Revisions during 1995

28 March 1995 A revision to the definition of tropopause for the humidity analysis was introduced.

4 April 1995 A new version of the forecast model (IFS cycle13r4) was implemented including:

• new prognostic cloud scheme;



• smoothed mean orography and new subgrid orography parametrization;
  
  
• modified quasi-regular (reduced) Gaussian grid.

Specific humidity on model levels and pressure levels was changed from spherical harmonic to quasi regular Gaussian grid field. In addition, three new quasi-regular Gaussian grid fields on model levels were introduced: cloud liquid water content, cloud ice water content and cloud cover. Fields of low, medium, high and total cloud cover continue to be produced, but convective cloud cover is no longer produced.

This change led to much improved representation of cloud cover, with large reductions from the previous systematic negative biases, especially during the day. A significant reduction in summertime warm bias of two-metre temperature, was achieved. The change improves the precipitation forecast, with smoother precipitation patterns and reduced spin-up problems in the early range of the forecast.

In addition, the parametrization of the ocean surface albedo has been changed to better represent the variation of reflectivity with solar zenith angle.

An article from the ECMWF Newsletter describing these changes is given in pdf(396 KB) file.

28 November 1995 A revised form of the continuity equation was implemented in the forecast model, resulting in a reduction of noise in all lower tropospheric fields previously experienced near mountains. In particular, the fields of mean sea level pressure, geopotential height and temperature are smoother, and the near surface wind is better represented.