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to be peer reviewed by: Paul Berrisford, 2018-07-24 approved by CUS team leader: 2018-05-01 |
This article applies to the ERA5 sub-daily datasets:
- ERA5 high resolution (HRES), containing
- sub-daily atmospheric data (
- stream oper)
- sub-daily ocean wave data (stream wave)
- ERA5 ensemble (EDA), containing
- sub-daily atmospheric data (
- stream enda)
- sub-daily ocean wave data (stream ewda)
This article
does not
apply to
ERA5 monthly data. What streams are available? (Section 'Data organisation').
Before continuing with this article you might want to read:
Analysis and forecast
If you access ERA5 data in the C3S Climate Data Store (CDS), you will not see the concept of type 'analysis' and 'forecast'. However, behind the scene, ERA5 data archive has two types of data available, 'analysis' (an) and 'forecast' (fc). This article will help you understand the differences between these two types.
- An analysis, of the atmospheric conditions, is a blend of observations with a previous forecast. An analysis can only provide instantaneous parameters (parameters valid at a specific time, e.g temperature at 12:00), but not accumulated parameters, mean rates or min/max parameters.
- A forecast starts with an analysis at a specific time (the 'initialization time'),
- and a model computes the atmospheric conditions for a number of 'forecast steps', at increasing 'validity times', into the future. A forecast can provide instantaneous parameters, accumulated parameters, mean rates, and min/max parameters.
To see which parameters are available as analysis (an) and/or forecasts (fc) see ERA5: data documentation, section 'Parameter listings'.
Instantaneous, accumulated, mean rate and min/max parameters
Each parameter is classed as either 'instantaneous', 'accumulated', 'mean rate' or '
min/max', depending on the
temporal properties of the parameter:
- Instantaneous parameters refer to a specific point in time
- , for example temperature at 12:00
- . For a list of available surface and single level instantaneous parameters see ERA5: data documentation, Table 2.
- Accumulated parameters are accumulations during a particular time period
- , for example precipitation between
- 17:00 and 18:00.
In the ECMWF model instantaneous parameters can be generated from an analysis or from a forecast. Accumulated parameters are always generated from a forecast.
instantaneous parameters,
e.g. 2m temperatureaccumulated parameters,
Data represents the average of a 30 minute window around the analysis time (t +/- 15min)
'step' is always 0
Data represents the average of 30 minute window around the forecast validity time (t + s +/- 15min)
'step'
data represents the accumulation up to the 'time' t + 'step' s, starting from the previous step, i.e. the accumulation covers the period from t + sx-1 to t + sx
For example, for hourly data, and t=08:00 and s=
Date and Time
In ERA5 'date' and 'time' indicate a specific point in time for which a data analysis is carried out, or at which a forecast starts. This time is also known as 'validity time' of an analysis or forecast.
The ERA5 dataset contains analyses (four times per day, at 00:00, 06:00, 12:00 and 18:00), as shown below.
analysis figure
Step
In ERA5 'Step' indicates the time period in hours from the beginning of a forecast to a specific forecast time. The 'step' concept only applies to forecast data. For analysis step is 0 by definition.
The ERA5 dataset contains forecasts (from 00:00 and 12:00, with 3, 6, 9, and 12-hour steps, and more, into the future), as shown below.
In ERA-Interim, forecasts are not available at step=0, so step=0 indicates analyses.
Instantaneous parameters
All the analysed fields and many forecast fields (e.g. temperature) are referred to as "instantaneous" parameters, but are actually representative of time scales equal to the model time step (30min in ERA-Interim).
Let's say you want to extract forecast 2m-temperature at 3pm (15:00). Select as start time 12:00 (midday), with step 3 (+3 hours). This gives you the temperature at 15:00.
Accumulated parameters
In ERA-Interim the forecast accumulations (e.g. total precipitation and radiation parameters) are accumulated from the start of the forecast, ie. from T=00:00 or T=12:00.
For example, Snowfall with Time=12:00 and Step=9 gives the accumulated Snowfall in the time period 12:00 to 21:00 (12:00+9h).
See also ERA-Interim data documentation, table 9, and examples 1 to 3 below..
In ERA5, the short forecast accumulations are accumulated from the end of the previous step.
Accumulated parameters are not available from the analyses.
Minimum/maximum parameters: named '... since previous post-processing'
In ERA5 there are some parameters named '...since previous post-processing', for example 'Maximum temperature at 2 metres since previous post-processing'.
In ERA5 there are two short forecasts per day, with start time T=06:00 and T=18:00 UTC. These forecasts produce hourly output up to T+18 hours. The 'maximum temperature at 2 metres since previous post-processing' is the maximum temperature in the hour up to the forecast 'Step'. For example,- For a list of available accumulated parameters see ERA5: data documentation, Table 3.
- Mean rate parameters are temporally averaged rates over a particular time period, for example the mean snowfall rate between 17:00 and 18:00. For a list of available mean rate parameters see ERA5: data documentation, Table 4.
- Min/max parameters are the minimum or maximum 'instantaneous' value within a particular time period, for example minimum temperature between 17:00 and 18:00. For a list of available min/max parameters see ERA5: data documentation, Table 5.
Time and Step
'time' in analyses
Each analysis has a validity time, i.e. the time the data values refer to (not the time when the analysis was computed).
All validity times are in hours UTC.
Depending on the selected stream, ERA5 daily analysis data is available hourly (i.e. for the HRES, with validity time 00:00, 01:00, 02:00, ... , 23:00) or 3-hourly (i.e. for the EDA, with validity time 00:00, 03:00, 06:00, ... , 21:00). See also the ERA5: data documentation, 'Temporal resolution' and the ERA5 Catalogue, streams.
The concept of 'step' does not apply to analyses.
'time' in forecasts
Each forecast starts with the atmospheric conditions at a specific 'initialization time'. In ERA5 a new forecast is computed twice a day, with initialization times of 06:00 and 18:00 UTC.
In the ERA5 data archive, for forecasts, 'time' (and date) refers to the initialization time.
'step' in forecasts
Each forecast computes the future atmospheric conditions, and at certain "points", or "steps", during this computation the data is post-processed, and stored in the ERA5 data archive. In ERA5 there is a step every 1 or 3 hours, depending on the selected stream. Note, when downloading data from the C3S Climate Data Store (CDS), 'step' does not need to be specified because data is selected according to the valid time automatically, assuming steps from 1 to 12 hours.
Steps are referenced in hours from the forecast initialization time. This is regardless of the step interval. For example, for time=06:00, step 3 is always at 09:00 (06:00+3h).
The step interval in ERA5 is:
- Every 1 hour in the HRES atmospheric (stream=oper), HRES wave (stream=wave) and EDA wave (stream=ewda) forecasts. Hence data is available with validity times 06:00 (step 0, initialization), 07:00 (step 1, i.e. initialization + 1h), 08:00 (step 2, i.e. initialization + 2h), and so on, and equivalent for the 18:00 initialization.
- Every 3 hours in the EDA atmospheric (stream=enda) forecasts. Hence data is available with validity times 06:00 (step 0, initialization), 09:00 (step 3, i.e. initialization +3h), 12:00 (step 6, i.e. initialization + 6h), and so on, and equivalent for the 18:00 initialization.
'step' and instantaneous, accumulated and min/max parameters
The interpretation of 'step' also depends on the parameter:
- Instantaneous parameters are valid at the time indicated by time+step. For example, temperature from the forecast at time=06:00, step=3, represents the temperature at 06:00 + 3h, i.e. at 09:00.
- Accumulated parameters are aggregated up to a Step Y, starting at the previous Step X:
Note that the interval between Step X and Step Y can be 1 hour or 3 hours, depending on the selected stream. At Step 0 all accumulated values and mean rates are zero, because there is no previous data to accumulate from. | Examples:
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- Mean rate parameters are similar to accumulated parameters, except that the quantities are averaged, instead of accumulated, up to a Step Y , from the previous Step X, so the units include "per second". For example:
- In HRES atmospheric (hourly steps), mean rate precipitation at time=06:00, step=3 represents the average precipitation rate in the 1-hour period from 08:00 to 09:00,.
- In EDA atmospheric (3-hourly steps), mean rate precipitation at time=06:00, step=3 represents the average precipitation rate in the 3-hour period from 06:00 to 09:00.
- Min/max parameters (parameters named 'Minimum/Maximum ... since previous post-processing' ) are similar to accumulated parameters, except that instead of accumulating, only the min/max value during the period from Step X to Step Y is archived.
Summary
- If you download ERA5 data hosted on the C3S Climate Data Store (CDS), you can either download the data using the web interface or CDS API. If you go for CDS API, you should use the web interface to help you build up the download script by making selections and then clicking the 'Show API request' button towards the end of a download form.
- If you download ERA5 data hosted outside of CDS, you can download the data using CDS API. You should then use the ERA5 catalogue to help you build up your script by making selections and then clicking the 'View the MARS request' link.
The following table summarizes the different parameter types available in ERA5 from analysis and forecast:
Instantaneous parameters, e.g. 2m temperature | Accumulated parameters, | Mean rate parameters | Minimum/maximum parameters named 'Minimum/Maximum ... since previous post-processing' | |
---|---|---|---|---|
Analysis Calculated from observations and previous forecasts 'time' indicates a specific point in time for which a data analysis is carried out 'time' is hourly, HH:00 'step' does not apply | For example '2 metre temperature'. Values are valid at 'time' | n.a. | n.a. | n.a. |
Forecast Calculated from analysis and the forecast model 'time' indicate a specific point in time at which a forecast starts (initialization time) 'time' can be 06:00 or 18:00 'step' indicates hours after the initialization time. | For example '2 metre temperature'. Values are valid at 'time'+'step' 'step' is in the range 0 to 18 (hours after initialization) | For example 'Total precipitation'. Values represent the accumulation up to 'time'+'step', from the previous 'step' 'step' is in the range 0 to 18 (hours after initialization) At 'step' 0 all data is zero. | For example 'Mean total precipitation rate'. Values represent the average rate up to 'time'+'step', from the previous 'step' 'step' is in the range 0 to 18 (hours after initialization) At 'step' 0 all data is zero. | For example 'Maximum temperature at 2 metres since previous post-processing' |
. Values represent the Min/Max in the period up to 'time'+'step', starting from the previous 'step' 'step' is in the range 0 to 18 (hours after initialization) At 'step' 0 all data is zero. |
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Examples ERA5 data should preferably be downloaded from the C3S Climate Data Store (CDS). There you only need to select the desired validity times; the CDS then maps the validity times to the corresponding analysis/forecast and step. The examples below apply to retrieving ERA5 data from the ECMWF data archive MARS. See How to download ERA5 data via the ECMWF Web API Example 1: hourly data, accumulated; e.g. total precipitation and evaporation
The resulting WebAPI Python script to download data: These specifications give you hourly 'total precipitation' data from 2015-01-01, 06:00 to 2015-02-01, 06:00. Discard the last 6 hours (2015-02-01, 00:01 to 06:00). The data does not cover the period 00:00 to 06:00 of the first date. Retrieve these 6 hours from the last forecast of the previous day ('date':'2014-12-31', 'time':'18:00', 'step':'7/8/9/10/11/12') Example 2: daily average, minimum and maximum of 2m temperatureIn ERA5 there is no daily data, so you have to download sub-daily data and aggregate to full days yourself.
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Examples
Example 1
Let's say you want to extract daily total precipitation. On the ECMWF data server you can select as start times 00:00 (midnight) and 12:00 (midday), both with step 12. This gives you two records:
* accumulated precipitation from 00:00 to 12:00 (midnight + 12 hours)
* accumulated precipitation from 12:00 to 24:00 (midday + 12 hours)
Then sum the two values to get the daily total.
(Note the units for total precipitation is meters.)
Example 2
You need the average precipitation per hour between 00:00 and 06:00.
To obtain the average of accumulated fields between two forecast steps (STEP1 and STEP2) it is necessary to retrieve the fields for the two steps: (Field(STEP1) and Field(STEP2)) then calculate the difference and divide by the time difference:
(Field(STEP2) - Field(STEP1)) / (STEP2 - STEP1).
To obtain average precipitation per hour between 00:00 and 06:00, download Total Precipitation with time 00:00 and steps 0 (tp0) and step 6 (tp6), then calculate:
( tp6 - tp0 ) / ( 6 - 0 )
(Note the units for total precipitation is meters.)
Example 3
You need total precipitation for every 6 hours.
Daily total precipitation (tp) is only available with a forecast base time 00:00 and 12:00, so to get tp for every 6 hours you will need to extract (and for the second and fourth period calculate):
tp(00-06) = (time 00, step 6)
tp(06-12) = (time 00, step 12) minus (time 00, step 6)
tp(12-18) = (time 12, step 6)
tp(18-24) = (time 12, step 12) minus (time 12, step 6)
(Note the units for total precipitation is meters.)
Example 4
You need the daily minimum and maximum of 2m temperature.
You can use the parameters '
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mx2t(06-07) = (time 06:00, step 1)
mx2t(07-08) = (time 06:00, step 2)
mx2t(08-09) = (time 06:00, step 3)
...
mx2t(17-18) = (time 06:00, step 12)
mx2t(18-19) = (time 18:00, step 1)
mx2t(19-20) = (time 18:00, step 2)
mx2t(20-21) = (time 18:00, step 3)
...
mx2t(05-06) = (time 12:00, step 12)
Sample Python script for the ECMWF WebAPI to retrieve Min and Max 2m temperature since previous post-processing:
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The resulting WebAPI Python script to download data: These specifications give you hourly data from 2015-01-01, 06:00 to 2015-02-01, 06:00. Discard the last 6 hours (2015-02-01, 00:01 to 06:00). The data does not cover the period 00:00 to 06:00 of the first date. Retrieve these 6 hours from the last forecast of the previous day ('date':'2014-12-31', 'time':'18:00', 'step':'7/8/9/10/11/12') To find the average 2 metre temperature per day, average the hourly 2t values per date. To find the daily max/min, find the the max/min of the hourly mx2t/mn2t per date. |
See also
How to download ERA5 data via the ECMWF Web API
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This document has been produced in the context of the Copernicus Climate Change Service (C3S). The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |
This script retrieves the maximum and minimum 2m temperature from 2016-01-01 06:00 to 2017-02-01 06:00, as one-hour periods. To identify the daily maximum/minimum 2m temperature, find the the maximum of these 3-hour maxima/minima.
Further information
About ERA-Interim: http://www.ecmwf.int/en/research/climate-reanalysis/era-interim
ERA-Interim documentation: http://www.ecmwf.int/en/elibrary/8174-era-interim-archive-version-20
Worth reading about spin-up in ERA-Interim: http://www.ecmwf.int/en/elibrary/10381-forecast-drift-era-interimRelated articles
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