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Improvement of cloud radar products for fog surveillance networks : fog life cycle analyses and calibration methodologies

Abstract : Visibility reduction caused by fog has a significant impact on human activities. In addition, fog is a complex phenomenon whose evolution depends on the delicate balance of several physical processes. Recent developments show that cloud radars are key instruments to improve observation of key fog processes. This, paired with new developments reducing their cost, opens the possibility of establishing networks of fog surveillance stations. Yet, some challenges must be addressed to secure the value of such networks:1.- Processes that drive fog life cycle occur throughout the fog layer, in particular at its lower boundary where the fog layer interacts with the soil, and at the top where it interacts with the unsaturated air above. In-situ sensors are well suited to monitor key variables in the lowest boundary. Remote sensing instruments, including cloud radars, can monitor the state of the fog layer and processes occurring at fog top. Fog Liquid Water Path (LWP) and fog top height are shown to be remotely sensed variables that are key drivers of fog temporal evolution throughout its life cycles.To address the challenge of assessing fog dissipation tendencies, the thesis proposes a novel approach that relies on remotely sensed variables combined with a new conceptual model. The approach provides two diagnostic variables: the Critical Liquid Water Path (CLWP), and the Reservoir Liquid Water Path (RLWP). CLWP is the minimum amount of LWP necessary to maintain a fog layer of a given thickness. RLWP is the excess of water that must be removed before fog dissipation at the surface can occur. The conceptual model establishes a new paradigm based on the observation of the fog column to assess its dissipation tendency.2.- Cloud radar calibration is an unsolved issue that hampers the performance of fog observation networks, by limiting the reliability of microphysical retrievals and of comparative studies between observation sites. This thesis researches a calibration strategy that could be applied on a fog surveillance network, based on the results of two calibration campaigns that took place at the SIRTA observatory, as part of ACTRIS infrastructure developments.The strategy consists of two steps: First, to calibrate a reference radar with a reliable estimation of uncertainty. Second, to transfer the calibration to other radars, by comparing reflectivity measurements. The reference radar (95 GHz BASTA mini) is calibrated using a new method, based on corner reflectors. Bias and uncertainty sources are discussed and quantified. At present, the method uncertainty is of 2 dB, limited by the use of a theoretical model to calculate the reflector radar cross section. Yet, calculations indicate that uncertainty could reach a theoretical minimum of 0.4 dB, depending on the characterization uncertainty of the reflector and on the experimental setup. The calibration transfer methodology is based on the analysis of simultaneous cloud measurements. Using two weeks of observations, it enabled the calibration transfer from the reference to a 94 GHz RPG cloud radar, with an added uncertainty of 0.9 dB with respect to the reference radar calibration.The technical and scientific work carried out in this thesis contribute to improving our capacity to monitor key variables that contribute to a better understanding of fog life cycle evolution.
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Submitted on : Friday, July 23, 2021 - 3:54:30 PM
Last modification on : Thursday, July 29, 2021 - 3:58:30 AM


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Felipe Toledo Bittner. Improvement of cloud radar products for fog surveillance networks : fog life cycle analyses and calibration methodologies. Meteorology. Institut Polytechnique de Paris, 2021. English. ⟨NNT : 2021IPPAX029⟩. ⟨tel-03298445v1⟩



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