子計畫三：人為溫室氣體排放與暖化作用對極端天氣系統變化影響的偵測與歸因

Whether or not we can found a distinct change in the recent weather and climate extremes that is detectable from natural variability is a rather controversial research topic. Even if the change is detectable, whether it is due to the anthropogenic climate change can still be in debate. Our project aimed at using probabilistic event attribution framework to study anthropogenic impact on recent weather extremes (e.g. tropical cyclone). Due to the model capability issue and limitation on computational resources, the attribution studies on weather extreme failed to provide quantitative assessment for the impact on tropical cyclone. We proposed to use cloud-resolving model that can properly capture the tropical cyclone evolution and also design unique numerical experiments to make the probabilistic estimate of human contribution possible. We estimated the anthropogenic impact from natural variability of the climate system using the CMIP5 climate model and evaluate the reliability of cloud-resolving model used in the study, especially whether it can simulate the rainfall extreme associated with typhoon. The large ensemble members of simulation for event attribution are generated from random perturbation added to the initial condition. They are required to make proper probabilistic sample and uncertainty assessment. By applying the probabilistic event attribution statistics to the ensemble simulations of 2009 typhoon Morokat that brought record breaking rainfall and caused huge damage and casualty, we found that while the anthropogenic impact did not change the typhoon track systematically, there is a significant increase of 5% on the intensity index of tropical cyclone. More importantly, our result suggests that there is a 10-15% increase in the risk for tropical cyclone rainfall extremes when anthropogenic forcing included in the Typhoon Morakot simulation. The increase is not only supported by the more abundance of water vapor. The additional dynamical impact from the enhancement of ascending motion corresponded well to the location of rainfall extreme increases.