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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 reduced Gaussian grid (with spectral truncation denoted by TL) to a cubic reduced Gaussian grid (denoted TC)
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. With the cubic
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reduced Gaussian grid, the shortest resolved wave is represented by four rather than two grid points.
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With this approach, the spectral truncation is unchanged
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but the
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grid-point space resolution is increased to more accurately represent the physical processes and advection. In the current operational configuration of the IFS (cycle 41r1) 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 cycle 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 reduced Gaussian grid (prefix 'O') with spectral truncation denoted by TCO has been adopted. 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 (prefix 'N'). 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.
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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. observation 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 a cubic octahedral reduced Gaussian grid for HRES, ENS and 4DVAR and EDA outer loopsouter loops. The spectral truncation is unchanged.
- For the EDA, the spectral truncation of the outer loop is increased from TL399 to TCO639 with the corresponding cubic octahedral reduced Gaussian grid.
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.
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IFS cycle 41r2 test data is accessible via the ecCharts layers menu, identified by the label "esuite 0069" as shown in the screenshot on the left. They can be displayed as any other layers of ecCharts, and they can be saved in the Dashboard, as seen in the screenshot on the right.
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