The Rossby Centre Regional Climate Model RCA3: Model description and performance

The physics in the Rossby Centre Regional Climate Model RCA3 has been improved with a tiled surface scheme. In general, RCA3 now shows equally good, or better, correspondence compared to previous model versions. Some biases with respect to observation still exist though.

Physics in RCA3, Rossby Centre Newsletter Nov 2009
Fig 1. The main development of physics in RCA3 concerns the surface processes but development has also been made for some atmospheric processes.

The Rossby Centre Regional Climate Model RCA3 has been applied in numerous applications for many years now. The model has been thoroughly described in a SMHI report but until now no publication has been available in a scientific journal describing the model system and its performance. In the upcoming Tellus RCA special issue one of the papers will be devoted to a description of the physics in RCA3 and results from model simulations will be evaluated against different observations to illustrate its performance on different scales in time and in space.

The major change in RCA3 compared to RCA2 was the introduction of a tiled surface scheme. It was recognised that only one surface temperature representing all kinds of different surfaces in RCA2 (snow, forest, grass, ice) was not sufficient to meet issues related to climate change and surface conditions. In RCA3 different sub-surfaces, tiles, in a grid box are represented by individual temperatures. Furthermore, the concept of double-energy balance was introduced for the forest tile which means that the canopy and the forest floor are given separate energy balances and temperatures. Along with these changes in the surface description updates were also made in the atmospheric physics related to the radiation, turbulence and cloud parameterisations.

In general, RCA3 shows equally good, or better, correspondence to climatologies as compared to the previous model versions. However, of course, biases with respect to observations still exist. On a seasonal basis the mean temperature errors are generally within ±1C except during winter when a positive bias is found in the north-eastern parts of the model domain and a negative bias appears in the Mediterranean region. The higher horizontal resolution used in RCA3 compared to forcing data, like e.g. ERA40, gives a better representation of topography and land-sea contrasts which have a beneficial impact on the precipitation distribution. However, RCA3 still tends to overestimate precipitation in northern Europe during summer and underestimate it in the south-east. A number of other evaluations will be presented and discussed in the upcoming paper.