ECMWF Integrated Forecasting System Operational (IFS Oper)

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.

Maximum 3‑second wind gust at 10 m height since the last post-processing step.

• Convection effects are included after 2008-10-01; before that, only turbulence is represented.
• Gusts are evaluated each time step and the maximum retained since the last post-processing.

Maximum 3‑second wind gust at 10 m height over the previous 3 hours.

• ECMWF estimates gusts within each time step using surface stress, friction, wind shear, and stability, then selects the maximum over the 3‑hour window.
• Comparisons to observations require care because observations are point measurements.

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 incoming shortwave radiation reflected by the snow-covered part of a grid box.

• IFS represents snow as a single layer above the uppermost soil level.
• Albedo varies with snow age and vegetation; typical ranges span 0.52–0.88 for low vegetation and 0.27–0.38 for high vegetation with snow underneath.
• See parameter 228032 for the equivalent value expressed in percent.

Horizontal divergence of velocity, indicating spreading or convergence of airflow.

• Positive values indicate divergence (air spreading out); negative values indicate convergence.

Accumulated eastward surface stress from turbulent atmosphere–surface interactions and sub‑5 km orographic form drag.

• Positive values denote stress toward the east; negative values denote westward stress.
• Accumulated over the integration period of the extracted data.

Height derived from geopotential divided by standard gravity (9.80665 m s-2).

• Used on constant pressure levels to diagnose synoptic features (e.g., troughs, ridges, cyclones).
• At the surface, it reflects orography (geopotential height of the surface).
• A geopotential metre is approximately 2% shorter than a geometric metre.

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).

Lowest 2 m temperature observed over the previous 3 hours.

• 2 m temperature is derived by interpolating between the lowest model level and the surface.
• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.

Lowest 2 m temperature observed over the previous 6 hours.

• 2 m temperature is derived by interpolating between the lowest model level and the surface.
• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.

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).

Convective Available Potential Energy for the most unstable parcel in the layer from the surface to 350 hPa.

• Relates to maximum potential updraft velocity.
• In the lowest 60 hPa, 30‑hPa mixed‑layer parameters are used.

Highest 2 m temperature observed over the previous 3 hours.

• 2 m temperature is derived by interpolating between the lowest model level and the surface.
• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.

Highest 2 m temperature observed over the previous 6 hours.

• 2 m temperature is derived by interpolating between the lowest model level and the surface.
• Units are kelvin (K); to convert to degrees Celsius (deg C), subtract 273.15.

Accumulated northward surface stress from turbulent atmosphere–surface interactions and sub‑5 km orographic form drag.

• Positive values denote stress toward the north; negative values denote southward stress.
• Accumulated over the integration period of the extracted data.

Surface precipitation type derived from predicted rain and snow fields plus atmospheric conditions.

• Use alongside precipitation rate to flag events such as freezing rain.
• IFS codes: 0 none, 1 rain, 3 freezing rain, 5 snow, 6 wet snow, 7 rain/snow mix, 8 ice pellets, 12 freezing drizzle.
• IFS does not diagnose WMO codes 2, 4, 9, 10, 11.

Mass of water vapour per kilogram of moist air.

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

Water vapour pressure as a percentage of saturation.

• For temperatures >0°C, saturation is over water; below -23°C, over ice.
• Between -23°C and 0°C, values are interpolated between ice and water using a quadratic function.

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

• Useful as a drought or flood indicator; comparisons to observations require care due to scale and unit differences.
• See parameter 231002 for the equivalent value expressed in kg m-2.

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.

Thickness of sea ice within a grid box.

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).

Net shortwave radiation at the surface (downward minus reflected).

• Depends on surface albedo and atmospheric absorption/scattering.
• Accumulated over the extraction period; divide by seconds to convert to W m-2.
• ECMWF convention for vertical fluxes is positive downward.

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.

Net longwave radiation at the surface (downward minus upward).

• Includes atmospheric and cloud emission to the surface and surface emission/reflectance upward.
• 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.

Eastward component of surface sea water velocity.

Northward component of surface sea water velocity.

Air temperature throughout the atmosphere.

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

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.

Mean rate of total precipitation (accumulated precipitation divided by the accumulation period).

• Total precipitation includes large-scale + convective rain and snow; excludes fog, dew, and precipitation that evaporates before reaching the surface.
• Units represent depth of water equivalent per second.
• Comparisons to observations require care due to spatial and temporal scale differences.

Net longwave radiation at the top of the atmosphere (negative of OLR).

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

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.

Rotation of horizontal flow about a vertical axis relative to the Earth’s surface.

• Negative/positive values correspond to clockwise/anticlockwise rotation depending on hemisphere; adding the Coriolis parameter gives absolute vorticity.

Volumetric soil moisture content.

• Time-mean volumetric soil moisture is encoded separately (parameter 235077).

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.

Sea surface height.