Universität zu Köln

Gierens, Rosa ORCID: 0000-0002-3879-3099 (2021). Observations of Arctic low-level mixed-phase clouds at Ny-Ålesund: Characterization and insights gained by high-resolution Doppler radar. PhD thesis, Universität zu Köln.

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Abstract

Low-level mixed-phase clouds (MPCs) play an important role in the Arctic climate system by contributing to the surface warming. The complexities of the mixed-phase microphysics combined with the multitude of ways the low-level MPCs interact with the surface and the boundary layer make these clouds difficult to represent in climate models, which contributes to the uncertainties in predicting future climate change in the Arctic. Observations are needed to provide constrains for model parameterizations on one hand, and to improve process understanding on the other hand. However, continuous observations in the high Arctic are sparse, particularly on the eastern hemisphere. This dissertation presents the first work investigating a multi-year dataset of remote sensing observations of persistent low-level mixed-phase clouds (P-MPC) above Ny-Ålesund, Svalbard. A state-of-the-art Doppler cloud radar providing highly vertically and temporally resolved cloud measurements was utilized in combination with further remote sensing and standard meteorological observations. Two complimentary approaches for addressing the observational gaps in measuring Arctic low-level mixed-phase clouds have been considered. The first study investigated the P-MPCs above Ny-Ålesund in the context of the complex fjord environment. The occurrence and properties of P-MPC in different seasons and under different regional free-tropospheric and surface wind conditions were analyzed. Furthermore, the influence of thermodynamical coupling with the surface was investigated considering both its effect on cloud properties and how coupling is related to the local wind in the fjord. P-MPCs were found to occur most commonly with westerly winds (from the direction of the sea), and these clouds had a lower liquid base height and higher mean liquid and ice water paths compared to the clouds associated with easterly winds (from the direction of the interior of the island). The increased height and rarity of P-MPCs with easterly free-tropospheric winds suggest the island and its orography have an influence on the studied clouds. Most P-MPCs were found at least partially decoupled from the surface, and the decoupled cases where found to have on average a lower liquid water path than the coupled P-MPCs. Decoupling was more common with surface wind directions associated with katabatic winds. The second study explored the potential of the cloud radar Doppler spectrum skewness for gaining insights in the microphysical properties of the P-MPCs. Combining case studies and statistical analysis of a 3-year dataset, a conceptual model relating the reflectivities of supercooled liquid and ice to the skewness profile in the mixed-phase layer at P-MPC top was developed and tested. The change from liquid dominated reflectivity at cloud top to ice dominating below was found to be associated with a skewness profile turning from positive to negative (when defining positive Doppler velocity down towards the radar), thus skewness is providing a reflectivity weighted measure of phase-partitioning in the mixed-phase layer. Although the approach is limited to profiles where the amount of liquid is sufficient to produce a clear signal in the Doppler spectrum, a third of all radar profiles obtained from P-MPCs were found to exhibit the described skewness feature. The analysis indicated that the height where skewness changes sign is to a large extend defined by the reflectivity of the ice phase, and skewness could therefore be useful for studying the early stages of precipitation formation. The statistical analysis carried out further revealed steady relationships between skewness and other cloud parameters, that could provide observational constrain for the evaluation of microphysical parameterizations applied in numerical models.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Gierens, Rosa rgierens@uni-koeln.de orcid.org/0000-0002-3879-3099 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-539008
Date:	2021
Place of Publication:	Köln
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 Earth sciences
Uncontrolled Keywords:	Keywords Language Arctic English mixed-phase clouds English ground-based remote sensing English radar Doppler spectrum English
Date of oral exam:	17 November 2020
Referee:	Name Academic Title Löhnert, Ulrich Prof. Dr. Neggers, Roel Prof. Dr.
Funders:	Deutsche Forschungsgemeinschaft
Projects:	Transregional Collaborative Research Center (TR 172) Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC)3
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/53900