First results of the snow-atmosphere coupling analysis in the CERISE project 

In early 2023, the European project CERISE started, under the auspices of the Copernicus Climate Change Service (C3S), devoted to enhancing the quality of the C3S reanalysis and seasonal forecasts with a focus on land-atmosphere coupling and land surface initialization. Studying snow cover is an important part of the CERISE project as it affects surface energy budget and hydrology, leading to shifts in atmospheric circulation and potential remote climate impacts. The impact of snow initialization on (sub-)seasonal atmospheric forecasts has received renewed attention in recent years.In this study, a first attempt is made to analyze snow-atmosphere coupling in seasonal forecasts produced by the European Center for Medium-range Weather Forecasts (ECMWF) as the phase 0 demonstrator in the CERISE project. In phase 0, the atmospheric and land initial conditions (including snow) were taken from the ERA5 and the experiments were run with Integrated Forecasting System (IFS) Cycle 48R1.1. The snow initial conditions in ERA5 comprise assimilation of Interactive Multi-Sensor Snow and Ice Mapping System (IMS) satellite observations and in-situ station data. The 4-month forecasts consist of 51 ensemble members, and 4 start dates are available per year (in months 2, 5, 8, and 11).  To assess the snow-atmosphere coupling, we map the temporal correlation between the daily snow depth and near-surface temperature over winters 2000 – 2022. Based on this (and other) metrics we identify regions of snow-atmosphere coupling. These “cold spots” are mainly situated in the snow transition regions at mid-latitudes, where the snow cover is highly variable. We further discuss the role of snow in large-scale land-atmosphere interactions. The accuracy and fidelity of the snow forecasts is also examined against satellite observations and re-analyses. The satellite observations include snow cover fraction and/or snow water equivalent products from the ESA-CCI (European Space Agency – Climate Change Initiative) dataset.