At CMCC, the regional climate model COSMO-CLM (Rockel and Geyer, 2008) is currently used to perform dynamical downscaling of global climate simulations (see figure 1).

Figure 1:  A RCM domain embedded in a GCM grid. Image source F. Giorgi, 2008.

Figure 1: A RCM domain embedded in a GCM grid. Image source F. Giorgi, 2008.

COSMO-CLM it is the climate version of the COSMO LM model (Steppeler et al., 2003), which is the operational non-hydrostatic mesoscale weather forecast model developed initially by the German Weather Service and then by the European Consortium COSMO.

The RCM COSMO CLM is currently developed by the CLM-Community, with which CMCC collaborates since 2008.

COSMO CLM can be used with a spatial resolution between 1 and 50 km, even if the non-hydrostatic formulation of the dynamical equations in LM makes it eligible especially for use at horizontal grid resolution lesser than 20 km (Bohm et al., 2006).

These values of resolution are usually close to those requested by the impact modelers; in fact the over mentioned resolutions allow to describe more correctly the terrain orography with respect to the global models, usually adopting lower resolution.

Moreover the non-hydrostatic formulation allows providing a better description of the convective phenomena (Holton, 2004), which are generated by vertical movement (through transport and turbulent mixing) of the properties of the fluid as energy (heat), water vapor and momentum. Convection can redistribute significant amounts of moisture, heat and mass on small temporal and spatial scales; furthermore convection can cause severe localized precipitation events (as thunderstorm or cluster of thunderstorms).

Another advantage related to the usage of COSMO CLM, with respect to other climate regional models available, is that the continuous development and validation of the meteorological version allows also continuous improvements in the code that are also adopted in the climate version,
ensuring the continuous update of the code.

The mathematical formulation of COSMO CLM is made up of the Navier-Stokes equations for a compressible flow. The atmosphere is treated as a multicomponent fluid (made up of dry air, water vapor, liquid and solid water) for which the perfect gas equation holds, and subject to the gravity and to the Coriolis forces. The model includes several parameterizations, in order to keep into account, at least in a statistical manner, several phenomena that take place on unresolved scales, but that have significant effects on the meteorological interest scales (for example, interaction with the orography).  The main features of the COSMO CLM simulation model are:

  • Non hydrostatic, full compressible hydro-thermodynamical equations in advection form.
  • Base state: hydrostatic, at rest
  • Prognostic variables: horizontal and vertical Cartesian wind components, pressure perturbation, temperature, specific humidity, cloud water content. Optionally: cloud ice content, turbulent kinetic energy and specific water content of rain, snow and graupel.
  • Coordinate System: generalized terrain-following height coordinate with rotated geographical coordinates and user defined grid stretching in the vertical. Options for (i) base-state pressure based height coordinate, (ii) Gal-Chen height coordinate and (iii) exponential height coordinate (SLEVE) according to Schär et al. (2002).
  • Grid Structure – Arakawa C-grid, Lorenz vertical grid staggering
  • Time integration: time splitting between fast and slow modes (Leapfrog, Runge-Kutta)
  • Spatial discretization: 2° order accurate Finite Difference technique
  • Parallelization: Domain Decomposition (MPI as message passing S/W)
  • Parameterizations:
    - Sub grid-Scale Turbulence
    - Surface Layer Parameterization
    - Grid-Scale Clouds and Precipitation
    - Sub grid-Scale Clouds
    - Moist Convection
    - Shallow Convection
    - Radiation
    - Soil Model
    - Terrain and Surface Data

For more information about the COSMO CLM model and the CLM-Community Assembly see the webpage:
For more details about the CMCC activities in the CLM-Community Assembly please visit the following link:

The most important papers and research papers published by CMCC using COSMO-CLM can be found in the publications section on CMCC web site.

Research Papers Contacts

Paola Mercogliano Edoardo Bucchignani

  • Bohm, U., M. Kücken, W. Ahrens, A. Block, D. Hauffe, K. Keuler, B. Rockel, and A. Will (2006), CLM- the climate version of LM: Brief description and long-term applications, COSMO Newsletter
  • Giorgi, F. (2008). Regionalization of climate change information for impact assessment and adaptation. WMO Bulletin 57 (2) – April 2008, 86 – 92. Retrieved April 15th, 2014 from
  • Holton, J.R. (2004). An Introduction to Dynamic Meteorology, Academic Press, International Geophysics Series Volume 88, Fourth Edition, 535 p., ISBN 0-12-354015-1, ISBN 978-0-12-354015-7
  • Rockel, B., and B. Geyer (2008).The performance of the regional climate model CLM in different Climate regions, based on the example of precipitation, Meteorologische Zeitschrift, 17(4), 487– 498.
  • Schär, C., D. Leuenberger, O. Fuhrer, D. Luthi, and C. Girard (2002), A new terrain-following vertical coordinate formulation for atmospheric prediction models, Mon. Wea. Rev., 130, 2459– 2480.
  • Steppeler, J., G. Doms, U. Schättler, H. Bitzer, A. Gassmann, U. Damrath, and G. Gregoric (2003), Meso-gamma scale forecasts using the nonhydrostatic model LM, Meteorol. Atmos. Phys., 82, 75–96.
  • Meso-gamma scale forecasts using the nonhydrostatic model LM, Meteorol. Atmos. Phys., 82, 75–96.