<p>Some confusion comes from the mixture of analysis data (such as 2m-air temperature) and reanalysis products (such as downward radiation) in the final reanalysis product. The following text directly from ECMWF may help</p>

<p><a href="https://confluence.ecmwf.int/pages/viewpage.action?pageId=56658233" rel="nofollow noreferrer">https://confluence.ecmwf.int/pages/viewpage.action?pageId=56658233</a></p>

CC BY-SA 4.0
2018-12-05T09:10:31.633Z
[Your answer is in another castle](http://meta.stackexchange.com/questions/225370/your-answer-is-in-another-castle-when-is-an-answer-not-an-answer). Can you [edit] this and quote the relevant parts here? The text as written now does not seem to answer the question.
Jan Doggen
CC BY-SA 4.0
2018-12-05T08:42:27.773Z
2018-12-05T08:42:27.773Z
Christian
<p>The "step" is the number of hours the ERA re-forecast has been run into the future from the "time" of the re-analysis.</p>

<p>For example, asking for ERA data with time = 12:00 and step = 0 means asking for data valid at 12:00 based on a re-analysis for 12:00. But asking for data with time = 12:00 and step = 6 means asking for data valid at 18:00, i.e. for data produced by a 6 h model run based on the 12:00 re-analysis.</p>

<p>To fill the gaps between the analysis times one can use steps from a forward run. Forward runs are needed to get parameters like time accumulated precipitation, which would not be available from an analysis. Sometimes a combination of steps and times might be appropriate for a joint analysis of instantaneous and time averaged parameters, i.e. to stagger them correctly in time.</p>

<p>Data valid 00:00 deviate less from the observations if they come from a 00:00 analysis compared to data from step 12 of the 12:00 analysis. But a series of steps from one initialization time should be more consistent in time compared to a series of independent analyses.</p>

CC BY-SA 3.0
2015-10-26T19:13:05.377Z
@EnergyNumbers why is it that some variables are not available at 6 and 12 time and not available at step 0?
Herman Toothrot
CC BY-SA 3.0
2015-10-27T06:45:58.223Z
I think it could be useful to address the availability of variables in a separate question.
BHF
CC BY-SA 3.0
2015-10-27T10:55:21.370Z
@user4050 ^ just as BHF says
410 gone
CC BY-SA 3.0
2015-10-15T15:02:38.110Z
thank you, it's still somewhat unclear though, all I need is the data at the time it was measured/modelled, what is the purpose of the step, why would someone want to use a step 6 or 12? It would be night instead of day.
Herman Toothrot
CC BY-SA 3.0
2015-10-16T08:59:53.630Z
@user4050 e.g. because sometimes it's useful to know the difference between "forecast" and "outturn", and comparing the 00:00 figures with the previous day's 12:00 + 12h forecast is one way to approach that.
410 gone
CC BY-SA 3.0
2015-10-12T19:50:06.017Z
2015-10-15T20:23:19.107Z
2015-10-15T20:23:19.107Z
BHF
<p>The webpage to <a href="http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc" rel="noreferrer">download ERA data</a> includes an option called "time" which is intuitive, but also an option called "step". I can't find an explanation of the meaning of this. It can't be a time step because it has also 0.</p>

CC BY-SA 3.0
2015-10-13T00:30:50.047Z
I suggest editing the question to define the acronym "ERA"
haresfur
<atmosphere-modelling><reanalysis><era>
What is the meaning of step in ERA reanalysis data?
Herman Toothrot
# What is the meaning of step in ERA reanalysis data?

(Asked by: username_0 on 2015-10-12)

The webpage to [download ERA data](http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc) includes an option called "time" which is intuitive, but also an option called "step". I can't find an explanation of the meaning of this. It can't be a time step because it has also 0.

**Tags:** atmosphere-modelling, reanalysis, era

> **Comments:**
> **username_1**: I suggest editing the question to define the acronym "ERA"  

---

## 2 answers

### Answer by username_4 on 2015-10-12. Score: 6 | Accepted answer

The "step" is the number of hours the ERA re-forecast has been run into the future from the "time" of the re-analysis.

For example, asking for ERA data with time = 12:00 and step = 0 means asking for data valid at 12:00 based on a re-analysis for 12:00. But asking for data with time = 12:00 and step = 6 means asking for data valid at 18:00, i.e. for data produced by a 6 h model run based on the 12:00 re-analysis.

To fill the gaps between the analysis times one can use steps from a forward run. Forward runs are needed to get parameters like time accumulated precipitation, which would not be available from an analysis. Sometimes a combination of steps and times might be appropriate for a joint analysis of instantaneous and time averaged parameters, i.e. to stagger them correctly in time.

Data valid 00:00 deviate less from the observations if they come from a 00:00 analysis compared to data from step 12 of the 12:00 analysis. But a series of steps from one initialization time should be more consistent in time compared to a series of independent analyses.

> **Comments:**
> **username_0**: thank you, it's still somewhat unclear though, all I need is the data at the time it was measured/modelled, what is the purpose of the step, why would someone want to use a step 6 or 12? It would be night instead of day.  
> **username_5**: @user4050 e.g. because sometimes it's useful to know the difference between "forecast" and "outturn", and comparing the 00:00 figures with the previous day's 12:00 + 12h forecast is one way to approach that.  
> **username_0**: @EnergyNumbers why is it that some variables are not available at 6 and 12 time and not available at step 0?  
> **username_4**: I think it could be useful to address the availability of variables in a separate question.  
> **username_5**: @user4050 ^ just as BHF says  

---

### Answer by username_2 on 2018-12-05. Score: 1

Some confusion comes from the mixture of analysis data (such as 2m-air temperature) and reanalysis products (such as downward radiation) in the final reanalysis product. The following text directly from ECMWF may help

<https://confluence.ecmwf.int/pages/viewpage.action?pageId=56658233>

> **Comments:**
> **username_3**: [Your answer is in another castle](http://meta.stackexchange.com/questions/225370/your-answer-is-in-another-castle-when-is-an-answer-not-an-answer). Can you [edit] this and quote the relevant parts here? The text as written now does not seem to answer the question.  

---

