underestimates near-surface (<1.89 m) permafrost area. This is thought to be due in part to the shallow soil column and coarse vertical discretization of the land surface model and to warmer simulated soil. The soil temperature bias in permafrost regions correlates well with the bias in air temperature and with maximum snow height. A review of the ERA5L snow parameterization and a simulation example both point to a low bias in ERA5L snow density as a possible cause for the warm bias in soil temperature. The apparent disagreement of station-based and areal evaluation techniques highlights challenges in our ability to test permafrost simulation models. While global reanalyses are important drivers for permafrost simulation, we conclude that ERA5L soil data are not well suited for informing permafrost research and decision making directly. To address this, future soil temperature products in reanalyses will require permafrost-specific alterations to their land surface models.

Highlights

Permafrost regions occupy more than one fifth of the exposed land area in the Northern Hemisphere (Gruber, 2012) and are subject to important temperature-dependent processes (Cheng and Wu, 2007; Westermann et al, 2009; Schuur et al, 2015; Walvoord and Kurylyk, 2016)
Soil temperature is found to have a warm bias in western Canada and Alaska but a cold bias in mid–low latitudes such as the Qinghai–Tibetan Plateau (QTP), leading to a near-zero wBIAS of −0.08 ◦C (Fig. 3)
The result suggests that mean annual air temperature (MAAT) and snow depth both influence ERA5L soil temperature

Summary

Introduction

Permafrost regions occupy more than one fifth of the exposed land area in the Northern Hemisphere (Gruber, 2012) and are subject to important temperature-dependent processes (Cheng and Wu, 2007; Westermann et al, 2009; Schuur et al, 2015; Walvoord and Kurylyk, 2016). Reanalysis products have been successfully used to analyze and simulate various permafrost phenomena at different scales, such as its spatial distribution (e.g., Cao et al, 2019b; Fiddes et al, 2015; Slater and Lawrence, 2013), thermal state (e.g., Guo and Wang, 2017; Koven et al, 2013), active-layer thickness (e.g., Tao et al, 2019; Qin et al, 2017), ground ice loss (e.g., Aas et al, 2019), and carbon release (e.g., Koven et al, 2015) These applications are mostly restricted to the use of atmospheric variables to drive models. Over the Qinghai–Tibetan Plateau (QTP), Hu et al (2019) and Yang and Zhang (2018) reported that the root mean squared error (RMSE) of daily soil temperature

Objectives

Methods

Results

Discussion

Conclusion