Cycle 41r2 summary of changes

Horizontal

• Atmospheric (changed)
  • HRES: O1280
  • ENS Days 1 - 15: O640
  • ENS extended (Days 16 - 46): O320
• Wave
  • HRES-WAM: 0.125° degrees
  • HRES SAW: 0.1° degrees
  • ENS-WAM: 0.25°degrees
  • ENS-WAM Extended: 0.5°

Vertical (unchanged)

• Atmospheric
  • HRES: L137
  • ENS: L91

• HRES
• ENS
• HRES-WAM
• HRES-SAW
• ENS-WAM

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).
• Assimilation of aircraft humidity data implemented.

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. 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 octahedral reduced Gaussian grid for HRES, ENS and 4DVAR and 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.
• Improved solar zenith angle calculation removes stratospheric temperature dependence on radiation timestep and reduces anomalous small amplitude fluctuations in incoming solar radiation around the equator.

Ensemble     

• Modified SKEB (Stochastic Kinetic Energy Backscatter) stochastic physics necessary for the new cubic grid, removing the numerical dissipation estimate from the dissipation rate. Reduces ensemble spread slightly, but this is then consistent with reduced error (RMSE) in the new cycle.
• Modified singular vector calibration to compensate for increased variance from the higher resolution EDA.

Upper Air

The new model cycle (41r2) provides improved HRES and ENS performance throughout the troposphere. In the HRES there is a significant reduction of forecast errors in upper-air fields in the extra-tropics. Error reductions on the order of 2-3% are found for most upper-air parameters and levels. The improvement in the primary headline score for the HRES (lead time at which the 500 hPa geopotential anomaly correlation drops below 80%) is about 2 hours (0.08 days). Improvements are seen both in verification against the model analysis and verification against observations. In the tropics, evaluation against model analysis shows an apparent degradation in the short and near-medium range, mostly due to a more active analysis resulting from the increase in resolution of the EDA. Verification against observations, however, gives neutral to positive results in the tropics, except for temperature at 500 hPa, which shows a slight degradation.

The root mean square error (RMSE) and anomaly correlation for temperature are both improved in the extra-tropics, but there is a small (0.2 K) mean cooling in the upper troposphere. As the mean geopotential in the lower stratosphere is sensitive to changes in the vertically integrated tropospheric temperature, this shows up as an increased RMSE for geopotential at 100 hPa. The upper air scores over East Asia are significantly better associated with an improved representation of the flow downstream of the Himalayas due to the new cubic grid and more stable numerics. The overall kinetic energy spectra of the model is significantly improved with an increase in the energy towards the smaller scales.

Changes in skill of the ENS are generally similar to the HRES, with improvements in the extra-tropics on the order of 2-3% (CRPS reduction), and degradations in the tropics when verified against analysis. The improvement in the primary headline score for the ENS (lead time at which the CRPSS of the 850 hPa temperature drops below 25%) is ~0.2 days. The overall kinetic energy spectra of the model is significantly improved with an increase in the energy towards the smaller scales.

Weather parameters

The increased resolution leads to a better representation of coastlines and orography with potential for improved local prediction. The new model cycle yields consistent gains in forecast performance in the tropics and extra-tropics for 2m temperature, 2m humidity, and 10m wind speed. Precipitation forecasts are slightly improved in the extra-tropics and slightly deteriorated in the tropics. Mostly neutral results are found for forecasts of total cloud cover.

The increase in forecast skill for 2m temperature as measured by the reduction of RMSE is about 3% in the northern hemisphere extra-tropics and 1% in the tropics. There is a mean cooling in the northern hemisphere of about 0.05 K. Changes to the calculation of radiative fluxes lead to particular improvements in near coastal 2m temperatures at places where surface conditions vary abruptly.

For 2m dewpoint, RMS error reductions of 2% are observed in the NH, and neutral results are obtained for the tropics. There is an overall reduction of 2m dewpoint on the order of 0.05 to 0.1 K.

The RMSE for 10m wind speed improves by about 2% overall. There is no significant change in the mean in the northern hemisphere, and a reduction on the order of 0.05 m s^-1 in the tropics.

Forecast skill for 24 hour precipitation totals shows an overall slight improvement in the northern hemisphere, and a small (1%) degradation in the tropics. This degradation in the tropics is seen in the SEEPS score but not in the RMSE.

Total cloud cover shows improvements in RMSE on the order of 0.5% in the tropics and neutral results in the extra-tropics.

There is a substantial reduction in localized (unrealistic) precipitation extremes over orography. The improvement is due to the cubic grid representation and modifications in the semi-Lagrangian advection scheme.

Tropical cyclones

The structural representation of tropical cyclones is improved with a more clearly defined eye and better resolved rainbands. Evidence from case studies shows that the increase in resolution leads to improved forecasts of tropical cyclone intensity in the ENS. Initial ensemble spread is also improved for tropical cyclones by the increased resolution in the EDA. For HRES, the tropical cyclone impact of the resolution change is smaller. Case studies show a better representation of the precipitation pattern around the core of tropical cyclones in the new cycle. This improvement is due to changes in model numerics (move to cubic grid and changes in the semi-Lagrangian scheme).

Wave forecast

Results for ocean wave height are positive, except for a deterioration in the very short range (day 1) in the tropics when verified against the analysis. A similar short-range degradation is seen for 10m wind speed over ocean areas. This is due to an increase in activity of the low-level wind and wave analyses associated with the move from T_L1279 to T_CO1279. When verified against observations (buoys), no degradation is seen. Wave period has a mixed signal and may require some retuning in the next cycle.

Monthly forecast

Results suggest a generally neutral effect on upper-air and near surface skill scores in the tropics and for the MJO. For extra-tropical skill there is a slight improvement coming from the change to a cubic grid. Tropical cyclone sub-seasonal prediction is also improved.

Data assimilation / analysis

The kinetic energy spectrum has changed in the analysis even more than for the HRES. Whereas the analysis used to have less energy in the smaller scales compared to the forecast, both now have the same improved energy spectra. The background error variances derived from the higher-resolution EDA are larger in many areas, particularly in the tropics, leading to closer analysis fit to observations. Observation-minus-background departure statistics have improved for wind profile data.

• Resolution changes
• New grid: the octahedral grid, for both HRES and ENS.  See Introducing the octahedral reduced Gaussian grid for further details.
• Changes to GRIB encoding
• Changes to forecast products
  • increased field size
  • new model output
  • correction to solar zenith angle for the sunshine duration diagnostic
  • correction to the interpolation method used for the precipitation type diagnostic

• Atmospheric: 146
• Ocean wave:  111  (unchanged)
• Standalone ocean wave: 211 (unchanged)

• discontinued ENS Calibration / Validation forecasts
• discontinued ENS variable resolution overlap products