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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-785
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Aug 2018

Research article | 16 Aug 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Cloud feedbacks in extratropical cyclones: insight from long-term satellite data and high-resolution global simulations

Daniel T. McCoy1, Paul R. Field1,2, Gregory S. Elsaesser3, Alejandro Bodas-Salcedo2, Brian H. Kahn4, Mark D. Zelinka5, Chihiro Kodama6, Thorsten Mauritsen7, Benoit Vanniere8, Malcolm Roberts2, Pier L. Vidale8, David Saint-Martin9, Aurore Voldoire9, Rein Haarsma10, Adrian Hill2, Ben Shipway2, and Jonathan Wilkinson2 Daniel T. McCoy et al.
  • 1Institute of Climate and Atmospheric Sciences, University of Leeds, UK
  • 2Met Office, UK
  • 3Department of Applied Physics and Applied Mathematics, Columbia University and NASA Goddard Institute for Space Studies, New York, NY, USA
  • 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 5Cloud Processes Research and Modeling Group, Lawrence Livermore National Laboratory, Livermore, California, USA
  • 6Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
  • 7Max Planck Institute for Meteorology, Hamburg, Germany
  • 8National Centre for Atmospheric Science-Climate, Department of Meteorology, University of Reading, Reading, UK
  • 9Centre National de Recherches Météorologiques (CNRM), Météo-France/CNRS, 42 Avenue Gaspard Coriolis, 31057 Toulouse, France
  • 10Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

Abstract. Extratropical cyclones provide a unique set of challenges and opportunities in understanding variability in cloudiness over the extratropics (poleward of 30°). We can gain insight into the shortwave cloud feedback from examining cyclone variability. Here we contrast global climate models (GCMs) with horizontal resolutions from 7km up to hundreds of kilometers with Multi-Sensor Advanced Climatology Liquid Water Path (MAC-LWP) microwave observations of cyclone properties from the period 1992–2015. We find that inter-cyclone variability in both observations and models is strongly driven by moisture flux along the cyclone's warm conveyor belt (WCB). Stronger WCB moisture flux enhances liquid water path (LWP) within cyclones. This relationship is replicated in GCMs, although its strength varies substantially across models. In the southern hemisphere (SH) oceans 28–42% of the observed interannual variability in cyclone LWP may be explained by WCB moisture flux variability. This relationship is used to propose two cloud feedbacks acting within extratropical cyclones: a negative feedback driven by Clausius-Clapeyron increasing water vapor path (WVP), which enhances the amount of water vapor available to be fluxed into the cyclone; and a feedback moderated by changes in the life cycle and vorticity of cyclones under warming, which changes the rate at which existing moisture is imported into the cyclone. We show that changes in moisture flux drive can explain the observed trend in Southern Ocean cyclone LWP over the last two decades. Transient warming simulations show that the majority of the change in cyclone LWP can be explained by changes in WCB moisture flux, as opposed to changes in cloud phase. The variability within cyclone composites is examined to understand what cyclonic regimes the mixed phase cloud feedback is relevant to. At a fixed WCB moisture flux cyclone LWP increases with increasing SST in the half of the composite poleward of the low and decreases in the half equatorward of the low in both GCMs and observations. Cloud-top phase partitioning observed by the Atmospheric Infrared Sounder (AIRS) indicates that phase transitions may be driving increases in LWP in the poleward half of cyclones.

Daniel T. McCoy et al.
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Status: final response (author comments only)
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Daniel T. McCoy et al.
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Short summary
The largest single source of uncertainty in the climate sensitivity predicted by global climate models is how much low-altitude clouds change as the climate warms. Models predict that the amount of liquid within and brightness of low-altitude clouds increases in the extratropics with warming. We show that increased fluxes of moisture into extratropical storms in the midlatitudes explain the majority of the observed trend and the modeled increase in liquid water within these storms.
The largest single source of uncertainty in the climate sensitivity predicted by global climate...
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