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This article applies to the ERA5 sub-daily datasets:

  • ERA5 high resolution (HRES), sub-daily data (streams oper/wave)
  • ERA5 ensemble (EDA), sub-daily data (streams enda/ewda).

This article is not applicable to other ERA5 streams. See here how data is organised in ERA5.



instantaneous parameters,

e.g. 2m temperature

accumulated parameters,

e.g. precipitation
analysis

Data represents the average of a 30 minute window around the analysis time (t +/- 15min)

step=0		n.a
forecast

Data represents the average of 30 minute window around the forecast time (t + s +/- 15min)

data represents the accumulation up to the 'time' + 'step', from the previous step, i.e. the accumulation covers the period [ t + sx-1 to t + sx ]

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, 'Maximum temperature at 2 metres since previous post-processing' with Time=06:00 and Step=3, is the max 2m temperature in the one-hour period leading up to 06:00+3h, ie, in the period 08:00 to 09:00. See Example 4 below.

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 'Maximum temperature at 2 metres since previous post-processing' (mx2t, parameter id 201) and 'Minimum temperature at 2 metres since previous post-processing' (mn2t, parameter id 202). These two parameters are available from the forecasts initialized at 06:00 and 18:00 in one-hour steps:

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:

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    "class": "ea",
    "dataset": "era5",
    "stream": "oper",
    "expver": "1",
    "date": "2016-01-01/to/2016-01-31",
    "type": "fc",
    "levtype": "sfc",
    "param": "201.128/202.128",             # 'Maximum temperature at 2 metres since previous post-processing' and 'Minimum temperature at 2 metres since previous post-processing'
    "time": "06:00:00/18:00:00",            # 2 forecasts per day, at T=06:00 and T=12:00
    "step": "1/2/3/4/5/6/7/8/9/10/11/12",   # For each forecast 18 hourly steps are available. Here we use only steps 1 to 12.
    "grid": "0.30/0.30",
    "area": "60/-10/20/50",
    "target": "output",                     # change this to your output file name
})

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-interim

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