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ERA-Interim production to stop on 31st August 2019

As ERA5 is now available (What are the changes from ERA-Interim to ERA5?), we are preparing to stop the production of ERA-Interim on 31st August 2019. This means that the complete span of ERA-Interim data will be from 1st January 1979 to 31st August 2019.

Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of 2019.

For the time being and until further notice, ERA-Interim shall continue to be accessible through the ECMWF Web API. ERA5 is available from the Climate Data Store (CDS).


The ERA-Interim dataset contains analyses (four times per day, at 00:00, 06:00, 12:00 and 18:00), as well as forecasts (from 00:00 and 12:00, with 3, 6, 9, and 12-hour steps, and more, into the future), as shown below.

In the ERA-Interim web interface, if you select step=0 only analysis parameters are available; if you select step>0 only forecast parameters are available.

 

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), for more information see the article Model grid box and time step.

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 ERA-Interim there are some parameters named '...since previous post-processing', for example 'Maximum temperature at 2 metres since previous post-processing'.

In ERA-Interim there are two forecasts per day, at T=00:00 and T=12:00 UTC. These forecasts produce 3-hourly output up to T+12 hours (and longer, though becoming less frequent, for surface and pressure level parameters). The 'maximum temperature at 2 metres since previous post-processing' is the maximum temperature between the preceding archived forecast 'Step' and the forecast 'Step'. For example, 'Maximum temperature at 2 metres since previous post-processing' with Time=00:00 and Step=9, is the max 2m temperature in the 3-hour period leading up to 00:00+9h, ie, in the period 06:00 to 09:00. See Example 4 below.

Examples

Example 1

Let's say you need data on daily total precipitation. Total precipitation is an accumulated parameter, so in the data archive this parameter is classed as forecast (fc). The data is available from two daily forecast start times (at 00:00 and 12:00), and from both you want data 12 hours into the future (step 12), as in this example:

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    "class": "ei",
    "dataset": "interim",
    "stream": "oper",
    "expver": "1",
    "date": "2017-01-01/to/2017-01-31",
    "type": "fc",
    "levtype": "sfc",
    "param": "228.128",             # parameter 'total precipitation'
    "step": "12",                   # accumulation over 12 hours
    "time": "00:00:00/12:00:00",    # from the two forecasts initialized at 00:00 and 12:00
    "grid": "0.75/0.75",
    "target": "output",             # change this to your output file name
})

This script retrieves:

* accumulated precipitation from 00:00 to 12:00 (time 00:00 + step 12)

* accumulated precipitation from 12:00 to 24:00 (time 12:00 + step 12)

Then sum the two values to get the daily total precipitation.

(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 00:00 and 12:00 in 3-hour steps, i.e. as eight 3-hour periods per day:

mx2t(00-03) = (time 00, step 3)

mx2t(03-06) = (time 00, step 6)

mx2t(06-09) = (time 00, step 9)

mx2t(09-12) = (time 00, step 12)

mx2t(12-15) = (time 12, step 3)

mx2t(15-18) = (time 12, step 6)

mx2t(18-21) = (time 12, step 9)

mx2t(21-24) = (time 12, step 12)

Sample Python script for the ECMWF WebAPI to retrieve Min and Max 2m temperature since previous post-processing, giving eight 3-hour periods per day:

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    "class": "ei",
    "dataset": "interim",
    "stream": "oper",
    "expver": "1",
    "date": "2017-01-01/to/2017-01-31",
    "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'
    "step": "3/6/9/12",             # 4 steps per forecast
    "time": "00:00:00/12:00:00",    # 2 forecasts per day
    "grid": "0.75/0.75",
    "target": "output",             # change this to your output file name
})

This script retrieves the maximum and minimum 2m temperature for the eight 3-hour periods per day. To identify the daily maximum/minimum 2m temperature, find the the maximum of these 3-hour maxima/minima.

Example 5

You need data in the highest possible temporal resolution. The highest possible resolution with ERA-Interim data is 3 hours.

If your parameter is instantaneous, for example 2 metre temperature, or wind u/v components:

  1. ERA-interim computes an analysis for every 6 hours (00:00, 06:00, 12:00, 18:00).  Download the analysis data, using 'type':'an', 'time':'00:00/06:00/12:00/18:00' , 'step':'0'
  2. For each 3 hours between the analysis, you can use forecast data. Download this using 'type':'fc', 'time':'00:00/12:00' , 'step':'3/9'. This gives you data for 03:00, 09:00, 15:00, 21:00
  3. Merge the two downloaded data files if needed.

If your parameter is accumulated, for example total precipitation:

  1. Accumulated data is archived as type 'forecast' and available from 00:00 and 12:00. Download data using 'type':'fc', 'time':'00:00/12:00' , 'step':'3'/6/9/12'. This gives you:
    • accumulation from 00:00 to 03:00 (00:00+3h)
    • accumulation from 00:00 to 06:00 (00:00+6h)
    • accumulation from 00:00 to 09:00 (00:00+9h)
    • accumulation from 00:00 to 12:00 (00:00+12h)
    • accumulation from 12:00 to 15:00 (00:00+3h)
    • accumulation from 12:00 to 18:00 (00:00+6h)
    • accumulation from 12:00 to 21:00 (00:00+9h)
    • accumulation from 12:00 to 00:00 (00:00+12h)
  2. Note that in ERA-Interim all accumulations are from the beginning of the respective forecast (i.e. from 00:00 or 12:00), so the accumulation periods overlap. Calculate 3- hour periods:
    • (accumulation from 00:00 to 03:00): no calculation required
    • (accumulation from 03:00 to 06:00) = (time 00:00, step 6) - (time 00:00, step 3)
    • (accumulation from 06:00 to 09:00) = (time 00:00, step 9) - (time 00:00, step 6)
    • (accumulation from 09:00 to 12:00) = (time 00:00, step 12) - (time 00:00, step 9)
    • (accumulation from 12:00 to 15:00): no calculation required
    • (accumulation from 15:00 to 18:00) = (time 12:00, step 6) - (time 12:00, step 3)
    • (accumulation from 18:00 to 21:00) = (time 12:00, step 9) - (time 12:00, step 6)
    • (accumulation from 21:00 to 00:00) = (time 12:00, step 12) - (time 12:00, step 9)
  3. Merge the calculated data files if needed

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

About ERA-Interim monthly means: ERA-Interim: monthly means

Worth reading about spin-up in ERA-Interim: http://www.ecmwf.int/en/elibrary/10381-forecast-drift-era-interim