Heavy precipitation events

The Alps particularly are exposed to both extremes of precipitation - i.e., heavy precipitation (including hail) and drought - according to the circulation patterns that are associated with extremes and their persistence. Many of the strong rainfall events result in flooding and geomorphologic hazards such as landslides, rock falls, and debris flows within regions of complex topography. If these events occur in the vicinity of populated regions, the impacts in human and economic terms can be enormous. Indeed, the August 2005 floods in Switzerland were estimated to be, on a gross domestic product (GDP) basis, as costly to the Swiss economy as the 2005 Katrina hurricane was to the US economy (unpublished figures from insurance firms).

Figure 8.5 shows the behavior of summer precipitation events, in the form of August precipitation totals recorded each year at Altdorf, a location in the central-northern Swiss Alps that is often subject to heavy precipitation events (Beniston, 2006). August is a prime month for strong convective downpours, especially when moist air converges into regions whose surfaces have been well heated during the summer months; in addition, explosive convection is exacerbated by forced uplift of air by the topography. Figure 8.5 shows that while

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Winter (DJF) Spring (MAM) Summer (JJA) Autumn (SON)

Figure 8.6. Seasonal precipitation change in the Swiss Alps as projected by four regional climate models for the IPCC A2 scenario. DJF, December-January-February; MAM, March-April-May; JJA, June-July-August; SON, September-October-November.

Winter (DJF) Spring (MAM) Summer (JJA) Autumn (SON)

Figure 8.6. Seasonal precipitation change in the Swiss Alps as projected by four regional climate models for the IPCC A2 scenario. DJF, December-January-February; MAM, March-April-May; JJA, June-July-August; SON, September-October-November.

record precipitation was registered during the devastating flood of August 2005, intense events also occurred during other periods of the twentieth century, such as in the late 1960s. Superimposed on the precipitation data in Figure 8.5 are the August temperature anomalies, which show that there is no discernible trend that could link rainfall trends to increasing temperatures, even though the latter, warmer part of the record shows a possible increase in variability.

Numerical models of the climate system have greater difficulty in simulating precipitation as opposed to temperature, because of the complex microphysics involved and the fact that sub-grid-scale features such as topography or land use are often inadequate for assessing the correct location and intensity of precipitation. However, recent studies with RCMs applied to Europe show that they capture the broad features of observed precipitation and their seasonal shifts, even in the complex Alpine domain (e.g., Frei et al, 2006; Christensen and Christensen, 2003; Beniston et al., 2007). When they are applied to the A2 and B2 scenario climates for the period 2071-2100, most RCMs show a distinct shift in the seasonal precipitation totals compared with 1961-90, in regions of the Alps that are prone to strong rainfall and flooding. Figure 8.6 shows the shift in precipitation in the central-northern Swiss Alps for four of the RCMs used in the PRUDENCE project. Although there is some disagreement as to the absolute levels of change, the RCMs nevertheless agree on the signs of change; i.e., increases in winter and spring, and reductions in summer and autumn (Beniston, 2006). The annual precipitation amounts

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