A4. RCMs’ added value in simulating significant weather events

The working assumption underlying regional climate modelling is that RCMs’ fine mesh permits the development of fine-scale structures that are required for an accurate description of local climates (e.g. Laprise et al., 2008). To this day however the quantification of the added value of RCMs remains a central issue. Frequency distribution analysis has confirmed the benefits of fine-mesh simulations in better reproducing the extreme values of key weather elements such as precipitation intensity (Mladjic et al., 2011). Scale separation has been used as a tool, revealing small-scale features that are induced by strong surface forcings as a result of sharp physiographic variations such as coastal or mountainous regions (e.g. Feser, 2006; Di Luca et al., 2011 and 2012 a, b). Several processes that are important for local climate are episodic or short-lived (such as the mesoscale convective systems or downslope wind storms), and hence do not contribute strongly to total variance and tend to be belittled by scale analysis.

In this project attention will rather be focused into the improvement of RCMs’ ability to simulate complex weather and climate phenomena as a result of increased resolution. The working hypothesis is that several processes result from complex interactions across a wide range of scales, and hence are expected to improve with high resolution, despite the fact their variance spectrum may be dominated by larger scales. Regional precipitation and temperature anomalies associated with synoptic-scale weather systems along the East Coast or with planetary-scale circulations such as the Arctic and North Atlantic Oscillations (AO/NAO) are examples of such phenomena we plan to investigate.