Last updated: August 2011
As a limited-area model, the COSMO-Model needs lateral boundary conditions from a driving model. Today it is possible to nest the COSMO-Model into the global models GME (from DWD) and IFS (from ECMWF). It is also possible to nest the COSMO-Model into itself. In the framework of the SRNWP Interoperability Programme work is underway to also allow the other european models (UM from UKMO, Aladin/Arpege from MeteoFrance, HIRLAM) as driving models.
The lateral boundary formulation is by the Davies (1976) relaxation technique, where the internal model solution is nudged against an externally specified solution within a narrow boundary zone by adding a relaxation forcing term to the equations.
The external solution is obtained by interpolation from the driving host model at discrete time intervals. The interpolated fields are hydrostatically balanced, i.e. a hydrostatic pressure is prescribed for the nonhydrostatic pressure variable in the COSMO-Model at the lateral boundaries. Within these specified time intervals, the boundary data are interpolated linearly in time (which is done inside the model). Normally the boundary update interval is chosen to be one hour for meso-beta scale applications of the COSMO-Model.
The boundary values (and initial values, if no data assimilation suite is operated) are obtained from the host model by a preprocessing program, the INT2LM. Originally, INT2LM has been a joint development within COSMO and originates from the former GME2LM. The climate mode has been added by members of the CLM-Community. The development tasks were distributed as follows: