Abstract
Abstract. Previous studies that investigated orographic precipitation have primarily focused on isolated mountain barriers. Here we investigate the influence of low-level blocking and shear-induced turbulence on the cloud microphysics and precipitation formation in a complex inner-Alpine valley. The analysis focuses on a mid-level cloud in a post-frontal environment and a low-level feeder cloud induced by an in-valley circulation. Observations were obtained from an extensive set of instruments including ground-based remote sensing instrumentation, in situ instrumentation on a tethered-balloon system and ground-based precipitation measurements. During this event, the boundary layer was characterized by a blocked low-level flow and enhanced turbulence in the region of strong vertical wind shear at the boundary between the blocked layer in the valley and the stronger cross-barrier flow aloft. Cloud radar observations indicated changes in the microphysical cloud properties within the turbulent shear layer including enhanced linear depolarization ratio (i.e., change in particle shape or density) and increased radar reflectivity (i.e., enhanced ice growth). Based on the ice particle habits observed at the surface, we suggest that riming, aggregation and needle growth occurred within the turbulent layer. Collisions of fragile ice crystals (e.g., dendrites, needles) and the Hallett–Mossop process might have contributed to secondary ice production. Additionally, in situ instrumentation on the tethered-balloon system observed the presence of a low-level feeder cloud above a small-scale topographic feature, which dissipated when the low-level flow turned from a blocked to an unblocked state. Our observations indicate that the low-level blocking (due to the downstream mountain barrier) created an in-valley circulation, which led to the production of local updrafts and the formation of a low-level feeder cloud. Although the feeder cloud did not enhance precipitation in this particular case (since the majority of the precipitation sublimated when falling through a subsaturated layer above), we propose that local flow effects such as low-level blocking can induce the formation of feeder clouds in mountain valleys and on the leeward slope of foothills upstream of the main mountain barrier, where they can act to enhance orographic precipitation through the seeder–feeder mechanism.
Highlights
Mountains can alter and reorganize incoming weather systems or force air masses to lift and produce a large proportion of the Earth’s annual precipitation (Roe, 2005)
The feeder cloud did not enhance precipitation in this particular case, we propose that local flow effects such as low-level blocking can induce the formation of feeder clouds in mountain valleys and on the leeward slope of foothills upstream of the main mountain barrier, where they can act to enhance orographic precipitation through the seeder–feeder mechanism
We studied the influence of low-level flow blocking and shear-induced turbulence on the microphysics of a mid-level cloud in a post-frontal environment and a lowlevel feeder cloud induced by an in-valley circulation
Summary
Mountains can alter and reorganize incoming weather systems or force air masses to lift and produce a large proportion of the Earth’s annual precipitation (Roe, 2005). Besides the total amount of precipitation, its spatial distribution across the mountain range becomes of increasing importance for public warning (e.g., avalanche, flash flood), water resources, hydropower production and winter tourism (Stoelinga et al, 2013). In addition to orographic lifting and the subsequent production of condensate, additional processes are required to efficiently form precipitation-sized particles within the lifetime of the cloud as it crosses the mountain barrier Extensive knowledge of these physical processes and the interplay between dynamics, microphysics and orography is essential to understand precipitation formation over complex terrain
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