Universität zu Köln

Chellini, Giovanni ORCID: 0000-0002-7303-0468 (2024). Precipitation formation in low-level mixed-phase clouds: determining relevant processes and drivers based on cloud radar observations from a high Arctic site. PhD thesis, Universität zu Köln.

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Abstract

Low-level mixed-phase clouds (LLMPCs) shroud large portions of Earth’s surface at high latitudes. They have been shown to dramatically affect the surface energy budget, yet, large uncertainties in their model representation remain, both in climate simulations, and in numerical weather prediction. Both computational limitations and poor understanding of a number of processes taking place in LLMPCs are thought to give rise to such uncertainties. In particular, precipitation formation processes have been relatively understudied in LLMPCs, and reaching a refined understanding is expected to lead to an improvement in model performance, as precipitation determines the cloud’s mass sink, and hence lifetime. In this dissertation, precipitation formation processes are investigated in LLMPCs at the high Arctic site of Ny-Ålesund, based on long-term cloud radar observations. Cloud radars are in fact especially suited for ice microphysical studies, due to the wide spectrum of observational fingerprints of ice microphysical processes that they provide. Doppler radar observations provide information on dynamics, multi-frequency radar observations on ice particle size, and polarimetric radar observations on particle shape and concentration. Radar data are combined with thermodynamic information, which further allows to discriminate between ice microphysical processes, due to their high sensitivity to temperature. In the first part of the dissertation, the relevance of the aggregation process for LLMPCs at Ny-Ålesund is assessed. Aggregation occurs when ice particles collide to form larger ice particles. A long-term dataset of dual-frequency radar observations, as well as thermodynamic information, is used to statistically assess the relevance of aggregation and its sensitivity to varying cloud thermodynamic conditions. The study finds that larger aggregate snowflakes are predominantly produced in LLMPCs whose mixed-phase layer is at temperatures compatible with the growth and subsequent mechanical entanglement of dendritic crystals. Surprisingly, the second enhanced aggregation zone close to the 0°C isotherm, typically observed in deeper cloud systems, is absent. In the second part, a novel state-of-the-art long-term dataset developed within this dissertation is presented. It combines dual-frequency and polarimetric Doppler cloud radar observations, together with thermodynamic information, and other auxiliary variables. After detailing the processing and curation approaches, the results on aggregation are confirmed, and expanded upon. Additionally, temperature regimes where columnar ice particles, riming, i.e., the collection of supercooled liquid droplets by ice crystals, and secondary ice production are likely to occur are identified. In the final part of the dissertation, the developed dataset is used to assess the effect of turbulence on aggregation and riming in LLMPCs at Ny-Ålesund. LLMPCs are in fact inherently turbulent, and maintained by turbulent overturning generated at cloud top. The turbulent kinetic energy dissipation rate (EDR) is retrieved, and the sensitivity of aggregation and riming to varying EDR conditions is investigated. It is shown that higher EDR regimes enhance the aggregation of particles, and are associated with signatures of increased ice particle concentration, possibly caused by fragmentation of ice particles. In temperature regimes more favorable to riming, turbulence dramatically enhances the particles’ fall velocities, denoting higher degrees of riming.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Chellini, Giovanni g.chellini@uni-koeln.de orcid.org/0000-0002-7303-0468 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-723208
Date:	2024
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Natural sciences and mathematics Physics Earth sciences
Uncontrolled Keywords:	Keywords Language Arctic low-level mixed-phase clouds English Cloud radar observations English Ice microphysics English
Date of oral exam:	8 January 2024
Referee:	Name Academic Title Kneifel, Stefan Dr. Löhnert, Ulrich Prof. Dr.
Funders:	Deutsche Forschungsgemeinschaft - Project No. 268020496
Projects:	TRR 172 - (AC)3: “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms ”
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/72320