# Clarification on the accumulation period of ERA5 accumulated variables retrieved at 3-hourly intervals

**URL:** <https://forum.ecmwf.int/t/clarification-on-the-accumulation-period-of-era5-accumulated-variables-retrieved-at-3-hourly-intervals/15301>\
**Category:** Datasets and Usage\
**Tags:** era5\
**Created:** [2 September 2026 14:20 UTC](https://forum.ecmwf.int/t/clarification-on-the-accumulation-period-of-era5-accumulated-variables-retrieved-at-3-hourly-intervals/15301 "2026-09-02T14:20:59Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![User\_pepeACFE\_at\_colorado](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/u/96bed5/32.png) [@User\_pepeACFE\_at\_colorado](https://forum.ecmwf.int/u/User_pepeACFE_at_colorado)\
**Post date:** [2 September 2026 14:20 UTC](https://forum.ecmwf.int/t/clarification-on-the-accumulation-period-of-era5-accumulated-variables-retrieved-at-3-hourly-intervals/15301/1 "2026-09-02T14:20:59Z")

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Dear ECMWF Support Team,

I am working with ERA5 reanalysis data and would like to clarify the temporal interpretation of the accumulated variables retrieved from the CDS.

I am using the dataset:

`reanalysis-era5-single-levels`

with the following request settings:

- Product type: reanalysis
- Data format: NetCDF
- Time: 00:00, 03:00, 06:00, …, 21:00 UTC
- Spatial resolution: 0.25° × 0.25°
- Region: 25°N–10°S, 70°E–105°E

Among the variables, I downloaded the following accumulated parameters:

- total precipitation (`tp`)
- evaporation (`e`)
- surface solar radiation downwards (`ssrd`)
- surface thermal radiation downwards (`strd`)
- surface net solar radiation (`ssr`)
- surface net thermal radiation (`str`)
- surface latent heat flux (`slhf`)
- surface sensible heat flux (`sshf`)

The downloaded NetCDF files contain a `valid_time` coordinate at 3-hourly intervals. For example:

```auto
2016-11-01 00:00
2016-11-01 03:00
2016-11-01 06:00
2016-11-01 09:00
...

```

The accumulated variables are identified in the file metadata as:

```auto
GRIB_dataType = "fc"
GRIB_stepType = "accum"

```

My question is about the exact accumulation period represented by each value.

For example, for `tp` at:

```auto
2016-11-01 03:00 UTC

```

does this value represent:

**(A)** precipitation accumulated over the preceding 3 hours, i.e. approximately 00:00–03:00 UTC,

or

**(B)** precipitation accumulated over a 1-hour forecast step, with the 03:00 valid time corresponding to approximately 02:00–03:00 UTC?

In other words, does requesting:

```auto
"time": ["00:00", "03:00", "06:00", ...]

```

cause the CDS to return 3-hour accumulated values, or does it simply select the corresponding forecast-valid times of the ERA5 HRES accumulated fields, whose accumulation period may remain 1 hour?

This distinction is important for my application because I am using these data as atmospheric forcing for a ROMS ocean model. If the values are 1-hour accumulations, I need to construct 3-hour accumulated fluxes before converting them to mean fluxes (W m⁻² or kg m⁻² s⁻¹). If they are already 3-hour accumulations, I should not perform an additional temporal summation.

I would therefore greatly appreciate it if you could clarify:

1. What is the exact accumulation period of an accumulated variable such as `tp` when retrieved from `reanalysis-era5-single-levels` at 3-hourly intervals?
2. What are the corresponding `time`, `step`, and `stepRange` values internally used by the CDS for such a request?
3. Does the temporal spacing specified in the CDS request (`00, 03, 06, ...`) affect the accumulation period, or does it only determine which valid times are returned?
4. If I need a true 3-hour accumulation ending at 03:00 UTC (00:00–03:00 UTC), what is the recommended way to retrieve it from ERA5?
5. For variables such as `ssrd`, `strd`, `ssr`, `str`, `slhf`, and `sshf`, should the same interpretation be applied?

If possible, could you provide a concrete example for `tp` at 03:00 UTC showing the exact forecast initialization time, step/stepRange, and accumulation interval represented by the value?

Thank you very much for your help.

Best regards,

Peng