Universität zu Köln

Schnitt, Sabrina ORCID: 0000-0002-3949-770X (2020). Advancing Ground-Based Water Vapor Profiling through Synergy of Microwave Radiometer and Dual-Frequency Radar. PhD thesis, Universität zu Köln.

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Abstract

Continuous water vapor profiling methods are crucial for advancing the understanding of the role of clouds and water vapor in Earth's climate system. Particularly in the maritime trade wind driven environment, where shallow cumulus clouds prevail, the interplay between cloud and convection processes is not quantified satisfactorily. Current instrumentation techniques are limited by low temporal resolution in the case of soundings, signal saturation at cloud boundaries in the case of optical methods, or too coarse vertical resolutions in the case of passive microwave measurements. Therefore, in this thesis, the feasibility of a novel synergy concept is assessed by combining synthetic microwave radiometer (MWR) and dual-frequency radar measurements. The synergy benefits are evaluated for a combination of seven MWR K-band brightness temperatures (TBs) with a Ka- and W-band radar combination (KaW), e.g. available at Barbados Cloud Observatory (BCO), and a Differential Absorption Radar (DAR) frequency combination of 167.0 and 174.8 GHz (G2). An optimal estimation framework retrieving the absolute humidity profile was selected to evaluate the synergy concept by deriving the retrieval uncertainty, information content through Degrees of Freedom of Signal (DFS), as well as the accuracy of the retrieved profile and partial water vapor amount. By varying the observation vector configuration to include both MWR TBs and radar Dual-Wavelength Ratio (DWR) in the synergistic configuration, or only TBs or only DWR in the single-instrument runs, the synergistic impacts were analyzed for an idealized single-cloud scenario frequently observed at BCO, and for three selected, more complex cases observed during the EUREC4A field study. Additional 2m humidity and cloud boundary measurements further constrain the retrieval. Based on the single-layered cloud scenario with varying water vapor conditions, the analyses show that the total information content of a MWR+KaW combination only increases marginally by less than 6%, while the DFS in case of the MWR+G2 synergy increases by 1.2 DFS on average compared to the MWR-only configuration. While the sub- and in-cloud information content is increased by 1 DFS, driven by the radar measurements, the synergistic information content above the cloud layer is enhanced by 13.5% compared to the MWR-only configuration. Meanwhile, the synergistic MWR+G2 retrieval uncertainty decreases around cloud base to 1.0gm-3, corresponding to a 28% reduction compared to the MWR-only configuration. The synergistic benefits are most sensitive to the assumed radar measurement error, leading to an uncertainty increase of 0.1gm-3 in the cloud layer when the DWR error is doubled, as well as to radar signal saturation before reaching cloud top. Case study analyses of two double-layered cloud scenarios confirm the findings of the single-cloud layer case as the information content above each cloud layer is increased in all cases by up to 0.3 DFS. A modified retrieval concept serves to evaluate the role of the synergy when reconstructing the atmospheric state at 12 hours between 24-hour spaced operational radiosondes based on the EUREC4A case scenarios. While the total synergistic information gain is reduced to 0.2 - 0.6 DFS due to the more accurate prior assumptions, the derived dry free tropospheric water vapor amount agrees better, by up to 3.6kgm-2, with the observed sounding reality than the interpolated prior amount. As expected, the addition of synthetic Raman lidar measurements improves the retrieval performance particularly in the sub-cloud layer, leading to increasing sub-cloud information content of 0.8 - 1.3 DFS, and decreasing optimal to prior uncertainty ratio of 13.6 - 26.2 percentage points compared to the MWR+G2 retrieval. A modified observation vector configuration including the simulated in-cloud humidity, as would e.g. be available by an independent direct inversion retrieval, further decreases the retrieval uncertainty in respect to the prior by 11.4 percentage points between the cloud layers. Under realistic instrument deployment, the simulated measurements suggest that current G-band radar signal sensitivity would impair profiling the whole vertical cloud extent for the simulated thin liquid clouds in the trades. First simulated cases show similar restrictions for an airborne deployment in the trades, for example on HALO. Simulated radar measurements for an idealized mixed-phase cloud scenario in the drier Arctic environment as observed at Ny-Ålesund, Spitsbergen, suggest that current G-band radar sensitivities would allow evaluating the concept in drier conditions than observed in the tropics. The analyzed benefits suggest that a synergy of MWR and G-band DAR could contribute to closing the current observational gap of continuous high-resolution water vapor profile measurements.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Schnitt, Sabrina UNSPECIFIED orcid.org/0000-0002-3949-770X UNSPECIFIED
URN:	urn:nbn:de:hbz:38-354376
Date:	2020
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 remote sensing UNSPECIFIED water vapor UNSPECIFIED shallow clouds UNSPECIFIED optimal estimation UNSPECIFIED synergy UNSPECIFIED microwave radiometer UNSPECIFIED radar UNSPECIFIED trade wind UNSPECIFIED
Date of oral exam:	20 November 2020
Referee:	Name Academic Title Löhnert, Ulrich Prof. Dr. Neggers, Roel Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/35437