The use of precipitation information from the CARRA dataset is briefly described in the CARRA Data User Guide and in the document “Copernicus Arctic Regional Reanalysis (CARRA): Added value to the ERA5 global reanalysis”
Since the use of precipitation data from CARRA is sometimes confusing for the users, hereafter we give some additional practical hands-on examples.
Precipitation in the CARRA dataset is based on forecasts started from the CARRA reanalysis. Precipitation data (‘Total precipitation’) is therefore available to download as accumulated precipitation since the start time (‘Time’ in the ‘download data’-page) of the forecast, and available for different ‘Leadtime hour’ (hours since the start time).
To get precipitation for a given period you therefore need to find two ‘Leadtime hours’ for a specific forecast start ‘Time’ that together defines the end and start of your requested period. After downloading these, you need to take the precipitation from the end of the period minus the precipitation at the start of your period to get the accumulated precipitation through your period of interest.
When downloading precipitation you should ideally balance the need for accurate estimates of precipitation but avoiding spin-up issues. Accuracy is in general higher for a smaller ‘Leadtime hour’ but spin-up issues can lead to systematic underestimation of the precipitation amount for the shortest ‘Leadtime hours’. However, after ‘Leadtime hours’ 3-6 spin-up issues are considered to be small.
Example 1: If you are interested in precipitation between 06 UTC and 06 UTC the next day (the observation precipitation day in most of Europe starts and ends at 06 UTC), you can choose the forecast start at ‘Time’ 00:00, download the precipitation at ‘Leadtime hour’ 30 and 6 and subtract these (see the related CDS download form below).
(Alternatively, to use more accurate forecasts with shorter lead times, the precipitation for the last 12 hours in the period could be replaced by data from lead time hours from 6 to 18 from the forecast start time 12:00. In this case you also need to take the forecast lead time 18 and 6 from the start ‘Time’ 00:00 to get the first 12h precipitation.)
Example 2: If you are interested in the precipitation between 00 UTC and 00 UTC the next day you can choose to download the forecast starting at ‘Time’ 12:00 the day before with lead time hours 18 and 12. Subtracting these gives you the precipitation for the first 6 hours of the day. Then you use ‘Time’ 00:00 for the same day with ‘Leadtime hour’ 18 and 6 to calculate precipitation for the next 12 hours and then finally ‘Time’ 12 with ‘Leadtime hour’ 12 and 6 to calculate precipitation for the last 6 hours of the day. In the end you add the precipitation for the three periods together to get the daily precipitation.
If you are interested in accumulation periods longer than a day you need to first split it into daily or shorter periods as in example 1 and 2 and add them together.
A separate product with daily and mean monthly accumulated precipitation will soon be available from the CARRA dataset.
