12 October 1999 Introduction of cy21r4 with 60 levels in the vertical for the reference model (T319) and 40 levels in the EPS (T159). Other changes include:

• new global orographics and associated subgrid orography fields;
• extensive changes in the cloud and convection schemes;
• a new post-processing of 10m winds aimed at a better representation of weather station environments;
• a revised evaluation of the model error cost function used in 4D-Var (Jb);
• a revised scheme for the correction of satellite radiance biases;
• active assimilation of SSM/I wind speed;
• bugfix of the humidity computation from SYNOP and TEMP observation below zero degree Celsius;
• introduction of ozone variable.

Please see article "The IFS cycle CY21r4 made operational in October 1999" from the ECMWF Newsletter No.87

ECMWF has issued the following warning about this data.

During the operation of cy21r4 of the ECMWF model from 12 October 1999 to 10 April 2000 the humidity fields in the lower stratosphere both in the analysis and forecast were seriously affected by erroneous moisture. These fields should not be used for research and it is recommended that the model results from the operational archive are not used to provide initial conditions for numerical experiments during this period.

The error is up to 1 to 2 orders of magnitude in a shallow layer just above the tropopause.

This may have a slight affect on the temperature in some regions.