The role of topography on projected rainfall change in mid-latitude mountain regions.

Change to precipitation in a warming climate holds many implications for water management into the future, and an enhance- ment of a precipitation decrease or increase on or around mountains would have numerous impacts. Here, an intermediate resolution regional climate model (RCM) ensemble projects enhanced precipitation decrease on the windward slopes of over many mid-latitude mountains in winter, consistent with theory and model studies of idealised mountain ranges. This ensemble projects that an increase in convective rainfall determines the sign of total rainfall change in many regions in sum- mer, only some of which are on or near mountains such as the European Alps. These same projected changes are present in inland slopes of the Australian Alps compared to surrounding regions as simulated by three RCM ensembles (the intermediate resolution and two high resolution ensembles), which agree on an enhanced precipitation decrease on the windward slopes in winter and spring, as well as an enhanced precipitation increase in summer driven by an increase in convective rainfall. The ensembles disagree on an enhanced precipitation decrease in autumn. The results represent regional-scale added value in the climate change signal of projections from high resolution models in cooler seasons, but suggest that the specific model components such as convection schemes strongly influence projections of summer rainfall change. Confidence in the simu- lation of change in convective rainfall, or convection-permitting modelling may be needed to raise confidence in summer rainfall projections over mountains.

Figure 5. Projected change in rainfall in the Australian Alps in WRF-10 between 1990–2009 and 2060–2079 under SRES A2 (%) by calendar season, a autumn (MAM); b winter (JJA) and c spring (SON). First row shows change in total rainfall in the mean of 4 CMIP3 models used as input; second row shows WRF-10 total rainfall; third row is WRF-10 stratiform rainfall; fourth row is WRF-10 convective rainfall; and fifth row plots modelled surface height against convec- tive and stratiform rainfall change. Stippling indicates where 9 to 12 models agree on the sign of change, lines show coastline and model topography contours at 400 m intervals