-------------------------------------------------------------------------------- DRAFT ---------------------------------------------------------------------------------
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
In the ERA5 data archive two types of data are available, 'analysis' (an) and 'forecast' (fc):
- 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 the ERA5 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 the ERA5 documentation, Table 2.
- Accumulated parameters are accumulations during a particular time period, for example precipitation between 17:00 and 18:00. For a list of available accumulated parameters see the ERA5 documentation, Table 3.
- Mean rate parameters are temporally averaged rates over a particular time period, for example the mean snowfall rate, per second, between 17:00 and 18:00. For a list of available mean rate parameters see the ERA5 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 the ERA5 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. with validity time 00:00, 01:00, 02:00, ... , 23:00) or 3-hourly (i.e. with validity time 00:00, 03:00, 06:00, ... , 21:00). See also ERA5 data documentation, 'Temporal resolution' and the ERA5 Catalogue, streams.
The concept of 'step' does not apply to analyses.
'time' in forecasts
ERA5 is largely a series of weather 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 time 06:00 and 18:00 UTC.
In the ERA5 data archive, for forecasts, 'time' specifies the initialization time.
'step' in forecasts
Each forecast computes the future atmospheric conditions, and at certain points during this computation a snapshot of the computed data is taken, post-processed, and stored in the ERA5 data archive. These snapshots are called 'steps'. In ERA5 there is a step every 1 hour or 3 hours, depending on the selected stream.
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:
|
- 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 average precipitation up to [06:00 + 3h], starting from the previous step (Step 2), i.e. average precipitation in the 1-hour period 08:00 to 09:00, per second.
- In EDA atmospheric (3-hourly steps), mean rate precipitation at time=06:00, step=3 represents average precipitation up to [06:00 + 3h], starting from the previous step (Step 0), i.e. average precipitation in the 3-hour period 06:00 to 09:00, per second.
- 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 of the period from Step X to Step Y is archived.
Summary
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' indicate 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. At each 'step' a snapshot of the forecast output is available | For example '2 metre temperature'. Values are valid at 'time'+'step' 'step' can be 0 (effectively giving analysis), or in the range 1 to 18 (hours after initialization) | For example 'Total precipitation'. Values represent the accumulation up to 'time'+'step', up from the previous 'step' 'step' is in the range 0 to 18 (hours after initialization) At 'step' 0 all data is zero though. | For example 'Mean total precipitation rate'. Values represent the average up to 'time'+'step', up from the previous 'step', and normalized to per-second units 'step' is in the range 0 to 18 (hours after initialization) At 'step' 0 all data is zero though. | 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 though. |
Examples
See also How to download ERA5 data via the ECMWF Web API
Example 1: hourly data, accumulated; e.g. total precipitation and evaporation
| Step | Your requirement | Select from the ERA5 data catalogue | Set in a WebAPI Python script | |
|---|---|---|---|---|
| 1 | You are interested in HRES data, not in the ensemble | > | Select Deterministic forecast, Atmospheric model | 'stream':'oper' |
| 2 | You need total precipitation and evaporation. See parameters available in ERA5 and ECMWF parameter definitions You find suitable parameters:
| > | Select Forecast | 'type':'fc' |
| 3 | Month | > | Select a month, e.g. 2015, January | |
| 4 | Total precipitation and and evaporation are 2-dimensional fields ('surface field') | > | Select Surface | 'levtype':'sfc' |
| 5 | Dates | > | Select dates | 'date':' 2015-01-01/to/2015-01-31' |
| 6 | ERA5 forecasts are initialized at 06:00 and 18:00, and in HRES the interval between forecast steps is one hour. Hence you need forecast steps 1 to 12, from each forecast | > | Select time 06:00 and 18:00 Select steps 1 to 12 | 'time':'06:00/18:00' 'step':'1/2/3/4/5/6/7/8/9/10/11/12' |
| 7 | Specify parameters | > | Select parameters 'Total precipitation' and 'Evaporation' | 'param':'tp/e' (short names) or 'param':'228/182' (param IDs) |
| 8 | > | Click 'View the MARS request' |
The resulting WebAPI Python script to download data:
#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
"class": "ea", # do not change
"dataset": "era5", # do not change
"expver": "1", # do not change
"stream": "oper", # do not change
"date": "2015-01-01/to/2015-01-31",
"param": "228.128/182.128", # parameter IDs (the .128 is optional and automatically by MARS)
"levtype": "sfc", # tp and e are 2-dimensional fields ('surface field')
"type": "fc", # tp and e are accumulated parameters, hence available from forecast (fc), not analysis (an)
"time": "06:00/18:00", # There are 2 forecasts per day, starting at 06:00 and 12:00
"step": "1/2/3/4/5/6/7/8/9/10/11/12", # From each forecast we retrieve steps 1(h) to 12(h).
"target": "output", # change this to your desired output file name
})
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')
Merge the two downloaded files
Example 2: daily average, minimum and maximum of 2m temperature
In ERA5 there is no daily data, so you have to download sub-daily data and aggregate to full days yourself.
| Step | Your requirement | Select from the ERA5 data catalogue | Set in a WebAPI Python script | |
|---|---|---|---|---|
| 1 | You are interested in HRES data, not in the ensemble | > | Select Deterministic forecast, Atmospheric model | 'stream':'oper' |
| 2 | You need average, minimum and maximum of 2m temperature. See parameters available in ERA5 and ECMWF parameter definitions You find suitable parameters:
You could retrieve the first parameter from analysis and the other two from forecast, or all three from forecast. You choose the latter. | > | Select Forecast | 'type':'fc' |
| 3 | Month | > | Select a month, e.g. 2015, January | |
| 4 | All 2 metre temperatures are by definition 2-dimensional fields ('surface field') | > | Select Surface | 'levtype':'sfc' |
| 5 | Dates | > | Select dates | 'date':' 2015-01-01/to/2015-01-31' |
| 6 | ERA5 forecasts are initialized at 06:00 and 18:00, and in HRES the interval between forecast steps is one hour. Hence you need forecast steps 1 to 12, from each forecast | > | Select time 06:00 and 18:00 Select steps 1 to 12 | 'time':'06:00/18:00' 'step':'1/2/3/4/5/6/7/8/9/10/11/12' |
| 7 | Specify parameters | > | Select parameters
| 'param':'2t/mx2t/mn2t' (short names) or 'param':'167/201/202' (param IDs) |
| 8 | > | Click 'View the MARS request' |
The resulting WebAPI Python script to download data:
#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
"class": "ea", # do not change
"dataset": "era5", # do not change
"expver": "1", # do not change
"stream": "oper", # do not change
"date": "2015-01-01/to/2015-01-31",
"param": "167/201/202", # parameter IDs
"levtype": "sfc", # tp and e are 2-dimensional fields ('surface field')
"type": "fc", # tp and e are accumulated parameters, hence available from forecast (fc), not analysis (an)
"time": "06:00/18:00", # There are 2 forecasts per day, starting at 06:00 and 12:00
"step": "1/2/3/4/5/6/7/8/9/10/11/12", # From each forecast we retrieve steps 1(h) to 12(h).
"target": "output", # change this to your desired output file name
})
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')Merge the two downloaded files
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.
