Statistical considerations concerning Probable Maximum Precipitation (PMP) in the Alpine Country of Austria

Summary

The statistical processing of the yearly extreme 1-day precipitation values from 504 observation stations in Austria confirmed known qualitative connections between extreme precipitation and orographic situations in a quantitative way. The results point indirectly to two different orographically induced precipitation enhancement mechanisms, particularly effective on mountain borders: (a) orographic precipitation by forced uplift while crossing a mountain barrier; (b) convective activity by baroclinic stratification due to the effects of “elevated heat sources” (“Plateaueffekt”). In the inner Alpine regions the maximum precipitation on average reached values which are slightly higher than those of the pre-Alpine regions.

The fitting of the Fisher-Tippett distributions to the data showed in most cases a type II distribution without an upper limit. Therefore no conclusions concerning the Probable Maximum Precipitation (PMP) could be made. Precipitation with a return period of 100 years depends, particularly in the case of short series, on the curvature parameterk. This parameter reacts in a sensible way to “outliers”. The results of the calculated precipitation amounts with a return period of more than 100 years are classified as less reliable. The fitting of the two-sided limited Beta-distribution results in variations of the upper limit but no useful result with respect to PMP. The precipitation calculated after Hershfield (PMPH) was in several cases far below the observed precipitation and can therefore not be regarded as PMP. Due to the decrease ofK _m with increasing average maximum precipitation, the PMPH results in a more uniform geographical distribution (e.g. precipitation with a 100 year return period) and is better suited to be a statistical parameter for the characterization of the regional maximum precipitation.