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For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km (note, this is inconsistent with what is actually used in the IFS),, as defined in the the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17.

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• The hydrological parameters have effective units of "m of water per day" and so they should be multiplied by 1000 to convert to kgm^-2day^-1 or mmday^-1.
• The energy (turbulent and radiative) and momentum fluxes should be divided by 86400 seconds (24 hours) to convert to the commonly used units of Wm^-2 and Nm^-2, respectively.

The monthly data in the CDS under 'ERA5 monthly averaged data' has been created by first creating monthly data on the native grid, then regridding this to the lat-lon grid used in the CDS. The hourly data in the CDS under 'ERA5 hourly data' has been created by regridding from the native to the lat-lon grid. Any calculation of monthly means using this hourly data takes place on the already regridded dataset.

In general, the monthly means calculated from the hourly data, or provided in the CDS should be identical, as regridding and averaging are both linear operations. However, when calculating wind speed, there is a nonlinear transformation sqrt(u*u+v*v) in between and then the order does matter. Therefore, small differences can be seen where the wind fields themselves vary quickly with location, like going from sea to the high volcanos over Hawaii.

Ensemble means and standard deviations

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