An assessment of future extreme precipitation in western Norway using a linear model

A Linear Model (Smith and Barstad, 2004) was used to dynamically downscale Orographic Precipitation over western Norway from twelve General Circulation Model simulations. The GCM simulations come from the AlB emissions scenario in IPCC's 2007 AR4 report. An assessment of the changes to future Orographic Precipitation (time periods: 2046-2065 and 2081-2100) versus the historical control period (1971-2000) was performed. Results showed increases in the number of Orographic Precipitation days and in Orographic Precipitation intensity. Extreme precipitation events, as defined by events that exceede the 99.5%-ile threshold for intensity for the considered period, were found to be up to 20% more intense in future time periods when compared to 1971-2000 values. Using station-based observations from the control period, the results from downscaling could be used to generate simulated precipitation histograms at selected stations. The Linear Model approach also allowed for simulated changes in precipitation to be disaggregated according to their causal source: (a) the role of topography and (b) changes to the amount of moisture delivery to the site. The latter could be additionaly separated into moisture content changes due to the following: (i) temperature, (ii) wind speed, and (iii) stability. An analysis of these results suggested a strong role of moist stability and warming in the increasing intensity of extreme Orographic Precipitation events in the area.