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Wastl, Clemens (2008): Klimatologische Analyse von orographisch beeinflussten Niederschlagsstrukturen im Alpenraum. Dissertation, LMU München: Fakultät für Physik



This dissertation deals with a climatological investigation of different orographic precipitation enhancement processes in the Alps. On the one hand, the database consists of observations of more than 1000 weather stations in Bavaria and western Austria for the time period of 1991 - 2000. On the other hand, a MM5-climate-mode-simulation driven with ERA-40 reanalysis data provides information about the environmental meteorological conditions. After a short introduction into the basics of orographic precipitation enhancement and into some simple linear models, the different station data are converted into a uniform temporal resolution of 6 hours. To estimate the relative contribution of different precipitation types, we distinguish between cold fronts, warm fronts, convection and a class carrying unclassified events. Convective precipitation in connection with fronts is attributed to the respective frontal class. Unclassified events predominantly consist of postfrontal upslope precipitation and quasi-stationary fronts. In addition, the wind direction at Alpine crest level (700 hPa) is considered. In a first step, the precipitation differences between the Alpine foreland and the northern Alps are analyzed. The investigation of the climatological importance of the 4 precipitation types in various regions shows that summertime convection and orographic lifting make the largest contribution to the precipitation gradient towards the Alps. Convective precipitation occurs predominantly in association with southwesterly flow and is more abundant in the Alps than in the adjacent forelands because convection is primarily triggered over the Alps. Orographic lifting is most active in case of northwesterly and northerly winds and intensifies both frontal and postfrontal precipitation. The climatological importance of orographic lifting is much larger in winter than in summer. A reversed precipitation gradient with systematically more precipitation in the foreland than in the Alps is found for fronts associated with a wind direction of exactly 270$^\circ$. Under these circumstances, the wind blows parallel to the mountain range and lee effects related to upstream topography reduce the precipitation intensity in the Alps. The climatological precipitation maximum is shifted from westerly towards northerly winds when moving from west to east in the northern Alps. The second part of this work comprises an investigation of the climatological precipitation decrease from the northern Alps to the inner-Alpine valleys. The comparison between the precipitation distribution of 3 regions in the northern Alps and 3 regions in the central Alps shows that especially for cold fronts and unclassified events in connection with northwesterly or northerly flow, a distinctive precipitation surplus can be found in the northern Alps. For convective precipitation and southwesterly or westerly winds the inner-Alpine regions show high convective activity. For southerly wind directions the showers formed over the central Alps are advected towards the northern Alps, where they sometimes even intensify due to a convergence with the inflow from the Alpine foreland. There is a strong west-east precipitation gradient in the central Alps. The decreasing crest level of the northern Alps and the decreasing north-south-extension of the Alps together with the topographical structure of the valleys are the main reasons for the higher precipitation amounts in the eastern parts of the central Alps than in the western parts. Additionally, an investigation of the precipitation gradient in dependence on the temperature level is made. However, a classification of the precipitation events into 3 classes (snow line $>$ 2500 m, 2500 m - 1000 m, $<$ 1000 m) does not show any clear results. The next part investigates altitudinal precipitation differences in the Alps. The analysis is performed for four station pairs, consisting of a mountain station and a nearby valley station each. The climatological precipitation distribution shows that the mountain stations usually receive substantially more precipitation than the valley stations, especially for northwesterly and northerly ambient flow in 700 hPa. However, the differences are regionally variable and indicate a strong influence of the local topography. Moreover, precipitation enhancement over mountains tends to be substantially more effective for low temperatures than for high temperatures. A more detailed investigation of some parameters affecting orographic precipitation enhancement is conducted for stratiform precipitation events. The magnitude of orographic precipitation enhancement markedly increases with the wind speed at 700 hPa. Moreover, precipitation enhancement increases with the depth of the moist layer in the approaching flow. High resolution simulations with the MM5 model are conducted for four climatologically representative precipitation events in the Zugspitze area. The first two wintertime cases are characterized by a strong northwesterly or northerly flow, associated with large precipitation differences between the mountain and the valley stations. For these cases the model validation shows good agreement with the observed precipitation patterns. The model results indicate a dominance of the classical seeder-feeder mechanism, with strong orographic lifting generating dense orographic clouds over the mountain ridges. The third precipitation event is also associated with a northerly flow, but in this case the low wind speeds do not cause enough orographical lifting so that the mountain-valley precipitation differences are rather small. The last case represents summertime precipitation events with northerly flow, a high snow line and almost no precipitation difference between the Zugspitze and the surrounding valley stations. The correlation between the model and the observations is not as good, because embedded convective cells are not reproduced in a deterministic sense in the MM5. The dynamical and microphysical fields show that the snow line around the peak causes a local precipitation minimum at the Zugspitze. Because of different fall speeds of snow/graupel and rain there is a divergence of the hydrometeorological trajectories near the peak and therefore the precipitation maximum is shifted towards the lee.