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Introduction

Accumulated variables are those which are aggregated over a particular time periods.

...

Dataset

Variables

What it is

Grib units

Netcdf Units

To get the total precipitation for an hour (mm)

To get the total precipitation for a day (mm)

To get the total precipitation for a month (mm)

To get the total precipitation for a year (mm)

ERA5 reanalysis (hourly data)

Total precipitation

Accumulations are over the hour (the processing period) ending at the validity date/time

m (of water equivalent per hour)

Mathdisplay
\text{tp}\ [\text{mm}] = \text{tp}\ [\text{m}] \cdot 1000

Total precipitation over the one hour ending at forecast step.

Mathinline
\text{tp}\ [\text{mm}] = \left( \sum_{h = 1}^{23}\text{tp}_{h}\ [\text{m}] + \text{tp}_{d+1\ 00\text{UTC}}\ [\text{m}] \right) \cdot 1000

where h is the hour and d the day of interest (d+1 is the following day).

The total precipitation over 24 hours is the sum of the individual total precipitation values for each hour.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{N} \left( \sum_{h = 1}^{23}\text{tp}_{h}\ [\text{m}] + \text{tp}_{d+1\ 00\text{UTC}}\ [\text{m}] \right)_{n} \cdot 1000

where N is the number of days in the month.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{N} \left( \sum_{h = 1}^{23}\text{tp}_{h}\ [\text{m}] + \text{tp}_{d+1\ 00\text{UTC}}\ [\text{m}] \right)_{n} \cdot 1000

where N is the number of days in the year.

ERA5 ensemble members (3 hourly data)

Total precipitation

Accumulations are over the 3 hours (the processing period) ending at the validity date/time

m (of water equivalent per 3 hours)

Mathinline
\text{tp}\ [\text{mm}] = \text{tp}\ [\text{m}] \cdot 1000

Mathinline
\text{tp}\ [\text{mm}] = \left( \sum_{h} \text{tp}_{h} [\text{m}] \right) \cdot 1000

where h are the day timesteps available: 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC and 00 UTC of the day after. The total precipitation over 24 hours is the sum of 8 3-hour total precipitation values.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{N} \left(\sum_{h} \text{tp}_{h}[\text{m}] \right)_{n} \cdot 1000

where h are the day timesteps available: 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC and 00 UTC of the day after and N is the number of days in the month.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{N} \left(\sum_{h} \text{tp}_{h}[\text{m}] \right)_{n} \cdot 1000

where h are the day timesteps available: 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC and 00 UTC of the day after, and N is the number of days in the year.

ERA5 monthly averaged reanalysis

Total precipitation

The accumulations in monthly means of daily means have been scaled to have units that include "per day"

m (of water equivalent per day)

N/A

Mathinline
\text{tp}\ [\text{mm}] = \text{tp}\ [\text{m/day}] \cdot 1000 \cdot N

where N is the number of days in the month.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{12}(\text{tp}\ [\text{m/day}] \cdot 1000 \cdot N)_{\text{n}}

where N is the number of days in the month and n is the number of the month.

ERA5 monthly averaged ensemble members

Total precipitation

The accumulations in monthly means of daily means have been scaled to have units that include "per day"

m (of water equivalent per day)

N/A

Mathinline
\text{tp}\ [\text{mm}] = \text{tp}\ [\text{m/day}] \cdot 1000 \cdot N

where N is the number of days in the month.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{12}(\text{tp}\ [\text{m/day}] \cdot 1000 \cdot N)_{\text{n}}

where N is the number of days in the month and n is the number of the month.

ERA5-Land hourly data

Total precipitation

Accumulations are from 00 UTC to the hour ending at the forecast step ¹

m (of water equivalent)

Mathinline

\text{tp}\ [\text{mm}] = \left\{
\begin{array}{ll}
    \text{tp}_{h}\ [\text{m}] \cdot 1000 & h = 01 \text {UTC} \\
    (\text{tp}_{h}\ [\text{m}]\ -\ \text{tp}_{h-1}\ [\text{m}])\ \cdot 1000 & \text{otherwise} \\
\end{array} 
\right.

Mathinline
\text{tp}\ [\text{mm}] = \text{tp}_{d+1\ 00\text{UTC}}\ [\text{m}]\ \cdot 1000

where d is the day for which the average flux total precipitation is being computed.

The time step labelled d+1 00UTC should also be taken because it contains the accumulated flux total precipitation over the previous 24 hours.

Mathinline
\text{tp}\ [\text{mm}] = \left( \sum_{n = 1}^{N}\text{tp}_{d+1\ 00\text{UTC},n}\ [\text{m}] \right)\cdot 1000

where d is the day for which the total precipitation is being computed and N is the number of days in the month.

The time step labelled d+1 00UTC should also be taken because it contains the accumulated total precipitation over the previous 24 hours. This implies that the sum goes from the 2^nd of the month to the 1^st of the next month (inclusive).

Mathinline
\text{tp}\ [\text{mm}] = \left( \sum_{n = 1}^{N}\text{tp}_{d+1\ 00\text{UTC},n}\ [\text{m}] \right) \cdot 1000

where d is the day for which the total precipitation is being computed and N is the number of days in the year.

The time step labelled d+1 00UTC because it contains the accumulated total precipitation over the previous 24 hours. This implies that the sum goes from 2^nd January of the year to 1^st January of the next year (inclusive).

ERA5-Land monthly averaged data

Total precipitation

The accumulations in monthly means of daily means have units that include "per day".

m (of water equivalent per day)

N/A

Mathinline
\text{tp}\ [\text{mm}] = \text{tp}\ [\text{m/day}] \cdot 1000 \cdot N

where N is the number of days in the month.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{n = 1}^{12}(\text{tp}\ [\text{m/day}] \cdot 1000 \cdot N)_{\text{n}}

where N is the number of days in the month and n is the number of the month.

Seasonal daily data

Total precipitation

24 hour aggregation since the beginning of the forecast

m

Mathinline
\text{tp}\ [\text{mm/hr}] = \frac{(\text{tp}_{t2}\ [\text{m}] - \text{tp}_{t1}\ [\text{m}])\cdot 1000}{24}

where t₂is the day of interest, t₁is the day before t2, tp_t2 is the total precipitation at t2, and tp_t1is the total precipitation at t1.

Mathinline
\text{tp}\ [\text{mm}] = (\text{tp}_{t2}\ [\text{m}] - \text{tp}_{t1}\ [\text{m}])\cdot 1000

where t₂is the day of interest, t₁is the day before t2, tp_t2 is the total precipitation at t2, and tp_t1is the total precipitation at t1.

Mathinline
\text{tp}\ [\text{mm}] = (\text{tp}_{t_2}\ [\text{m}] - \text{tp}_{t_1}\ [\text{m}])\cdot 1000

where t₂ is the 1st day of the month after the month of interest, t₁= is the 1st day of month before t2, tp_t2= is the total precipitation at t2, and tp_t1is the total precipitation at t1.

N/A For seasonal forecasts, as data only cover 7 months

Seasonal Monthly data

Total precipitation

The accumulations in monthly means have units that include "per second".

m/s

N/A

Mathinline
\text{tp}\ [\text{mm}] = \text{tp}_{t1}\ [\text{m}]\cdot 1000

where t₁is the month of interest, N1 is the number of days in t_1,and tp_t1is the result of (total precipitation at t₁ * N1 * 24 * 60 * 60).

N/A For seasonal forecasts, as data only cover 7 months

ERA-Interim sub-daily data

Total precipitation

Accumulations are from the beginning of the forecast

m (of water equivalent)

N/A

NOT AVAILABLE HOURLY

Mathinline
\text{tp}\ [\text{mm}] = (\text{tp}_{12\ 00\text{UTC}}\ + \text{tp}_{12\ 12\text{UTC}}\ [\text{m}])\ \cdot 1000

where the time step labelled 12 has to be selected from time reference 00UTC plus the time step labelled 12 selected from time reference 12UTC because they contain the accumulated total precipitation over the 24 hours for each day.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{d = 1}^{N}(\text{tp}_{12\ 00\text{UTC}}\ + \text{tp}_{12\ 12\text{UTC}}\ [\text{m}])_{d} \cdot 1000

where d is the day for which the total precipitation is being computed and N is the number of days in the month.

Mathinline
\text{tp}\ [\text{mm}] = \sum_{d = 1}^{N}(\text{tp}_{12\ 00\text{UTC}}\ + \text{tp}_{12\ 12\text{UTC}}\ [\text{m}])_{d} \cdot 1000

where d is the day for which the total precipitation is being computed and N is the number of days in the year.

...

¹Please, note that the convention for accumulations used in ERA5-Land differs with that for ERA5. The accumulations in the short forecasts of ERA5-Land (with hourly steps from 01 to 24) are treated the same as those in ERA-Interim or ERA-Interim/Land, i.e., they are accumulated from the beginning of the forecast to the end of the forecast step. For example, runoff at day=D, step=12 will provide runoff accumulated from day=D, time=0 to day=D, time=12. The maximum accumulation is over 24 hours, i.e., from day=D, time=0 to day=D+1,time=0 (step=24).

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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

_{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.}

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