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Browsing PCIC Publications by Author "Andrews, Timothy"
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Item Extreme wet and dry conditions affected differently by greenhouse gases and aerosols(npj Climate and Atmospheric Science, 2019) Sillmann, Jana; Stjern, Camilla W.; Myhre, Gunnar; Samset, Bjørn H.; Hodnebrog, Øivind; Andrews, Timothy; Boucher, Olivier; Faluvegi, Gregory; Forster, Piers M.; Kasoar, Matthew R.; Kharin, Viatcheslav V.; Kirkevåg, Alf; Lamarque, Jean-Francois; Olivié, Dirk J. L.; Richardson, Thomas B.; Shindell, Drew; Takemura, Toshihiko; Voulgarakis, Apostolos; Zwiers, Francis W.Global warming due to greenhouse gases and atmospheric aerosols alter precipitation rates, but the influence on extreme precipitation by aerosols relative to greenhouse gases is still not well known. Here we use the simulations from the Precipitation Driver and Response Model Intercomparison Project that enable us to compare changes in mean and extreme precipitation due to greenhouse gases with those due to black carbon and sulfate aerosols, using indicators for dry extremes as well as for moderate and very extreme precipitation. Generally, we find that the more extreme a precipitation event is, the more pronounced is its response relative to global mean surface temperature change, both for aerosol and greenhouse gas changes. Black carbon (BC) stands out with distinct behavior and large differences between individual models. Dry days become more frequent with BC-induced warming compared to greenhouse gases, but so does the intensity and frequency of extreme precipitation. An increase in sulfate aerosols cools the surface and thereby the atmosphere, and thus induces a reduction in precipitation with a stronger effect on extreme than on mean precipitation. A better understanding and representation of these processes in models will provide knowledge for developing strategies for both climate change and air pollution mitigation.Item PDRMIP: A precipitation driver and response model intercomparison project—Protocol and preliminary results(Bulletin of the American Meteorological Society, 2017) Myhre, Gunnar; Forster, Piers M.; Samset, Bjørn H.; Hodnebrog, Øivind; Sillmann, Jana; Aalbergsjø, S. G.; Andrews, Timothy; Boucher, Olivier; Faluvegi, Gregory; Fläschner, D.; Iversen, T.; Kasoar, Matthew M.; Kharin, Viatcheslav V.; Kirkevåg, Alf; Lamarque, Jean Francois; Olivié, Dirk J. L.; Richardson, Thomas B.; Shindell, Drew; Shine, K. P.; Stjern, Camilla W.; Takemura, Toshihiko; Voulgarakis, Apostolos; Zwiers, Francis W.As the global temperature increases with changing climate, precipitation rates and patterns are affected through a wide range of physical mechanisms. The globally averaged intensity of extreme precipitation also changes more rapidly than the globally averaged precipitation rate. While some aspects of the regional variation in precipitation predicted by climate models appear robust, there is still a large degree of intermodel differences unaccounted for. Individual drivers of climate change initially alter the energy budget of the atmosphere, leading to distinct rapid adjustments involving changes in precipitation. Differences in how these rapid adjustment processes manifest themselves within models are likely to explain a large fraction of the present model spread and better quantifications are needed to improve precipitation predictions. Here, the authors introduce the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where a set of idealized experiments designed to understand the role of different climate forcing mechanisms were performed by a large set of climate models. PDRMIP focuses on understanding how precipitation changes relating to rapid adjustments and slower responses to climate forcings are represented across models. Initial results show that rapid adjustments account for large regional differences in hydrological sensitivity across multiple drivers. The PDRMIP results are expected to dramatically improve understanding of the causes of the present diversity in future climate projections.