ERA5 Land: Conversions required to comply with ALMA/CF

[AlisonBennett]

Unit conversions (ERA5 Land to CABLE):

• Shortwave radiation: convert from accumulation in J m-2 to W m-2
• Longwave radiation: convert from accumulation in J m-2 to W m-2
• Precip: convert from accumulation in m to kg m-2 s-1
• Snowfall: convert from accumulation in m to kg m-2 s-1

NOTE: shortwave radiation, longwave radiation, precip and snowfall are all provided as accumulated totals from the beginning of the forecast. See conversion notes here.

Missing variable conversions:

• Specific humidity: calculate from dewpoint_temperature and surface_pressure at the same height (must be in units of kg kg-1).
• Wind speed: calculate from 10m_u_component_of_wind and 10m_v_component_of_wind (must be in units of m s-1)

Time-units:

• hours since 1900 01-01-01 - gregorian calendar, integer hours.