The microphysics and dynamics of mixed-phase clouds in the Swiss Alps: insights from balloon-borne and remote sensing observations

Abstract

Mountains can trigger the formation of orographic clouds or modify the structure of incoming cloud systems and thereby account for a large fraction of the Earth's annual precipitation. During winter, most of the precipitation-forming clouds are mixed-phase clouds (MPCs), which consist of a mixture of supercooled cloud droplets and ice crystals. Although the coexistence of liquid and ice is thermodynamically unstable, persistent MPCs have frequently been observed in mountain regions, where the local topography produces sufficiently large updrafts to provide a continuous source of condensate. Despite significant advances in the fundamental understanding of MPCs and orographic precipitation over the past decades, large uncertainties remain regarding ice initiation, the partitioning between the liquid and ice phase and the glaciation time in MPCs. This limits the ability to obtain accurate forecasts of precipitation formation and amounts in complex terrain. Innovative measurement strategies are required to improve our understanding of the numerous microphysical and dynamical processes in orographic MPCs and their convoluted interactions, which govern orographic precipitation. The aim of this thesis is to improve the process understanding of orographic MPCs and precipitation through a multi-instrumental approach by combining observations from an extensive set of in situ and remote sensing instrumentation. Since the ability of aircrafts to fly in cloudy conditions in complex terrain is limited (e.g., in winding and narrow valleys) and cloud observations at mountain-tops are influenced by surface processes, a new measurement platform on a tethered balloon system (HoloBalloon) was developed to study boundary layer clouds in complex terrain. The major component of the measurement platform is a newly developed HOLographic Imager for Microscopic Objects (HOLIMO 3B), which was designed for balloon-borne measurements. HOLIMO 3B uses digital in-line holography to image an ensemble of cloud particles in the size range from small cloud droplets to precipitation-sized particles in a three-dimensional sample volume. Based on two-dimensional images, information about the phase-resolved particle size distribution and particle shape can be obtained. The velocity-independent sample volume of HOLIMO 3B makes it particularly well suited for balloon-borne applications, which have to deal with fluctuations in wind speed and direction. Additionally, the measurement platform is equipped with an optical particle counter, a 3D ultrasonic anemometer, and a temperature, a humidity and a pressure sensor. The HoloBalloon platform was successfully deployed in different environments and atmospheric conditions. In winter 2018, the HoloBalloon platform demonstrated its feasibility by obtaining vertical profiles of microphysical and meteorological cloud properties up to 700 m above the surface in low stratus clouds over the Swiss Plateau. In winter 2019, the HoloBalloon platform was deployed in orographic clouds within the framework of the Role of Aerosols and CLouds Enhanced by Topography on Snow (RACLETS) campaign, which took place in the Swiss Alps in the region around Davos. A wide variety of orographic effects can enhance precipitation over complex terrain and influence its spatial distribution along the mountain barrier. In the present thesis, the complex interplay between orography, dynamics, microphysics and precipitation is explored on the basis of several case studies of the RACLETS campaign. The role of low-level blocking and shear-induced turbulence on the cloud microphysics and precipitation formation was investigated in an inner-Alpine valley. We suggest that aggregation, needle growth and secondary ice production occurred within the turbulent shear layer, which enhanced ice growth and precipitation formation. The amount of precipitation was determined by the strength of the cross-barrier flow and of the low-level blocking, as precipitation was only observed at the ground when the blocking was weakest and the altitude of the cloud base and shear layer were lowest. Additionally, a persistent low-level liquid cloud was observed by the HoloBalloon platform, when the low-level flow was forced to rise over a small-scale topographic feature in the mountain valley. This shallow low-level cloud did not generate significant precipitation by itself, but was found to "feed" on precipitation particles that formed at higher altitudes (i.e., in the seeder region of the cloud) and to produce conditions favorable for secondary ice production processes. Seeder regions were observed in connection with cloud top generating cells, within which enhanced ice formation and growth occurred. In the present thesis, we propose different mechanisms that potentially increase ice formation and growth within generating cells (convective overshooting, radiative cooling, droplet shattering) by considering the ground-based ice nucleating particle concentration and the ice crystal number concentration measured near cloud base. We found that increased ice formation and growth within the seeder region can induce the full glaciation of a MPC and suggest that secondary ice production may have been partly responsible for the elevated ice crystal number concentrations that have been previously observed in feeder clouds at mountain-top observatories. The synergy of aerosol, cloud, dynamics and precipitation observations can improve our understanding of the microphysical pathways of precipitation formation in orographic MPC. These microphysical processes (e.g., secondary ice production, glaciation of MPC) and the dynamical response of the flow to the orography need to be represented accurately in atmospheric models in order to improve the reliability of precipitation forecasts in complex terrain.