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QUESTIONS RELATED TO THE CARRA DATA

Can you give a tutorial, please on how to obtain hourly precipitation timeseries? Why is it left to the user to produce this?

It is difficult to download and calculate daily accumulated precipitation, would this be simplified? What are the interpretation of the precipitation forecast "leadtime" and "time" variables?


What would be your guess why CARRA snow thickness (on sea ice) is usually much larger than from altimetry?

It is not clear which altimetry product the question is referring to and how accurate it is. However, a comparison study of such data with CARRA data would be of interest. The modelling of snow on sea ice in CARRA applies the SICE scheme (Batrak et al, 2018, https://doi.org/10.5194/gmd-11-3347-2018). This scheme has been shown to give advances over previous schemes, which don't attempt to take the snow on sea ice into account, however it still does not give a perfect representation of the snow on sea ice. To speculate about sources for possible biases, one could mention the absence of ice dynamics in the scheme: If there is an area of intensive snow accumulation, ice has no chance to transport this snow and it keeps piling up in the same grid cells. Also the absence of snow-ice formation in the model (even though it isn't as important in the Arctic as in the Antarctic). Additionally, the CARRA system doesn't represent the process of snow precipitation loss to the open sea portion of a grid cell when ice concentration is below 100%.


QUESTIONS RELATED TO THE CARRA DATA

Can you give a tutorial, please on how to obtain hourly precipitation timeseries? Why is it left to the user to produce this?

It is difficult to download and calculate daily accumulated precipitation, would this be simplified? What are the interpretation of the precipitation forecast "leadtime" and "time" variables?

The precipitation amount is stored as accumulated precipitation since the forecast start time. This is a The precipitation amount is stored as accumulated precipitation since the forecast start time. This is a standard procedure in Numerical Weather Prediction (NWP) although some centres re-compute the precipitation amounts for shorter time intervals (like ERA5 for 1h intervals). In CARRA this was not considered, instead it was thoroughly explained in the CARRA Data User Guide (Copernicus Arctic Regional Reanalysis (CARRA): Data User Guide#Singlelevelvariables) and even a designated page was created to explain all the details for the users (https://confluence.ecmwf.int/pages/viewpage.action?pageId=338485588). The latter document also includes some recommendations and examples. 

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This is a valid question what we are aware of. First of all we tried to speed up the access to the data, i.e. faster download speeds for the full (whole domain) data files. We think that with the migration of our archiving system to Bologna this gave some improvements. We are also planning to put the most used parts of the CARRA datasets into spinning discs instead of tapes. The next step will be to make possible for the users to have any geographic subset of the CARRA data. This is work in progress (the complications are related to the distorted Lambert projection geometry towards the Pole and the fact that the netcdf files don't have a good data compression capabilities).improvements. We are also planning to put the most used parts of the CARRA datasets into spinning discs instead of tapes. The next step will be to make possible for the users to have any geographic subset of the CARRA data. This is work in progress (the complications are related to the distorted Lambert projection geometry towards the Pole and the fact that the netcdf files don't have a good data compression capabilities).


When will monthly and daily means be available?

The software computing CARRA daily and monthly mean values are ready and will be deployed soon for the entire CARRA dataset. Since we need to get through the entire more than 30-year sub-daily CARRA dataset for the daily/monthly mean computations it will take a bit of time to complete the process and open the dataset for public use. Our latest estimate is that the means will be available for the users around mid-2024. We will give news on our progress in the CARRA User Forum (see at Copernicus Arctic Regional Reanalysis (CARRA)).