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Cloud-resolving ensemble simulations of the August 2005 Alpine flood


Hohenegger, Walser, Langhans, and Schär, 2008, Quart. J. Roy. Meteor. Soc.
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Taylor diagrams of daily precipitation on (top) August 20 and (bottom) August 22. A Taylor diagram compares a pair of integrations (m1,m2) on the basis of their spatial correlation r and of the ratio of their spatial standard deviations, σ = σm2m1. The azimuthal location of an entry in the Taylor diagram defines its correlation, while the radial distance gives σ. Blue dots show pairs drawn from the 10 km ensemble (90 pairs), while black circles show 10-2.2 km pairs driven by the same initial and boundary conditions (20 pairs). Red boxes indicate statistics from the comparison of the ensemble means (2 pairs). Pair (m2,m1) is the mirror of pair (m1,m2). On the first day (August 20), the correlation between a 2.2 km simulation (or ensemble mean) and its 10 km counterpart is smaller than between two 10 km members. The ratio σ is also smaller for the 10 km ensemble. On the third day of the integration (August 23), the correlations between corresponding 2.2 and 10 members are generally higher than between different 10 km simulations.
Heavy precipitation events over the European Alps are responsible for major floods and damage. For example, the total damage caused by a flood on the northern side of the Alps during August 2005 was estimated as 3.3 billion USD. This particluar event was ranked as the 5th most costly insurence loss globally in 2005 (after hurricanes Katrina, Rita, and Wilma and European winter storm Erwin). In this study, the potential benefits of using a cloud-resolving ensemble prediction system (EPS) over its driving lower resolution EPS is assessed.
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