# Description about ensemble members ERA5

**URL:** <https://forum.ecmwf.int/t/description-about-ensemble-members-era5/1349>\
**Category:** C3S - Datasets and Usage\
**Tags:** c3s, era5\
**Created:** [19 August 2021 15:32 UTC](https://forum.ecmwf.int/t/description-about-ensemble-members-era5/1349 "2021-08-19T15:32:42Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![thomas\_cordonnier](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/t/ecb155/32.png) [@thomas\_cordonnier](https://forum.ecmwf.int/u/thomas_cordonnier)\
**Post date:** [19 August 2021 15:32 UTC](https://forum.ecmwf.int/t/description-about-ensemble-members-era5/1349/1 "2021-08-19T15:32:42Z")

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Dear all, I have a question about ERA5 EDA ensemble members mean and Spread.

When downloading those files in NetCDF experimental format from CDS (Copernicus data storage) I can see that the members are identified by a number ranging from 0 to 9.

If I well understand there is 9 members and an unperturbed control. My question is how can I identify the control ? Is it the number 0 in the ranging?

Also how is calculated the ensemble mean? Is it the mean of the 10 members or the mean of the 9 members excluding the control member?

Thanks for your help.

Thomas

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**Author:** ![Michela](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/m/898d66/32.png) [@Michela](https://forum.ecmwf.int/u/Michela)\
**Post date:** [10 September 2021 13:31 UTC](https://forum.ecmwf.int/t/description-about-ensemble-members-era5/1349/2 "2021-09-10T13:31:32Z")

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Hi,

please have a look at the following link:

[ERA5: data documentation#Ensemblemeansandstandarddeviations](https://confluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation#ERA5:datadocumentation-Ensemblemeansandstandarddeviations)

Regards

Michela

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**Author:** ![thomas\_cordonnier](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/t/ecb155/32.png) [@thomas\_cordonnier](https://forum.ecmwf.int/u/thomas_cordonnier)\
**Post date:** [13 September 2021 13:27 UTC](https://forum.ecmwf.int/t/description-about-ensemble-members-era5/1349/3 "2021-09-13T13:27:37Z")

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Hi Michela,&nbsp;

Thanks for your help and the link. But I can't find all the information that I am looking for.&nbsp;

I understand that "Both these quantities are calculated from all the 10-members (i.e., including the control).", so ok, it is the mean and standard deviation ("spread") of the 10 members, including control.&nbsp;

However I still don't know how to identify the control member&nbsp;(number"0"?) when downloading Ensemble members. Also when I do the mean or stdev of the 10 members (manual calculation from the 10 members) at a specific time and lon/lat I don't get exacly the same value as the mean or standard deviation directly downloaded from CDS.

So I am wondering how are calculated those values.&nbsp;

Best regards

Thomas

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