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von Kármán vortex street over Canary Islands
Credit: NASA

Philip Stier

Professor of Atmospheric Physics

Research theme

  • Climate physics

Sub department

  • Atmospheric, Oceanic and Planetary Physics

Research groups

  • Climate processes
philip.stier@physics.ox.ac.uk
Telephone: 01865 (2)72887
Atmospheric Physics Clarendon Laboratory, room 103
  • About
  • Research
  • Teaching
  • CV
  • Publications

Glaciation of liquid clouds, snowfall and reduced cloud cover at industrial aerosol hot spots

Science American Association for the Advancement of Science 386:6723 (2024) 756-762

Authors:

Velle Toll, Jorma Rahu, Hannes Keernik, Heido Trofimov, Tanel Voormansik, Peter Manshausen, Emma Hung, Daniel Michelson, Matthew Christensen, Piia Post, Heikki Junninen, Benjamin J Murray, Ulrike Lohmann, Duncan Watson-Parris, Philip Stier, Norman Donaldson, Trude Storelvmo, Markku Kulmala, Nicolas Bellouin

Abstract:

The ability of anthropogenic aerosols to freeze supercooled cloud droplets remains debated. In this work, we present observational evidence for the glaciation of supercooled liquid-water clouds at industrial aerosol hot spots at temperatures between −10° and −24°C. Compared with the nearby liquid-water clouds, shortwave reflectance was reduced by 14% and longwave radiance was increased by 4% in the glaciation-affected regions. There was an 8% reduction in cloud cover and an 18% reduction in cloud optical thickness. Additionally, daily glaciation-induced snowfall accumulations reached 15 millimeters. Glaciation events downwind of industrial aerosol hot spots indicate that anthropogenic aerosols likely serve as ice-nucleating particles. However, rare glaciation events downwind of nuclear power plants indicate that factors other than aerosol emissions may also play a role in the observed glaciation events.
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ICON-HAM-lite: simulating the Earth system with interactive aerosols at kilometer scales

Atmospheric Chemistry and Physics Preprints European Geosciences Union (2024)

Authors:

Philipp Weiss, Ross Herbert, Philip Stier

Abstract:

Aerosols strongly influence Earth's climate as they scatter and absorb radiation and serve as condensation nuclei for cloud droplets and ice particles. New Earth system models that run at kilometer resolutions allow us to examine long-standing questions related to these interactions. To perform kilometer-scale simulations with the Earth system model ICON-MPIM, we developed the one-moment aerosol module HAM-lite. HAM-lite was derived from the two-moment module HAM. Like in HAM, aerosols are represented as an ensemble of log-normal modes. Unlike in HAM, aerosol sizes and compositions are prescribed, which reduces the computational costs significantly. Here, we present a first global simulation with four aerosol modes at a resolution of five kilometers and over a period of one year. The simulation captured key aerosol processes including, for example, the emission of dust aerosols by convective storms in the Sahara and the interactions between sea salt aerosols and tropical cyclones in the Pacific.
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Synoptic Scale Controls and Aerosol Effects on Fog and Low Stratus Life Cycle Processes in the Po Valley, Italy

Geophysical Research Letters Wiley 51:20 (2024) e2024GL111490

Authors:

Eva Pauli, Jan Cermak, Jörg Bendix, Philip Stier

Abstract:

Fog and low stratus clouds (FLS) form as a result of complex interactions of multiple factors in the atmosphere and at the land surface and impact both the anthropogenic and natural environments. Here, we analyze the role of synoptic conditions and aerosol loading on FLS occurrence and persistence in the Po valley in northern Italy. By applying k‐means clustering to reanalysis data, we find that FLS formation in the Po valley is either based on radiative processes or moisture advection from the Mediterranean sea. Satellite‐based data on FLS persistence shows longer persistence of radiatively formed FLS events, likely due to air mass stagnation and a temperature inversion. Ground‐based aerosol optical depth observations further reveal that FLS event duration is significantly higher under high aerosol loading. The results underline the combined effect of topography, moisture advection and aerosol loading on the FLS life cycle in the Po valley.
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Multifractal Analysis for Evaluating the Representation of Clouds in Global Kilometre-Scale Models

Geophysical Research Letters American Geophysical Union (2024)

Authors:

Lilli Freischem, Philipp Weiss, HANNAH CHRISTENSEN, Philip STIER
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Combined Impacts of Temperature, Sea Ice Coverage, and Mixing Ratios of Sea Spray and Dust on Cloud Phase Over the Arctic and Southern Oceans

Geophysical Research Letters Wiley 51:20 (2024) e2024GL110325

Authors:

Barbara Dietel, Hendrik Andersen, Jan Cermak, Philip Stier, Corinna Hoose

Abstract:

We analyze the importance of cloud top temperature, dust aerosol, sea salt aerosol, and sea ice cover for the thermodynamic phase of low‐level, mid‐level, and mid to low‐level clouds observed by CloudSat/CALIPSO over the Arctic and the Southern Ocean using an explainable machine learning technique. As expected, the cloud top temperature is found to be the most important parameter for determining cloud phase. The results show also a predictive power of sea salt and sea ice on the phase of low‐level clouds, while in mid‐level clouds dust shows predictive power. Over the Southern Ocean, strong zonal winds coincide with the aerosol distribution. While they can produce high mixing ratios of sea spray at lower levels, the strong zonal winds may prevent the pole‐ward transport of dust. Sea ice may prevent the release of sea salt aerosols and marine organic aerosols leading to higher liquid fractions in clouds over sea ice.
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