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The method we have used for estimating the uncertainties is described here in details: https://datastore.copernicus-climate.eu/documents/reanalysis-carra/CARRAUncertainty%20estimationFinal.pdf Copernicus Arctic Regional Reanalysis (CARRA): Uncertainty estimation method
The methodology does not account for systematic errors or for uncertainties associated with the forecasting model used for numerical simulations, such as for example parameterization of unresolved processes. In addition to the climatological upper bound analysis error standard deviation that we have discussed above, we provide a refined climatological analysis error standard deviation. The refined climatological analysis error standard deviation is the climatological upper bound analysis error standard deviation multiplied by a rescaling factor. The rescaling factor is obtained by confronting the climatological upper bound forecast error variance against observation error variance in observation space. The multiplication by rescaling factor allows compensating for the limitations of the methodology mentioned above to some extent. The refined climatological analysis error standard deviation is a more reliable estimate of the uncertainty of the CARRA datasets than the climatological upper bound analysis error standard deviation in the areas with dense observation coverage. At the same time, the short-range forecast error variance is always lower in the areas covered by observations than in the areas of sparse observation coverage. Thus, the refined climatological analysis error standard deviation can underestimate the actual uncertainty of the Arctic reanalysis data in the areas with poor observation coverage. We propose to use the refined analysis error standard deviation in data rich areas and to use the upper bound standard deviation in data sparse areas as a measure of uncertainty.
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For further details see the test and verification report for CARRA: https://datastore.copernicus-climate.eu/documents/reanalysis-carra/CARRATestVerificationFinal.pdf Copernicus Arctic Regional Reanalysis (CARRA): Complete test and verification report on fully configured reanalysis and monitoring system and the full system documentation: https://datastore.copernicus-climate.eu/documents/reanalysis-carra/CARRAFullSystemDocumentationFinal.pdf Copernicus Arctic Regional Reanalysis (CARRA): Full system documentation
Known Issues
Since the regional reanalysis is run nested into the ERA5 global reanalysis, it is affected by the known issues of ERA5. In addition to those issues, we have found that ERA5 uses incorrect glacier masks for most of the glaciers in the regional Arctic reanalysis domain, and the glaciers in ERA5 always have an analysis albedo of 0.85. This is wrong, since for instance exposed glacier ice albedos during summer are unaccounted for. These areas affect the general circulation and thermodynamic state in ERA5 and can affect the quality of the Arctic regional reanalysis.
Additionally, the Arctic reanalysis has the following known issues:
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The details about the Numerical Weather Prediction model system used can be found in the full system documentation: https://datastore.copernicus-climate.eu/documents/reanalysis-carra/CARRAFullSystemDocumentationFinal.pdf Copernicus Arctic Regional Reanalysis (CARRA): Full system documentation
To supplement this we provide in the following a short description of the service as well as a description of basic principles of data assimilation in numerical weather prediction aimed at a non-specialist audience.
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