12 June 2001 Introduction of Cy23r4. This version includes the following modifications:
1. several changes in the use of satellite data over land and sea ice (more AMSU data and use of skin temperature rather than upper-layer soil temperature in processing radiances);
2. radiation parametrization routines are called on an hourly (instead of 3-hourly) basis during the data assimilation;
3. the time integration of the surface skin temperature over land has been improved;
4. the horizontal diffusion on vorticity has been increased to bring it in line with that applied to divergence and temperature without any noticeable impact on the forecast fields;
5. an improved ozone model developed by Meteo France has been introduced with a new parametrization of the destruction of ozone by heterogeneous chemical processes.