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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  01 Sep 2020

01 Sep 2020

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This preprint is currently under review for the journal ACP.

Influence of low-level blocking and turbulence on the microphysics of a mixed-phase cloud in an inner-Alpine valley

Fabiola Ramelli1, Jan Henneberger1, Robert Oscar David2, Annika Lauber1, Julie Thérèse Pasquier1, Jörg Wieder1, Johannes Bühl3, Patric Seifert3, Ronny Engelmann3, Maxime Hervo4, and Ulrike Lohmann1 Fabiola Ramelli et al.
  • 1Department of Environmental System Sciences, Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
  • 2Department of Geosciences, University of Oslo, Oslo, Norway
  • 3Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 4Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland

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, by combining observations 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 a turbulent shear layer, which separated the blocked layer near the surface from the stronger cross-barrier flow aloft. Cloud radar observations indicate changes in the microphysical cloud properties within the turbulent shear layer including enhanced linear depolarization ratio (i.e., change in particle shape) and increased radar reflectivity (i.e., enhanced ice growth). Based on the ice particle habits observed at the surface, we suggest that needle growth and aggregation occurred within the turbulent layer and that collisions of fragile ice crystals (e.g., dendrites, needles) 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) caused the low-level flow to ascend the leeward slope of the local topography in the valley, thus producing a low-level feeder cloud. Although the feeder cloud did not enhance precipitation in the present 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.

Fabiola Ramelli et al.

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Fabiola Ramelli et al.

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Publications Copernicus
Short summary
Interactions between dynamics, microphysics and orography can enhance precipitation. Yet the exact role of these interactions is still uncertain. Here we investigate the role of low-level blocking and turbulence for hydrometeor growth by combining remote sensing and in situ observations. The observations show that blocked flow can induce the formation of liquid layers and that turbulence can enhance ice growth, demonstrating the importance of local flow effects for orographic precipitation.
Interactions between dynamics, microphysics and orography can enhance precipitation. Yet the...