Abstract Hurricane Ophelia was a category 3 hurricane which underwent extratropical transition and made landfall in Europe as an exceptionally strong post-tropical cyclone in October 2017. In Ireland, Ophelia was the worst storm in 50 years and resulted in significant damage and even loss of life. In this study, the different physical processes affecting Ophelia’s transformation from a hurricane to a mid-latitude cyclone are studied. For this purpose, we have developed software that uses OpenIFS model output and a system consisting of a generalized omega equation and vorticity equation. By using these two equations, the atmospheric vertical motion and vorticity tendency are separated into the contributions from different physical processes: vorticity advection, thermal advection, friction, diabatic heating, and the imbalance between the temperature and vorticity tendencies. Vorticity advection, which is often considered an important forcing for the development of mid-latitude cyclones, is shown to play a small role in the re-intensification of the low-level cyclone. Instead, our results show that the adiabatic upper-level forcing was strongly amplified by moist processes, and thus, the diabatic heating was the dominant forcing in both the tropical and extratropical phases of Ophelia. Furthermore, we calculated in more detail the diabatic heating contributions from different model parameterizations. We find that the temperature tendency due to the convection scheme was the dominant forcing for the vorticity tendency during the hurricane phase, but as Ophelia transformed into a mid-latitude cyclone, the microphysics temperature tendency, presumably dominated by large-scale condensation, gradually increased becoming the dominant forcing once the transition was complete. Temperature tendencies caused by other diabatic processes, such as radiation, surface processes, vertical diffusion, and gravity wave drag, were found to be negligible in the development of the storm.

中文翻译：

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用广义欧米茄方程和涡度方程诊断的飓风奥菲利亚 (2017) 的温带过渡

摘要 飓风奥菲莉亚（Hurricane Ophelia）是 3 级飓风，经历温带过渡，于 2017 年 10 月作为特强热带后气旋登陆欧洲。在爱尔兰，奥菲莉亚是 50 年来最严重的风暴，造成重大破坏甚至人员伤亡。 . 在这项研究中，研究了影响奥菲莉亚从飓风到中纬度气旋转变的不同物理过程。为此，我们开发了使用 OpenIFS 模型输出和由广义欧米茄方程和涡度方程组成的系统的软件。通过使用这两个方程，大气垂直运动和涡度趋势被分离为来自不同物理过程的贡献：涡度平流、热平流、摩擦、非绝热加热、以及温度和涡度趋势之间的不平衡。涡度平流通常被认为是中纬度气旋发展的重要推动力，但在低层气旋的重新强化中只发挥了很小的作用。相反，我们的结果表明，绝热上层强迫被潮湿过程强烈放大，因此，非绝热加热是奥菲利亚热带和温带相的主要强迫。此外，我们更详细地计算了来自不同模型参数化的非绝热加热贡献。我们发现对流方案引起的温度趋势是飓风阶段涡度趋势的主导作用力，但随着奥菲利亚转变为中纬度气旋，微物理温度趋势，大概以大规模凝结为主，一旦过渡完成，逐渐增加成为主导强迫。其他非绝热过程（如辐射、表面过程、垂直扩散和重力波阻力）引起的温度趋势在风暴的发展过程中可以忽略不计。