Revisions during 2008

11 March 2008 Introduction of modified Cycle 32r3. With these changes ECMWF has integrated its Monthly Forecasting System with the medium-range Ensemble Prediction System (EPS). The changes will mainly affect the Monthly Forecasts, but some changes will also have an impact on the medium range, both the EPS and the deterministic forecasts.

The main changes in this revision are:

• Use of persisted SST anomalies in all atmospheric forecasts
• Daily ocean-coupling of days 10 to 15 of 0000 UTC EPS forecasts
• Monthly Forecast run once per week from 0000 UTC on a Thursday as an extension of the 15 day EPS forecast from this base time
• Modified EFI products using the new unified re-forecasts
• New GRIB description for all Monthly Forecast products, analogous to the current medium-range EPS data

Previously, the sea surface temperatures were persisted from the initial conditions during both the deterministic and the medium-range EPS forecasts. With the implementation of the unified VarEPS monthly system, the sea surface temperature anomalies relative to the 1979-2001 ERA-40 daily climates are persisted. The anomalies are added to the ERA-40 daily climate to produce the full SST field.

The new GRIB model identifiers are: 131 (atmospheric model). Model numbers for the Global wave model will remain at 116. Also for the European waters wave model the number will remain 216.

3 June 2008 Introduction of Cy33r1. The resolution of the limited area wave model was increased from 25 km to 10 km. The main changes included in this cycle are:

• Improved moist physics in tangent linear/ adjoint model used in 4D- Var assimilation.
• Re-tuned entrainment in convection scheme.
• Bug fix to scaling of freezing term in convection scheme.
• Additional shear term in diffusion coefficient of vertical diffusion.
• Increased turbulent orographic form drag.
• Fix for soil temperature analysis in areas with 100% snow cover.
• Change in surface roughness for momentum, and change in post-processing of two-metre temperature and specific humidity.
• Assimilation of AMSR-E and TMI radiances in 1D+4D-Var; assimilation OMI ozone data.
• Usage of all four wind solutions for QuikSCAT in assimilation, rather than only two previously.
• Extended coverage and increased resolution for the limited area wave model.
• Improved shallow water physics and modified advection scheme for ocean wave models.
• Introduction of two new wave model parameters: maximum wave height and corresponding wave period.

Evaluation of Cy33r1

Evaluation from four months of deterministic forecasts shows a modest but significant positive impact on northern hemisphere and European scores in the troposphere. Similar results were obtained for three weeks of EPS testing.

The new GRIB model identifiers are: 132 (atmospheric model), 117 (global wave model), 217 (European waters wave model).

30 September 2008 Introduction of Cy35r1 (Cy33r2). This cycle includes improvements to the handling of melting snow as it falls, resulting in less snow on the ground in marginal situations. This addresses a specific shortcoming from last winter. In addition, a new sea-surface temperature analysis product is being implemented, providing higher-resolution and more frequently updated information, in particular in cloudy situations, based on a range of satellite and in-situ observations. The corresponding new sea-ice analysis product is also being implemented. Significant ice differences occur in the areas with broken ice and the largest SST differences are also in the Polar Regions, as well as in the meanders of the major currents such as the Gulf Stream.   

The main changes included in this cycle are:

• The OSTIA (Operational Sea Surface Temperature and Sea Ice Analysis) high-resolution sea surface temperature, produced by the Met Office, and corresponding sea ice analysis (from EUMETSAT Ocean and Sea Ice SAF)
• Conserving interpolation scheme for trajectory fields in 4D- Var
• New variational bias correction ( VARBC) bias predictors to allow the correction of infrared shortwave channels affected by solar effects
• Cleaner cold-start of AMSUA channel 14 bias corrections
• Changes to physics for melting of falling snow, albedo of permanent snow cover (e.g. over Antarctica), diurnal variation of sea surface temperature, and linear parametrization schemes
• Convective contribution added to wind gusts in post-processing
• Monitoring of MERIS total-column water vapour data

Evaluation of Cy35r1

The impact of the new cycle on the performance of the forecasting system has been tested in research mode for the months of February/March and May/June 2008, and in pre-operational runs over the period July to September 2008. The new cycle shows essentially neutral scores for the extra-tropics, and improved scores for upper tropospheric winds in the tropics. There is some improvement in both hemispheres for the late medium range.

The new cycle is technically transparent. The new GRIB model identifiers are: 133 (atmospheric model), 118 (global wave model) and 218 (limited area wave model).