Interaction between Dynamics and Cloud Microphysics in Orographic Precipitation Enhancement: A High-Resolution Modeling Study of Two North Alpine Heavy-Precipitation Events

Abstract

Interactions of atmospheric dynamics and cloud microphysics with the Alpine orography are investigated for two north Alpine heavy-precipitation cases (20?22 May 1999 and 22?23 August 2005). Both cases were related to a deep cyclone propagating slowly eastward along the Alps, advecting moist air of Mediterranean origin toward the northern side of the Alps. A validation against high-resolution rain gauge data reveals that the average model bias is below 15% in the region of interest, but there is a tendency to systematically underestimate very heavy precipitation. A scale decomposition of the discrepancies between model and observations reveals that errors on the meso-?-scale contribute at least as much to the total model error as discrepancies on the meso-?-scale. On the scale of single mountain ridges and valleys, the formation of precipitation maxima at valley locations is investigated, with particular emphasis on a region in which a valley receives systematically more precipitation than the adjacent mountain ridges. It is found that the downstream advection of precipitation hydrometeors generated in an orographic feeder cloud is essential for the development of valley maxima. Strong ambient winds and (due to the fall speed difference between snow/graupel and rain) a low freezing level favor a large distance of the precipitation maximum from the upstream mountain ridge. Under suitable geometrical conditions, downstream advection of hydrometeors can even lead to systematically more precipitation in the valley than over the adjacent ridges. Another mechanism capable of generating a systematic rainfall maximum at valley locations requires a freezing level between valley bottom and crest height and a mountain wave flow penetrating into the valley. Under such conditions, the increase in the fall velocity related to melting of snow or graupel into rain leads to a locally intensified fallout of hydrometeors and thus to a maximum in the precipitation rate.