Rossby Centre regional atmospheric model, RCA4

RCA4 is the fourth version of the Rossby Centre Regional Atmospheric Climate Model (RCA4). Since RCA3, RCA has undergone both physical and technical changes. RCA4 was the version used in the downscaling of CMIP5 simulations for CORDEX. RCA is no longer undergoing any developments; the current regional climate model at Rossby Center is HARMONIE-Climate (HCLIM).

On the technical side we have enhanced the correctness of RCA by removing the use of global variables and moving over to the use of modern fortran 90/95 and C/C++. Most code in RCA is now modular in fortran modules (C++ parts are object oriented) and uses extensively MPI\_Datatypes to avoid cumbersome and complicated packing during inter-processor communication. The code is easy to port to different architectures via standard "gmake", which also obsoletes the scripting system of RCA v.3.

No pre-processing is needed to run RCA4 over any limited area of the globe, since all data used in the simulation are read from global databases. RCA4 needs not be recompiled when changing domain size, number of levels, position of grid as was the case for RCA3. Everything is controlled via namelist files.

Global physiography from ECOCLIMAP

The global physiography data read by RCA include Gtopo30 orography, ECOCLIMAP land-use and soil information [2], soil carbon [3] and lake depth [4]. Inclusion of ECOCLIMAP is one of the main reason why RCA4 can be applied globally. On the physical side we have modified mainly the land and hydrological processes plus a few modifications in the atmosphere.

The soil hydrology in RCA4 is divided into a forest and an open land tile, respectively. The depth of each tile soil column with respect to soil moisture is defined by the ECOCLAIMP root depth and the root density decreases exponentially with depth. Inclusion of soil carbon in RCA4 reduces the overestimated soil-heat transfer in RCA3.
In addition the heat transfer in the top soil layer is further reduced in presence of vegetation. Both these parameterisations act to increase the diurnal temperature range.
Modifications in prognostic snow albedo reduces warm bias in in cold climate conditions.

Globally applicable routing scheme

The lake model FLake [5] replaces PROBE and is now consistently flux coupled to the atmosphere.
A globally applicable routing scheme has been introduced in RCA4. The routing scheme uses runoff from forest, open land and lake tiles and rout this runoff through a river system as defined by HYDRO1k [6]. River discharge for predefined river mouths can be processed on or offline.
The Kain-Fritsch convection scheme has been updated to the Bechtold Kain-Fritsch scheme which separates the shallow and deep convection processes.


[1] Special issue on Regional climate studies using the SMHI-Rossby Centre models, Tellus Volume 63A, Number 1, Jan. 2011