Small‐scale variability of orographic precipitation in the Alps: Case studies and semi‐idealized numerical simulations

Abstract

This study presents high-resolution numerical simulations in order to investigate the small-scale precipitation variability in a north Alpine region with an extraordinarily high density of operational rain-gauge stations. Some of the simulations have been conducted with realistic analysis data, considering two heavy-precipitation cases that occurred in July and August 2005. Both cases were related to cyclonic northeasterly flow and exhibited a small-scale precipitation distribution differing substantially from the climatological mean. Most notably, a station lying on the southwestern flank of a major mountain massif recorded precipitation amounts far exceeding all surrounding stations in both cases, although its precipitation climatology does not show outstanding values. Despite some quantitative differences, all real-case simulations exhibit a precipitation pattern consistent with the observed one. They indicate that the small-scale precipitation variability, including the unusual local maximum, was dominated by the classical seeder-feeder mechanism combined with downstream drift of the precipitation hydrometeors.In addition, a large number of semi-idealized simulations have been performed, combining realistic topography with synthetic large-scale flow conditions. Their main purpose is a systematic investigation of the factors controlling the structure and amplitude of the small-scale precipitation variability. It is found that the ambient wind direction has the largest impact on the small-scale precipitation patterns. Selecting a 700 hPa wind direction that differs by about 60°from the actual one yields a precipitation pattern that is completely uncorrelated with the observed data. The pattern correlation with the observed data is also degraded when the ambient wind speed is moved away from its real value, but the impact is not as large as for the wind direction. A less systematic impact is found for temperature, or freezing level, but the most realistic precipitation pattern is still obtained when the specified values approximately match those observed in the real cases. Interestingly, when choosing large-scale flow conditions that approximately match the real ones, the pattern correlation with the observed rain-gauge data becomes in both cases at least as high as for the best real-case simulation.

Role of Lateral Terrestrial Water Flow on the Regional Water Cycle in a Complex Terrain Region: Investigation With a Fully Coupled Model System

“…The range in ensemble basin‐averaged precipitation is particularly increased on 9–10 July and 22–23 August, as shown in Figure . The first event was caused by a midtropospheric low‐pressure system moving from the Northern Sea toward the Alps, as documented by Zängl (). In both model ensembles, the lower part of the trough on 9 July is considerably stronger in comparison to the reanalysis data (not shown), thus increasing the northeasterly moisture inflow toward the Alps and increasing simulated precipitation in the study region.…”

Numerical simulation of the falling snow deposition over complex terrain

“…Generally, the main processes governing snow accumulation at different scale in alpine terrain are orographic precipitation [ Roe , ], preferential deposition [ Lehning et al ., ], and wind‐induced snow transport [ Liston and Sturm , ]. In recent years, the simulation studies on snow distribution in the complex terrain have been also conducted [ Pomeroy et al ., ; Zängl , ; Lehning et al ., ; Mott and Lehning , ], and a series of achievements have been established. However, it is still very difficult to quantitatively predict the orographic precipitation because many mechanisms and physical processes together contribute to this issue [ Roe , ], such as the interactions between precipitation and terrain [ Colle , ; Houze , ], the effect of updraft on the morphologic change of snow particles [ Houze and Medina , ], and seeder‐feeder mechanism [ Bergeron , ; Dore and Choularton , ; Stoelinga et al ., ], in which the seeder‐feeder mechanism describes the increase in particle volume in the process of passing through an orographically generated feeder cloud.…”

Orographic effects on snow deposition patterns in mountainous terrain

“…There are a large number of studies relating small-scale precipitation patterns to air flow dynamics and/or microphysical processes in mountainous terrain [Anquetin et al, 2003;Colle, 2004;Anders et al, 2007;Zängl, 2007aZängl, , 2007bZängl, , 2008Zängl et al, 2008;Lehning et al, 2008;Minder et al, 2008]. Some of these studies [Choularton and Perry, 1986;Dore and Choularton, 1992;Medina and Houze, 2003;Colle, 2004;Houze and Journal of Geophysical Research: Atmospheres 10.1002/2013JD019880 Medina, 2005;Purdy et al, 2005;Zängl, 2007bZängl, , 2008Zängl et al, 2008] showed that the orographic enhancement of precipitation due to the seeder-feeder mechanism [Bergeron, 1965;Colle, 2004;Colle et al, 2013;Stoelinga et al, 2013] is a key process in producing local precipitation maxima over mountains.…”