Grid and Projections

COSMO-REA6 uses a rotated-pole coordinate system where the grid appears regular (evenly spaced in the rotated longitude/latitude), but the pole is shifted from the geographic North Pole to reduce metric distortion over Europe.

Why No Reprojection Is Needed

At first glance the rotated-pole grid might seem to require reprojection to WGS84 before use. In practice this is not necessary for BuEM, for two reasons:

  1. Auxiliary WGS84 coordinates are already embedded in the GRIB files. Each grid point carries pre-computed latitude and longitude values in standard WGS84 (EPSG:4326). When the NetCDF is opened with xarray, these appear as 2-D coordinate arrays alongside the native rotated indices. Point extraction uses these WGS84 coordinates directly — no coordinate transformation is required at query time.

  2. Scalar fields are projection-invariant. Temperature, irradiance, and wind speed are scalar quantities. Their numeric values do not change under coordinate rotation — a grid cell holding 293.15 K holds the same value regardless of how the grid is oriented. Only vector fields (wind U/V components) are direction-dependent, and BuEM uses only the scalar wind speed (WS_10M = sqrt(U² + V²)), which is rotationally invariant.

Wind Components (U_10M, V_10M)

The raw U and V wind components are defined relative to the rotated-pole grid north, not geographic north. This means the direction implied by U and V is rotated relative to true north. However:

  • WS_10M = sqrt(U² + V²) is the same in any orthogonal coordinate system.

  • BuEM’s thermal model only uses wind speed, not direction.

  • If wind direction were needed in the future, a rotation matrix using the pole coordinates (available in the GRIB metadata) would convert rotated U/V to geographic U/V. This is a simple linear transform, not a grid resampling.

Grid Dimensions

Property

Value

Grid size

848 × 824 (longitude × latitude)

Resolution

~0.055° rotated (~6 km)

Temporal resolution

Hourly

Coverage

1995–2019, Europe

Pole (rotated)

(−170.0, 40.0) — as encoded in GRIB metadata

CRS

Rotated latitude-longitude (non-standard EPSG)

Auxiliary coords

WGS84 latitude, longitude per grid point