Implementation of IFS cycle 41r2

ECMWF plans to upgrade the horizontal resolution of its integrated forecasting system (IFS) including its high-resolution (HRES) and ensemble (ENS) forecasts.The upgraded horizontal resolution will be about 9 km for the HRES and the data assimilation (the outer loop of the 4D-Var) and about 18 km for the ENS up to day 10 and about 36 km for the extended rage (monthly). The resolution of the ensemble of data assimilations (EDA) will be increased to 18 km.

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. The main contents of IFS cycle 41r2 are:

• The horizontal resolution is increased by changing from linear (T_L) to cubic (T_C) spectral truncation and introducing an octahedral reduced Gaussian grid. With the cubic spectral truncation the shortest resolved wave is represented by four rather than two grid points and the octahedral grid is globally more uniform than the previously used reduced Gaussian grid.
• The realism of the kinetic energy spectrum is significantly improved with more energy in the smaller scales due to a reduction of the diffusion and removal of the dealiasing filter, enabled by the change to using a cubic truncation for the spectral dynamics.
• There is a significant revision to the specification of background error covariances (B) used in the HRES data assimilation due to the increased resolution of the EDA and the introduction of scale-dependence of the hybrid B (climatological and EDA), thereby relying more on the EDA "errors of the day" for the smaller scales.
• There are improvements in the use and coverage of assimilated satellite data due to changes in observation selection and error representation (for GPS radio occultation data, all-sky microwave, AMSU-A, IASI and AMVs) and improved observation operators for radiance data from microwave sounders.
• The stability of the semi-Lagrangian scheme near strong wind gradients is improved, reducing noise downstream of significant orography and in tropical cyclones.
• The radiative heating/cooling at the surface is improved by introducing approximate updates on the full resolution grid at every timestep. This leads to a reduction in 2-metre temperature errors, particularly near coastlines.
• Additionally there are changes to the triggering of deep convection, non-orographic wave drag and improvements to the linear physics in the data assimilation (for gravity wave drag, vertical diffusion and the surface exchange).

These upgrades

• do not include any increase in the vertical resolution;
• do not apply to the ECMWF seasonal forecasts;
• do not apply to the standalone wave model (HRES-SAW);
• do apply to products from the Boundary Condition Optional Programme.