ECMWF Artificial Intelligence/Integrated Forecasting System Ensemble (AIFS Ensemble)

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).

Fraction of a grid box covered by high-level cloud.

• Computed from model levels with pressure < 0.45 of surface pressure (~6 km and above).
• Cloud fractions range from 0 to 1; overlap assumptions are applied across levels.
• See parameter 3075 for the equivalent value in percent.

Fraction of a grid box covered by low-level cloud.

• Computed from model levels with pressure > 0.8 of surface pressure (~2 km and below).
• Cloud fractions range from 0 to 1; overlap assumptions are applied across levels.
• See parameter 3073 for the equivalent value in percent.

Fraction of land within a grid box relative to ocean and inland waters.

• Values range from 0 to 1 and are dimensionless.
• From IFS CY41R1 (May 2015), grid boxes >0.5 may mix land and inland water; <=0.5 are water only.
• Before CY41R1, >0.5 implied land only; <=0.5 implied ocean only (no inland water differentiation).

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).

Mass of water vapour per kilogram of moist air.

• Moist air includes dry air, water vapour, cloud liquid, cloud ice, rain, and falling snow.

Total surface + subsurface runoff accumulated over the output period, expressed as water equivalent mass.

• Useful for drought/flood assessment; comparisons to observations require care due to scale differences.
• See parameter 205 for the equivalent value expressed in metres.

Standard deviation of sub-grid orography within a grid box (valleys, hills, and mountains below model resolution).

• Derived from ~1 km land-surface data for features between 5 km and the model grid resolution.
• Used by the sub-grid orography scheme for low-level blocking and gravity-wave drag.

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).

Slope metric for sub-grid orography derived from ~1 km terrain data.

• Represents slopes of valleys, hills, and mountains unresolved by the model grid.
• A flat surface has value 0; a 45° slope corresponds to 0.5.

Soil temperature.

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.

Air temperature throughout the atmosphere.

• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.
• Available on multiple vertical levels.

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.

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.

Eastward wind component; negative values indicate westward flow.

• Combine with the V component to derive wind speed and direction.

Northward wind component; negative values indicate southward flow.

• Combine with the U component to derive wind speed and direction.

Vertical motion in pressure coordinates; negative values indicate upward motion.

• Useful for diagnosing large-scale ascent (negative) and subsidence (positive).

Gravitational potential energy per unit mass relative to mean sea level.

• Geopotential height = geopotential / g (9.80665 m s-2).
• At the surface, reflects orography (geopotential height of the surface).
• Constant-pressure charts of geopotential height are key for synoptic analysis.