Evaluation of snow cover properties in ERA5 and ERA5-Land with several satellite-based datasets in the Northern Hemisphere in spring 1982–2018

Abstract. Seasonal snow cover of the Northern Hemisphere (NH) greatly influences surface energy balance, hydrological cycle, and many human activities, such as tourism and agriculture. Monitoring snow cover at continental scale is only possible from satellites or using reanalysis data. The aim of this study is to analyze timeseries of surface albedo, snow water equivalent (SWE), and snow cover extent (SCE) in spring in ERA5 and ERA5-Land reanalysis data and to compare the timeseries with several satellite-based datasets. As satellite data for the SWE intercomparison, we use bias-corrected SnowCCI v1 data for non-mountainous regions and the mean of Brown, MERRA-2 and Crocus v7 datasets for the mountainous regions. For surface albedo, we use the black-sky albedo datasets CLARA-A2 SAL, based on AVHRR data, and MCD43D51 based on MODIS data. Additionally, we use Rutgers and JAXA JASMES SCE products. Our study covers land areas north of 40° N and the period between 1982 and 2018 (spring season from March to May). The analysis shows that both ERA5 and ERA5-Land overestimate SWE. ERA5-Land shows larger overestimation, which is mostly due to very high SWE values over mountainous regions. The analysis revealed a discontinuity in ERA5 around year 2004, since adding IMS (Interactive Multisensor Snow and Ice Mapping System) from year 2004 onwards considerably improves SWE estimates but makes the trends less reliable. The negative NH SWE trends in ERA5 range from −249 Gt decade⁻¹ to −236 Gt decade⁻¹ in spring, which is two to three times larger than the trends detected by the other datasets (ranging from −124 Gt decade⁻¹ to −77 Gt decade⁻¹). Albedo estimates are more consistent between the datasets with a slight overestimation in ERA5 and ERA5-Land. SCE is accurately described in ERA5-Land, whereas ERA5 shows notably larger SCE than the satellite-based datasets. The negative trends in albedo and SCE are strongest in May, when albedo trend varies from −0.011 decade⁻¹ to −0.006 decade⁻¹ depending on the dataset. The negative SCE trend detected by ERA5 in May (−1.22 million km² decade⁻¹) is about twice as large as the trends detected by other datasets (ranging from 0.66 million km² decade⁻¹ to −0.50 million km² decade⁻¹). The analysis also shows that there is a large spatial variability in the trends, which is consistent with other studies.