The webpage to download ERA data 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.
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asked Oct 12, 2015 at 17:57
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2$\begingroup$ I suggest editing the question to define the acronym "ERA" $\endgroup$– haresfurOct 13, 2015 at 0:30
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2 Answers
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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.
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$\begingroup$ 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. $\endgroup$ Oct 15, 2015 at 15:02
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$\begingroup$ @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. $\endgroup$– 410 goneOct 16, 2015 at 8:59
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$\begingroup$ @EnergyNumbers why is it that some variables are not available at 6 and 12 time and not available at step 0? $\endgroup$ Oct 26, 2015 at 19:13
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1$\begingroup$ I think it could be useful to address the availability of variables in a separate question. $\endgroup$– BHFOct 27, 2015 at 6:45
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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
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2$\begingroup$ Your answer is in another castle. Can you edit this and quote the relevant parts here? The text as written now does not seem to answer the question. $\endgroup$ Dec 5, 2018 at 9:10