Atmospheric Forcing Datasets

As of 2026-10-04. Which atmospheric datasets can force a coastal ocean model through this service or through NumericalEarth, how they compare, and which one the CoastalSim configurations use.

An ocean model’s surface fluxes need, at every hour of the run: 10 m wind (eastward and northward), 2 m air temperature and specific humidity, surface pressure, precipitation, and downwelling shortwave and longwave radiation. The bulk formulae in NumericalEarth turn these into momentum, heat and freshwater fluxes; any dataset missing one of them cannot drive the model on its own.

Summary

Dataset

Type

Grid

Output step

Coverage

Latency

Path in our stack

HRRR (NOAA NCEP)

Hourly-cycling forecast, 3 km convection-allowing, radar assimilation

3 km, Lambert conformal over CONUS

1 h (15 min for some fields)

CONUS; archive on AWS from 2014-07-30

About an hour after each cycle

forcingkit /api/v1/atmosphere (schema hrrr-atm-v1); default for CoastalSim

ERA5 (ECMWF, Copernicus C3S)

Global reanalysis (4D-Var)

0.25 degrees (about 31 km)

1 h

1940 to present

ERA5T about 5 days; final ERA5 2 to 3 months

NumericalEarth ERA5PrescribedAtmosphere (coastal-sim atmosphere = "era5")

RRFS v1 (NOAA NCEP)

Hourly-cycling forecast, FV3 limited-area, 3 km

3 km, North America

1 h

Operational from 2026-10-06 (12 UTC cycle)

Hourly cycles to 18 h; to 84 h every 6 h

Not integrated; replaces NAM, SREF, HREF and HiresW, while HRRR stays in operations

NAM CONUS Nest (NOAA NCEP)

Forecast, 3 km

3 km, CONUS

1 h

Retired 2026-10-06 in favour of RRFS

Four cycles a day, to 60 h

Not integrated; do not adopt

GFS (NOAA NCEP)

Global forecast, FV3

0.25 degrees

1 h to 120 h, then 3 h

Global; NODD archive on AWS

Four cycles a day, to 384 h

Not integrated; the natural extension past HRRR’s 48 h in forecast mode

ECMWF IFS open data

Global forecast

0.25 degrees

3 h, then 6 h

Global; real time

Four cycles a day, to 15 days (00 and 12 UTC)

Not integrated; CC-BY-4.0 since 2025-10-01

JRA55-do (JMA, MRI)

Reanalysis adjusted for ocean-sea-ice models

About 0.5 degrees

3 h

1958-01-01 to 2024-02-01, final version 1.6.0

Discontinued (JRA-55 ended January 2024; successor JRA-3Q)

NumericalEarth JRA55PrescribedAtmosphere (its catalogue ends 2019-12-31)

ECCO v4 (NASA JPL)

Ocean state estimate’s adjusted forcing

About 1 degree

Monthly

Multi-decadal

Not real time

NumericalEarth ECCOPrescribedAtmosphere; climate scale only

Datasets in use

HRRR (default)

Provenance. NOAA NCEP’s High-Resolution Rapid Refresh, version 4, a 3 km convection-allowing model that assimilates radar every 15 minutes and starts a new forecast every hour. Distributed through the NOAA Open Data Dissemination programme in the noaa-hrrr-bdp-pds bucket on AWS (us-east-1), anonymous, from 2014-07-30 to the present. NOAA open data: free to use; NOAA asks for attribution and that modified products not be presented as NOAA’s.

How this service builds it (fetchers/hrrr_atmosphere.py). Hour t is the one-hour forecast (wrfsfcf01) of the cycle that started at t - 1 h. Every record is therefore a short forecast from the latest analysis, and cycles are chained, so a run of any length is covered without reaching into the long-range end of any single forecast. Eight GRIB2 messages are read per hour by byte range from the .idx sidecar:

Message

Store variable

Treatment

UGRD, VGRD at 10 m

u10, v10 (m/s)

Rotated from the Lambert grid axes to east and north (HRRR winds are grid-relative), angle sin(38.5 deg) * (lon + 97.5 deg), checked against pyproj

TMP at 2 m

t2m (K)

SPFH at 2 m

q2m (kg/kg)

Used directly; no dewpoint conversion

PRES at the surface

sp (Pa)

Surface, not sea-level, pressure

APCP 0-1 h accumulation

prate (kg m-2 s-1)

Mean of the accumulations ending at t and t + 1 h, divided by 3600 s: a rate centred on t

DSWRF, DLWRF at the surface

dswrf, dlwrf (W m-2)

Instantaneous at t; hourly sampling resolves the diurnal cycle but aliases short cloud transients. Hourly means exist only for shortwave, in the sub-hourly files

Fields are interpolated (barycentric, weights built once per request) to a regular 0.03 degree longitude-latitude grid covering the bbox plus 0.25 degrees, which is the grid type NumericalEarth’s atmosphere regridder accepts. Records run from one hour before the run start to one hour after its end; a missing message or hour raises, and the store is published only when complete. Building one hour takes about 6.5 s on a warm connection, so a 168 h window takes about 19 minutes.

Strengths. Resolves the land-sea contrast, sea breezes and frontal timing at the scale of our domains (MAB is 15 by 17 km; LIS 85 by 65 km). Hourly radiation and precipitation. No credentials.

Limits. CONUS only. A forecast product, not a reanalysis: it carries forecast-model biases and is not homogeneous across HRRR versions (v4 since December 2020). Radiation is instantaneous.

ERA5 (fallback)

Provenance. ECMWF’s fifth-generation global reanalysis for the Copernicus Climate Change Service: 0.25 degree grid (about 31 km), hourly, 1940 to the present. ERA5T, the initial release, appears about five days behind real time and is overwritten by the final ERA5 two to three months later. Requires a free Copernicus Climate Data Store account (credentials in ~/.cdsapirc); Copernicus licence, attribution required.

How it is used. CoastalSim reads it through NumericalEarth’s ERA5PrescribedAtmosphere and ERA5PrescribedRadiation over the bbox padded by 0.5 degrees, with linear time indexing and one hour of padding past the end. Accumulated fields (precipitation, radiation) are hour-ending means that NumericalEarth places at the centre of their hour. NumericalEarth 0.8.1’s catalogue stops at 2025-12-31; coastal-sim extends it to six days before today until upstream rolls the date.

Strengths. Homogeneous, global, long, assimilates far more observations than any forecast; the standard against which forcing biases are judged.

Limits. At 31 km a domain like MAB spans one or two ERA5 cells, so the forcing is nearly uniform and smears the coast: in the first hours of 2026-04-02 over MAB, the ERA5 box (which includes New Jersey land) was about 3 K warmer at 2 m and had about half HRRR’s wind speed. Latency rules out anything closer than five days to the present.

Choosing

Need

Dataset

Hindcast of a CONUS coastal domain from 2014-07-30 on

HRRR (default)

Hindcast before 2014-07-30, outside CONUS, or a reanalysis-grade comparison run

ERA5

Forecast to 48 h

HRRR forecast mode (planned, Sprint 4): one cycle’s f01 to f48 from the 00, 06, 12 or 18 UTC cycle

Forecast beyond 48 h

HRRR to 48 h, then GFS or ECMWF IFS open data (not integrated); RRFS to 84 h once it is established in operations

Multi-decade or climate-scale forcing

ERA5, or JRA55-do through NumericalEarth (to 2019 in its catalogue)

References