ECMWF Reanalysis v5 (ERA5)

Eastward component of the 100 m wind, describing near-surface flow aloft.

• Comparisons to observations require care because observations represent point measurements rather than grid-box means.
• Combine with the 100 m V component to derive wind speed and direction.

Northward component of the 100 m wind, describing near-surface flow aloft.

• Comparisons to observations require care because observations represent point measurements rather than grid-box means.
• Combine with the 100 m U component to derive wind speed and direction.

Eastward (U) component of near-surface wind.

• Combine with the 10 m V component to derive wind speed and direction.
• Observations can diverge due to terrain, vegetation, and local exposure.

Northward (V) component of near-surface wind.

• Combine with the 10 m U component to derive wind speed and direction.
• Observations can diverge due to terrain, vegetation, and local exposure.

Dewpoint temperature describing near-surface moisture conditions. Useful for humidity diagnostics.

• Often combined with temperature and pressure to estimate relative humidity.
• Computed by interpolating between the lowest model level and the surface.

Near-surface air temperature representative of conditions over land, sea, or inland waters. Derived using surface and lowest model level information.

• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.
• Mean 2 metre temperature is encoded separately (parameter 228004).

Atmospheric pressure adjusted to mean sea level, reflecting the weight of the air column.

• Maps of mean sea level pressure highlight cyclones, anticyclones, and wind strength via contour spacing.
• Often expressed in hPa or millibars (1 hPa = 1 mb = 100 Pa).

Depth of snow expressed as the equivalent depth of water if the snow melted.

• Represents snow over the snow-covered part of the grid box.
• See parameter 228141 for the equivalent value expressed in kg m-2.

Accumulated snowfall at the surface (large-scale + convective).

• Accumulated over the extraction period; depth in metres of water equivalent.
• See parameter 228144 for the equivalent value expressed in kg m-2.

Temperature of the Earth's surface required to satisfy the surface energy balance.

• Represents the uppermost surface layer with no heat capacity, so it responds instantly to surface flux changes.
• Computed differently over land and sea.
• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.
• Mean skin temperature is encoded separately (parameter 235079).

Atmospheric pressure at the surface of land, sea, and inland water.

• Often used with temperature to estimate air density.
• Mean sea level pressure is typically preferred for identifying large-scale pressure systems in mountainous regions.
• Often expressed in hPa or millibars (1 hPa = 1 mb = 100 Pa).

Downward shortwave radiation at the surface (direct + diffuse).

• Approximate model equivalent of pyranometer measurements.
• Accumulated over the extraction period; divide by seconds to convert to W m-2.
• ECMWF convention for vertical fluxes is positive downward.

Downward longwave radiation at the surface emitted by the atmosphere and clouds.

• Accumulated over the extraction period; divide by seconds to convert to W m-2.
• ECMWF convention for vertical fluxes is positive downward.

Fraction of a grid box covered by cloud across all model levels.

• Computed from cloud across different heights with overlap assumptions.
• See parameter 228164 for the equivalent value in percent.

Total column water including vapour, liquid water, cloud ice, rain, and snow.

• Older IFS versions did not include rain and snow in the total.

Total water vapour in a column from the surface to the top of the atmosphere.

• Represents a grid-box mean value.

Accumulated rain + snow at the surface (large-scale + convective).

• Does not include fog, dew, or precipitation that evaporates before reaching the surface.
• Accumulated over the extraction period; depth in metres of water equivalent.
• See parameter 228228 for the equivalent value expressed in kg m-2.