Universität zu Köln

Lauer, Melanie ORCID: 0000-0001-8168-1933 (2024). The influence of synoptic features on precipitation in the Arctic - lessons learned from case studies and climatology. PhD thesis, Universität zu Köln.

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Abstract

During the last decades, the Arctic has warmed faster than the rest of the globe. This phenomenon is known as Arctic amplification (AA). Many processes contribute to changes in the Arctic, including the increased poleward moisture transport. The poleward moisture transport increases the cloud cover and water vapor content in the Arctic which amplifies the near-surface warming and sea ice melting. In addition, the increased water vapor transport influences the precipitation in the Arctic. Precipitation is a key climate variable and plays a crucial role in the Arctic climate system. It can affect the atmosphere due to thermodynamic processes and surface characteristics such as the surface mass balance, surface energy budget, surface albedo, and also the formation of sea ice. As a warmer atmosphere has a higher moisture capacity, it is expected that Arctic precipitation will increase in the future. Further, a phase transition from snow to rain will lead to a rain-dominated Arctic which could accelerate the sea ice melting and consequently near-surface warming (the so-called ice-albedo feedback). In this thesis, the goal is to investigate the influence of poleward moisture transport related to atmospheric rivers (ARs), cyclones, and fronts on precipitation in the Arctic. For the analyses, the ERA5 global reanalysis data with its high spatial and temporal resolution is used. The ERA5 data is initially evaluated by comparing it with ground-based and spaceborne remote-sensing measurements for two AR events. For the first AR event, the vertical profile of radar reflectivities and hydrometeors are compared, while for the second AR event, the focus was on the surface snowfall rate. Generally, ERA5 reanalyses represent the vertical structure of the precipitation systems very well. There are also deviations, especially over complex terrain, and in representing small-scale features. Nevertheless, ERA5 represents larger precipitation features and AR positioning that are related to synoptic features in a satisfying way. Within the framework of this thesis, a new methodology has been established to distinguish between AR-, cyclone-, and front-related precipitation. This method takes into account whether these synoptic features are co-located or separated (not co-located) from each other. First, the method is applied for two distinct periods from two airborne campaigns: ACLOUD (Arctic Cloud Observations Using airborne measurements during polar day; May/June 2017) and AFLUX (Aircraft campaign Arctic Boundary Layer Fluxes; March/April 2019). The contribution of ARs, cyclones, and fronts to precipitation differs between the periods. During the early summer campaign, the precipitation is mainly associated with AR- (40%) and frontrelated (55%) components, especially when they are co-located. The early spring campaign shows another pattern. During this campaign, the precipitation was mainly concentrated in the cyclone-related components (62%), especially when they were not co-located. The difference in contribution during the early summer and early spring campaigns was later confirmed in the climatological analyses. Throughout the ERA5 period (1979 – 2021), the precipitation is not evenly distributed over the Arctic. 45% of the total precipitation is concentrated over the Arctic North Atlantic sector and the Kara and Barents Seas. The remaining precipitation (55%) is more evenly distributed over the other regions. The analyses showed also spatio-temporal variations in the total accumulated precipitation. During summer, the precipitation is evenly distributed over the entire Arctic and has its maximum in August. In the other seasons, the Arctic North Atlantic and the Kara and Barents Seas are most affected by precipitation. Further, there are differences in the type and form of precipitation. Considering the type of precipitation, 86% is classified as large-scale precipitation, and only 14% is convective precipitation. 70% of the precipitation is in the form of snow, while 30% is in the form of rain. Generally, snowfall is the dominant type of precipitation from October to May, and rainfall in July and August. Further, rainfall is located mostly over the Arctic North Atlantic. Slightly more than half of the total precipitation (56%) is attributable to ARs, cyclones, and fronts. The precipitation is mainly associated with cyclones (30%). Only 13% of the total precipitation is associated with ARs and 19% with fronts. The sum of these values exceeds the 56% mentioned, as part of the precipitation occurs when the synoptic features are co-located. The AR-, cyclone- and front-related precipitation shows also spatio-temporal differences as well as differences between rain and snow. Throughout the year, cyclones contribute similary to the total precipitation, while ARs and fronts become more effective in summer. During winter, the cyclonerelated precipitation is mainly concentrated over the Arctic North Atlantic and the Kara and Barents Seas. In summer, the AR-, cyclone-, and front-related precipitation is more evenly distributed with a higher contribution over the Barents, Kara, and Laptev Seas. Generally, cyclones and fronts contribute more to precipitation than ARs. This correlates with the frequency of their occurrence. Cyclones occur three times and fronts two times more often than ARs, and they also cover a greater area. Nevertheless, AR-related precipitation is more intense compared to cycloneand front-related precipitation. It is worth noting that 44% of the precipitation cannot be assigned to any of the synoptic features. This amount is classified as residual. The residual is higher for snowfall (50%) than for rainfall (24%) and has its maximum in winter over the Arctic North Atlantic. Further analyses reveal that the residual over the Arctic North Atlantic can mainly be attributed to marine cold air outbreaks (CAOs). Considering CAOs, the occurrence of the residual would decrease from 15% to 5% over this region. The residual over the other regions is less affected by CAOs. Applying a threshold of 0.1 mm hr−1 has shown that these regions are mainly affected by light precipitation rates which are not associated with synoptic features. Especially over the Arctic Ocean, up to 50% of the total precipitation has precipitation rates lower than 0.1 mm hr−1. This highlights the importance of light precipitation, especially with regard to snowfall. Applying a threshold would not only decrease the residual, but it would also increase the contribution rates of synoptic features, especially ARs, to the total precipitation in the Arctic. Trend analyses show a significant increase in convective and large-scale rainfall over the Arctic North Atlantic and the Kara and Barents Seas. The positive trend over the Arctic North Atlantic is mainly related to ARs and fronts, while the positive trend over the Kara and Barents Seas is mainly related to cyclones and CAOs. There are also differences between convective and large-scale precipitation. A significant increase in convective precipitation over the Kara and Barents Seas is evident which could be associated with sea ice retreat leading to local evaporation. For the large-scale precipitation, the significance is less pronounced because of compensating trends, i.e., a negative trend in snowfall and a positive trend in rainfall. This highlights a phase change from snow to rain in these regions and no increase in total precipitation.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language UNSPECIFIED German
Creators:	Creators Email ORCID ORCID Put Code Lauer, Melanie melanie.lauer@uni-koeln.de orcid.org/0000-0001-8168-1933 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-749423
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 Earth sciences
Uncontrolled Keywords:	Keywords Language Arctic UNSPECIFIED precipitation UNSPECIFIED Atmospheric rivers UNSPECIFIED Extratropical cyclones UNSPECIFIED Atmospheric fronts UNSPECIFIED
Date of oral exam:	3 May 2024
Referee:	Name Academic Title Crewell, Susanne Prof. Dr. Vercauteren, Niki 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/74942