Universität zu Köln

Radovan, Ana (2021). Variability and trends of Arctic water vapour from passive microwave satellites Special role of Polar lows and Atmospheric rivers. PhD thesis, Universität zu Köln.

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Abstract

Water in the vapour phase is the most important component of the hydrological cycle. It is formed by processes of evaporation and sublimation during which a lot of energy as latent heat is absorbed from the atmosphere. Through atmospheric large and small scale circulation, this energy is transported and released elsewhere through the process of condensation. Water vapour is the most important greenhouse gas (GHG) due to its abundance and its effectiveness in absorbing longwave radiation. In the light of global climate change, it is of great importance to identify trends of water vapour amounts in the atmosphere and its variability. Climate change in terms of the near-surface temperature is most pronounced in the Arctic, known as Arctic Amplification. Since most of the Arctic are either open ocean or sea-ice covered surfaces, only sparse ground-based observations, mostly confined to land areas are available. Therefore, one must resort to usage of the satellite based observations which offer a great advantage by their large spatial coverage. For water vapour assessment, passive microwave satellites are well suited due to their ability to sense water vapour under clear and cloudy sky conditions independent of sun light. A number of products of integrated water vapour (IWV) from various satellites are available. However, these are often inconsistent and prone to have biases due to various assumptions and uncertainties of a priori data included in the retrieval algorithms. According to the Clausius-Clapeyron relation, water vapour is constrained by the saturation vapour pressure which is constrained only by the temperature. Therefore, this thesis investigates the hypothesis that brightness temperatures (Tbs) from spaceborne passive microwave instruments can be used as a proxy for water vapour trends. To test this hypothesis, satellites based Tbs are compared to synthetic Tbs derived from the Arctic System Reanalysis (ASR). To enable the comparison, the ASR has been evaluated in Tb space by employing the Passive and Active Microwave TRAnsfer forward model (PAMTRA). Moreover, Tbs from sounding channels were correlated with corresponding IWV based on the weighted absolute humidity profiles peaks. The hypothesis is tested for the dry, cold and sun-absent winter season (January) and the sun-return transitional spring season (May). The results show that Tbs from frequency channels can explain trends in the corresponding IWV columns derived from ASR for regions with significant positive trends for both, Tb and IWV since high correlation coefficients, reaching 0.98, have been found. This is true for different time scales, daily, monthly and for the period of 17 years (2000-2016). The exception to this has been found for May for daily time scale for frequency channel dominated by the signal from the upper troposphere lower stratosphere (UTLS). For this combination of Tbs and IWV correlations tend to be weaker and at some locations even negative. This is consistent with theoretical calculations and observational studies which report a cooling in the UTLS region for increasing IWV. However, Tbs from the corresponding channel seem less reliable in explaining trends of the corresponding IWV derived from the ASR. This indicates the importance of other processes relevant in the UTLS region during spring. Furthermore, this thesis investigates synoptic features which are associated with water vapour transport and precipitation. Previous studies have shown that Arctic cyclone activity during winter has a large impact on the sea ice melt in the following seasons making them important players in the complex feedback mechanism of the climate change in the Arctic. However, the life cycle of the most intense of such cyclones, also known as polar lows (PL) are not yet fully understood. To analyse their dynamics, this thesis investigates different environmental conditions (and their combination) between genesis and maturity stage of January PLs. PLs with overall lower thermal instability between the surface and 500 hPa during formation stage are typically accompanied by higher and steeper lapse rates throughout the boundary layer. Therefore these PLs were fostering convective development. However, as observed for a few cases, a decreased thermal instability alongside a simultaneous decrease of convection coincides with high relative humidity (mostly above 90%). Furthermore, higher relative humidity at lower levels during genesis stage promoted stronger winds at the maturity stage. Besides water vapour turnover associated with Arctic cyclones, atmospheric rivers (ARs) transport major amounts of moisture from tropical and extratropical regions into the Arctic. Studies have shown that about 90% of the total mid-latitude vertically integrated water vapour transport (IVT) is related to these synoptic features. To study the influence of ARs on PL precipitation, an event with a coupled AR and PL is compared to an event which featured only a PL. The AR had a strong influence on the PL resulting in higher snow amounts on the order of ∼ 4 kg/m2 higher wind speeds and a longer distance traveled during its life cycle, compared to the PL only case.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Radovan, Ana ana.radovan11@gmail.com UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-536096
Date:	October 2021
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 water vapour English microwave satellites English polar lows English atmospheric rivers English
Date of oral exam:	5 October 2020
Referee:	Name Academic Title Crewell, Susanne prof.Dr.sc.
Projects:	(AC)3
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/53609