Information about implementation of ECMWF upgrade March 2016 to 9-km (HRES) and 18-km (EPS)
Detailed information of implementation of IFS cycle 41r2
Created by Umberto Modigliani, last modified by Paul Dando on Nov 06, 2015
ECMWF plans to upgrade the horizontal resolution of its analyses and forecasts.The upgrade will have a horizontal resolution that translates to about 9 km for HRES and the data assimilation (the outer loop of the 4D-Var) and to about 18 km for the ENS up to day 10.
A new cycle of the IFS will be introduced to implement the horizontal resolution upgrade. This cycle is labelled 41r2, and includes a number of enhancements to the model and data assimilation llisted herein. The detailed specification of the resolution upgrades included in IFS cycle 41r2 are:
Horizontal resolution of the HRES increased from TL1279 / N640 to TCO1279 / O1280, where subscript CO stands for cubic octahedral;
Horizontal resolution of the ENS increased from TL639 / N320 to TCO639 / O640 for Leg A (to day 10) and from TL319 / N160 to TCO319 / O320 for Leg B;
Introduction of a new form of the reduced Gaussian grid, the octahedral grid, for both HRES and ENS;
Increase of the HRES wave resolution from 0.25 to 0.125 degrees and the ensemble wave from 0.5 to 0.25 degrees;
Horizontal resolution of the EDA outer loop is increased from TL399 to TCO639 with the inner loops increased from TL159 / TL159 to TL191 / TL191, respectively;
Horizontal resolution of the 4DVar inner loops is increased from TL255 / TL255 / TL255 to TL255 / TL319 / TL399, respectively.
These upgrades
do not include any increase in the vertical resolution;
do not apply to the ECMWF seasonal forecasting system;
do not apply to the standalone wave model (HRES-SAW);
do apply to products from the Boundary Condition Optional Programme.
Timetable for implementation
Horizontal resolution upgrade
Meteorological content of IFS cycle 41r2
Meteorological impact of the new cycle
Technical details of the new cycle
Impact on users
Availability of test data from the cycle 41r2 pre-operational e-suite
Document versions
Timetable for implementation
The planned timetable for the implementation of IFS cycle 41r2 is as follows:
4 Nov 2015 Initial announcement to Member States and other customers
early Dec 2015 Availability of test data in dissemination
end Mar 2016 Expected date of implementation
The timetable represents current expectations and may change in light of actual progress made.
Horizontal resolution upgrade
The 2016 horizontal resolution upgrade has been developed with a trade-off between resolution and computational costs in mind. A number of options of how to produce the most effective combination of horizontal resolutions between 4D-Var, EDA, HRES and ENS have been tested to establish computing costs and to derive possible efficiency gains.
The most viable option found was to change from the current linear (TL) to cubic (TC) spectral truncation. With the cubic spectral truncation the shortest resolved wave is represented by four rather than two grid points. While keeping the spectral truncation unchanged, the resolution is increased in grid-point space to more accurately represent the physical processes and advection. In the current operational configuration of the IFS a build-up of energy at the shortest scales is mitigated by a lower-than-nominal resolution of the orography, strong horizontal diffusion and a de-aliasing filter. In IFS 41r2 this is much less of an issue. The TC configuration also substantially improves mass conservation.
In order to reduce the computational cost further, the use of a new octahedral grid with spectral truncation denoted by TCO has been investigated. The octahedral grid applies a new rule for computing the number of points per latitude circle and is globally more uniform than the previously used reduced Gaussian grid. It is based on a new mesh that also allows for future implementations of a hybrid spectral – grid point model. The computational cost is reduced by about 25% compared to the cubic grid as fewer grid point calculations are needed.
Meteorological content of IFS cycle 41r2
Data assimilation:
Compute scale-dependent hybrid B (background error covariance) by adding samples from latest EDA forecast to static climatological B with increasing weight of today's EDA for smaller wavelengths (30% up to T63, growing to a maximum 93% at T399).
The EDA now cycles its own background error and covariance estimates, rather than using climatological estimates.
Change to use the Sonntag equation for saturation vapour pressure in humidity observation operators to improve saturation calculation for very cold temperatures (colder than -40C).
Satellite:
GPSRO (radio occultation) observation errors based on a physical error propagation model are increased by 25% to account for missing sources of error (e.g. obs error correlations, forecast model error). Improves lower stratosphere/tropopause winds and temperatures.
Activated SSMIS F-18 humidity sounding channels over ocean and extended all-sky assimilation to snow covered land surfaces.
Improved specification of AMSU-A observation errors based on satellite (due to instrument noise characteristics and ageing) and situation (cloud, orography) thereby increasing the number of observations assimilated.
Improved aerosol detection and screening for IASI infrared satellite data.
Increased use of Atmospheric Motion Vectors (AMVs), including extension in latitudinal coverage from geostationary platforms from 60 to 64 degrees zenith angle and addition of Meteosat mid-height AMVs derived from infrared imagery.
Revised data selection (screening) of cold-air outbreaks in low-peaking all-sky microwave channels to allow more data to be assimilated.
Updated microwave observation operator coefficient files (54-level RTTOV files with latest spectroscopy)
Numerics:
Changed from linear to cubic truncation for the spectral dynamics and from a linear reduced Gaussian to an octahedral reduced Gaussian grid for HRES, ENS and DA/EDA outer loops.
Increased semi-lagrangian departure point iterations from 3 to 5 to remove numerical instabilities near strong wind gradients, particularly improving East Asia (downstream of the Himalayas) and improved representation of tropical cyclones.
Changed formulation of the horizontal spectral diffusion to a spectral viscosity with significantly reduced damping at the small scales.
Removed dealiasing filter on rotational part of the wind as no longer needed for cubic grid (no aliasing).
Reduced diffusion in the sponge layer near the top of the model (above level 30) scaled by grid resolution rather spectral resolution, due to new cubic grid.
Physics:
Improved representation of radiation-surface interactions with approximate updates every timestep on the full resolution grid leads to a reduction in 2m temperature errors near coastlines.
Included surface-tiling for long-wave radiation interactions to reduce occasional too cold 2m temperature errors over snow.
Improved freezing rain physics and an additional diagnostic for freezing rain accumulation during the forecast.
Introduced resolution dependence in the parametrization of non-orographic gravity wave drag, reducing with resolution and improving upper stratospheric wind and temperature for HRES and ENS.
Changed the parcel perturbation for deep convection to be proportional to the surface fluxes, reducing overdeepening in tropical cyclones.
Increased cloud erosion rate when convection is active, to reduce cloud cover slightly and improve radiation, particularly over the ocean.
Improvements of linear physics used in the data assimilation for gravity wave drag, surface exchange and vertical diffusion, improving near-surface winds over ocean in the short-range.
Correction to solar zenith angle for the sunshine duration diagnostic. For clear sky days the sunshine duration increases by 2 hours, now in good agreement with observations. For cloudy days, sunshine duration may now be overestimated due to an existing underestimation of cloud optical thickness.
Ensemble:
Modified SKEB (Stochastic Kinetic Energy Backscatter) necessary for the new cubic grid, removing the numerical dissipation estimate from the dissipation rate. Reduces spread slightly, but this is then consistent with reduced RMSE in the new cycle.
Modified singular vector calibration to compensate for increased variance from the higher resolution EDA.
Meteorological impact of the new cycle
Information about the meteorological impact of the new cycle will be provided at a later date.
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