Units and time conventions of the input data
This page describes the IFS and GLO12 packages that oceanbench.datasets.input exposes: what each package covers in time, and the units of each variable.
The script that produced the IFS packages is not available. The IFS conventions below were established by comparing the packages with IFS operational 6-hourly analyses and forecasts, on the same grid, for packages from late December 2023 and January 2024. The mean sea level pressure was also compared with ERA5 daily means, to check which day each lead covers. The GLO12 conventions come from the metadata of the stores.
IFS forecast packages
oceanbench.datasets.input.ifs_forcings() opens one package per week, from January 3, 2023 to December 30, 2025. Each package is a Zarr store with one group per variable:
https://s3.waw3-1.cloudferro.com/oceanbench-bucket/dev/additionnal-data/IFS/ifs_forcing_rg_forecasts_R{YYYYMMDD}.zarrR is a Tuesday, the first day of the forecast. A package has the dimensions first_day_datetime (1), lead_day_index (10), lat (2560) and lon (5120). first_day_datetime is day R. lead_day_index is a plain index with no date attached: lead k covers the UTC day R + k, from 00:00 to 24:00. So package R20240102 covers January 2 to January 11, 2024.
The grid is the ECMWF F1280 regular Gaussian grid. Latitudes go from 89.95 to -89.95 (north to south) and longitudes from 0 to 359.93.
Each value is a mean over its UTC day, except cp, ewss and nsss (see the table). For the instantaneous variables, the daily mean is consistent with an average of the hourly forecast output.
| Variable | ECMWF parameter | Units | Meaning |
|---|---|---|---|
somslpre |
msl (151) |
Pa | Mean sea level pressure, daily mean. |
sp |
sp (134) |
Pa | Surface pressure, daily mean. |
sotemair |
2t (167) |
K | 2 m air temperature, daily mean. |
sotemhum |
2d (168) |
K | 2 m dewpoint temperature, daily mean. This is a temperature, not a humidity. |
sohumspe |
derived from 2d and msl |
kg kg-1 | 2 m specific humidity, computed from the 2 m dewpoint temperature and the mean sea level pressure, daily mean. |
skt |
skt (235) |
K | Skin temperature, daily mean. |
sowinu10 |
10u (165) |
m s-1 | 10 m eastward wind, daily mean. |
sowinv10 |
10v (166) |
m s-1 | 10 m northward wind, daily mean. |
sosudosw |
ssrd (169) |
W m-2 | Surface downward shortwave radiation, daily mean flux. |
sosudolw |
strd (175) |
W m-2 | Surface downward longwave radiation, daily mean flux. |
ssr |
ssr (176) |
W m-2 | Surface net shortwave radiation, daily mean flux. |
strd |
strd (175) |
W m-2 | Surface downward longwave radiation, daily mean flux. Close to sosudolw, see the note below. |
sowaprec |
tp (228) |
kg m-2 s-1 | Total precipitation, daily mean rate. |
sosnowfa |
sf (144) |
kg m-2 s-1 | Snowfall, in water equivalent, daily mean rate. |
cp |
cp (143) |
m | Convective precipitation, accumulated since 00:00 UTC of day R. Not a rate: it grows with the lead day. The values are consistent with metres of water. |
ewss |
ewss (180) |
N m-2 s | Eastward turbulent surface stress, accumulated over each hour. Not usable: each hourly value was clipped to the range 0 to 1e4 before the daily mean, so negative stress is lost. Do not use. |
nsss |
nsss (181) |
N m-2 s | Northward turbulent surface stress, accumulated over each hour. Not usable: each hourly value was clipped to the range 0 to 1e4 before the daily mean, so negative stress is lost. Do not use. |
The fluxes do not all come from the same forecast runs. On the first lead day, sosudosw, sosudolw, sowaprec and sosnowfa match fluxes taken 6 hours at a time from the 00:00 and 12:00 UTC runs of that day. ssr and strd come from the 00:00 UTC run alone. This is why strd and sosudolw differ slightly, by about 3 W m-2.
Dividing a valid hourly stress accumulation by 3600 gives a stress in N m-2. This cannot be done with ewss and nsss because of the clipping.
GLO12 nowcast packages
oceanbench.datasets.input.glo12_nowcasts() opens one package per week, from January 4, 2023 to December 31, 2025, and joins them along time. Each package is a Zarr store with one group per variable:
https://s3.waw3-1.cloudferro.com/oceanbench-bucket/dev/additionnal-data/GLO12/glo12_rg_1d-m_nwct_R{YYYYMMDD}.zarrR is a Wednesday. A package holds the 7 daily means before R, from day R - 7 to day R - 1. So package R20240103 covers December 27, 2023 to January 2, 2024.
A package has the dimensions time (7), depth (50), latitude (2041) and longitude (4320). The grid is regular at 1/12 degree, from -80 to 90 degrees north and from -180 to 179.92 degrees east. Each time is 12:00 UTC of its day, the centre of the daily mean. It is stored in hours since 1950-01-01.
Land cells hold the fill value 9.96921e+36, declared as _FillValue. xarray turns them into NaN when it opens the data.
| Variable | Units | Meaning |
|---|---|---|
thetao |
°C | Sea water potential temperature, daily mean. |
so |
1e-3 | Sea water salinity, on the practical salinity scale, daily mean. |
uo |
m s-1 | Eastward sea water velocity, daily mean. |
vo |
m s-1 | Northward sea water velocity, daily mean. |
zos |
m | Sea surface height above the geoid, daily mean. |
siconc |
1 | Sea ice area fraction, daily mean. |
sithick |
m | Sea ice thickness, daily mean. |
usi |
m s-1 | Eastward sea ice velocity, daily mean. |
vsi |
m s-1 | Northward sea ice velocity, daily mean. |