20 Oct 2021
20 Oct 2021
Status: this preprint is currently under review for the journal TC.

Stability of long-term satellite and reanalysis products to monitor snow cover trends

Ruben Urraca and Nadine Gobron Ruben Urraca and Nadine Gobron
  • European Commission, Joint Research Centre, Via Fermi 2749, I-21027 Ispra, Italy

Abstract. Monitoring snow cover extent is now feasible using Earth Observation (EO) data together with reanalysis products (derived from earth system model and data assimilation) to infer climate change impacts. Temporal stability is essential but can be altered by the combination of multiple satellite sensors and their degradation, or by the assimilation of new observations at a certain period in the case of reanalysis. This study evaluates the stability of some longest satellite and reanalysis products (ERA5, 1950–2020, ERA5-Land, 1981–2020, and NOAA CDR, 1966–2020) by using 470 ground stations as reference data (1950–2020). Temporal stability is assessed with the time series of the annual bias in snow depth and snow cover duration of the products at the different stations.

Results show that the assimilation of new observations in ERA5 improved significantly its accuracy during the recent years (2005–2020) but introduced three negative step discontinuities starting in 1980, 1992, 2004. By contrast, ERA5-Land is more stable due to the lack of data assimilation, but at expense of worsening its accuracy despite having a finer spatial resolution. In the NOAA CDR, the increasing number of satellite data used introduces a positive trend since 1990–1995 that leads to artificial recovery of snow cover in fall and winter. The magnitude of most of these artificial trends/discontinuities is larger than actual snow cover trends and Global Climate Observing System (GCOS) stability requirements. The stability challenge of reanalysis products is linked to the assimilation of new observations to improve their accuracy or extend their temporal coverage.

The study also updates snow trends (1950–2020) over local sites in the North Hemisphere (NH) corroborating the retreat of snow cover, driven mainly by an earlier melt and recently by a later snow onset. In warmer regions such as Europe, snow cover decrease is aggravated by a decreasing snow depth due to less snowfall, while in drier regions such as Russia snow cover retreats despite the increasing snow depth observed.

Ruben Urraca and Nadine Gobron

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2021-281', Chris Derksen, 24 Nov 2021
    • AC1: 'Reply on RC1', Ruben Urraca, 07 Mar 2022
  • RC2: 'Comment on tc-2021-281', Álvaro Ayala, 25 Nov 2021
    • AC2: 'Reply on RC2', Ruben Urraca, 07 Mar 2022

Ruben Urraca and Nadine Gobron

Ruben Urraca and Nadine Gobron


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Short summary
We evaluate the fitness of some of the longest satellite (NOAA-CDR 1966–2020) and reanalysis (ERA5 1950–2020, ERA5-Land 1981–2020) products currently available to monitor the Northern Hemisphere snow cover trends using 477 stations as the reference. We found different artificial trends and step-wise discontinuities in all the products that hinder the accurate monitoring of snow trends, at least without bias correction. The study also updates on the snow cover trends during 1950–2020.