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Large-scale circulation drivers and stable water isotope characteristics of low-level clouds over the tropical North Atlantic

English title Large-scale circulation drivers and stable water isotope characteristics of low-level clouds over the tropical North Atlantic
Applicant Scholder-Aemisegger Franziska
Number 188731
Funding scheme Project funding (Div. I-III)
Research institution Institut für Atmosphäre und Klima ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Climatology. Atmospherical Chemistry, Aeronomy
Start/End 01.11.2019 - 31.10.2022
Approved amount 177'918.00
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All Disciplines (2)

Discipline
Climatology. Atmospherical Chemistry, Aeronomy
Meteorology

Keywords (11)

atmospheric water cycle; meteorology; shallow cumulus cloud feedbacks; atmospheric dynamics; stable water isotopes; numerical weather and climate models; lagrangian diagnostics; cavity ring-down spectrometer; field experiment; lower tropospheric mixing; radiation

Lay Summary (German)

Lead
Quantifizierung des Einflusses der grossskaligen Zirkulation auf tiefliegende Wolken über dem tropischen Nordatlantik mittels stabiler Wasserisotope Flache Kumuluswolken über dem tropischen Nordatlantik sind kleinskalige Wolken, welche wegen ihrer hohen Albedo eine zentrale Rolle spielen für das Erdklima. Mit diesem Projekt möchten wir den Einfluss der nordhemisphärischen Windsysteme auf die grösserskalige Organisation dieser Wolken quantifizieren. Als Werkzeug verwenden wir Messungen und Modellsimulationen stabiler Wasserisotope als Indikatoren der Feuchteherkunft und vergangenen Phasenübergängen in der Atmosphäre.
Lay summary

Inhalt und Ziel des Forschungsprojekts

Das Ziel dieses Projekts ist es, die Interaktion von flachen Kumuluswolken über dem tropischen Nordatlantik mit der atmosphärischen Zirkulation besser zu verstehen. Spezifisch werden wir 1) den Einfluss extratropischer Wettersysteme wie subtropische Zyklonen und Antizyklonen auf die flache Bewölkung in den Tropen quantifizieren und 2) den Einfluss niederschlagsproduzierender Wolken auf die Feuchte- und Isotopenmassenbilanz der marinen Grenzschicht untersuchen. Wir verwenden dafür eine Kombination von innovativen Mess- und Modellierungsmethoden. Während der französisch-deutschen Feldkampagne EUREC4A im Januar-Februar 2020 in Barbados werden wir flugzeugs- und bodengestützte Messungen stabiler Wasserisotope durchführen. Die Flugzeugmessungen werden uns Aufschluss über die Bedingungen in und unter den Wolken geben. Die bodengestützten Messungen werden uns erlauben, die Niederschlagsprozesse einzelner Wolken zeitlich hochaufgelöst zu beschreiben. Die erhobenen Messungen werden wir kombinieren mit globalen (Re-)Analyse Daten, trajektorienbasierten Untersuchungen, sowie Simulationen mit einem numerischen Wetter- und Klimamodell. 

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Unsere Arbeit wird neue und wichtige Informationen zu den physikalischen und dynamischen Prozessen liefern, die für die Bewölkung über dem tropischen und subtropischen Nordatlantik wichtig sind. Die Ergebnisse werden für die Grundlagenwissenschaft relevant sein. Sie werden stark eingebettet sein in das internationale EUREC4A Projekt, dessen Ziel es ist, die zurzeit grossen Unsicherheiten in der Quantifizierung der Klimasensitivität zu reduzieren.

Direct link to Lay Summary Last update: 08.10.2019

Responsible applicant and co-applicants

Employees

Name Institute

Associated projects

Number Title Start Funding scheme
177996 Lagrangian analysis of ice cloud formation pathways and their isotopic signals in high-resolution COSMO-iso simulations of the African and Asian monsoon 01.07.2018 PIRE

Abstract

Shallow cumulus clouds are the most frequently occurring cloud type in the atmosphere. But despite the prevalence of these fluffy, fair weather clouds, our knowledge about their formation, organisation and interaction with the global circulation is still fragmentary. The change in low clouds due to anthropogenic warming is currently the greatest source of uncertainty in climate projections. Due to their high albedo, these clouds strongly modulate Earth’s energy budget, particularly over the dark, low albedo, tropical oceans. Over the tropical North Atlantic, shallow cumulus clouds are embedded in a large-scale easterly trade wind flow. Therein, they act as small localised engines fuelled by convection. These engines drive mesoscale circulations induced by cloud-radiative and evaporative cooling effects that in turn modulate the trade wind circulation. This complex interplay between clouds, radiation and circulation at different scales represents a key feedback mechanism in our climate system, i.e., a grand challenge in climate science that needs to be tackled. To disentangle the different scales of interaction between shallow cumulus clouds and the circulation, this project proposes to use a new kind of moisture tracing tool. Stable water isotopes in the atmosphere are powerful tracers of moisture sources and sinks. Large-scale descending air masses from the upper troposphere above the trade wind inversion carry an isotope signature that is distinct from moisture in the boundary layer that freshly evaporated from the ocean or from falling rain droplets. In the past, these tracers have been successfully used as proxies for paleoclimate conditions. The variations in their abundance at the short-timescale from minutes to weeks is however still poorly understood. Although the number of available isotope observations, particularly in the vapour phase has exploded in the last decade, the dynamical drivers of these short-term variations only start to be unveiled. The role of cloud processes in this context is a practically unexplored topic due to the lack of cloud-scale in situ observations. The potential of new isotope measurements to help us unravel the mysteries about how clouds and the atmospheric flow are coupled is therefore very large. In this project, we propose to combine high precision in situ measurements with a numerical modelling approach for advancing our understanding of trade wind cumulus cloud patterns. We address two questions related to the scale of interaction between shallow cumulus clouds and the circulation: 1) How does the large-scale orchestrated advection of moisture impact the shallow convection regime and the observed cloud patterns? and 2) How much is the formation of trade wind cumulus clouds controlled by local mesoscale moisture cycling? We attribute the high time resolution (1min to 6h) fluctuations in the cloud and sub-cloud moisture fields to source, transport and sink processes. For doing so, we participate in the coordinated international field campaign EUREC4A in Jan-Feb 2020 on the Caribbean island Barbados. During this campaign, we perform in situ aircraft and ground-based laser spectrometric isotope measurements. As a complement to the observational dataset, we use a sophisticated isotope-enabled numerical model (COSMOiso) to perform convection-permitting simulations. We thus produce a cloud-scale water isotope characterisation of the environment, in which trade wind cumuli form. The measured and simulated isotope signals are related to different large-scale flow configurations, as well as to different cloud patterns and their mesoscale circulations. By taking a unique approach, combining measurements and modelling of stable water isotopes, in and around low-level cumulus clouds, this project will sharpen our understanding of the complex and climate-relevant coupling mechanism between cumulus clouds and the large-scale circulation.
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