The co-occurrence of wind and rainfall extremes can yield larger impacts than when either hazard occurs in isolation. This study assesses compound extremes produced by Extra-tropical cyclones (ETCs) during winter from two perspectives. Firstly, we assess ETCs with extreme footprints of wind and rainfall; footprint severity is measured using the wind severity index (WSI) and rain severity index (RSI) which account for the intensity, duration, and area of either hazard. Secondly, we assess local co-occurrences of 6-hourly wind and rainfall extremes within ETCs. We quantify the likelihood of compound extremes in these two perspectives and characterise a number of their drivers (jet stream, cyclone tracks, and fronts) in control (1981–2000) and future (2060–2081, RCP8.5) climate simulations from a 12-member ensemble of local convection-permitting 2.2 km climate projections over the UK and Ireland. Simulations indicate an increased probability of ETCs producing extremely severe WSI and RSI in the same storm in the future, occurring 3.6 times more frequently (every 5 years compared to every 18 years in the control). This frequency increase is mainly driven by increased rainfall intensities, pointing to a predominantly thermodynamic driver. However, future winds also increase alongside a strengthened jet stream, while a southward displaced jet and cyclone track in these events leads to a dynamically-enhanced increase in temperature. This intensifies rainfall in line with Clausius-Clapeyron, and potentially wind speeds due to additional latent heat energy. Future simulations also indicate an increase in the land area experiencing locally co-occurring wind and rainfall extremes; largely explained by increased rainfall within warm and cold fronts, although the relative increase is highest near cold fronts suggesting increased convective activity. These locally co-occurring extremes are more likely in storms with severe WSI and RSI, but not exclusively so as local co-occurrence requires the coincidence of separate drivers within ETCs. Overall, our results reveal many contributing factors to compound wind and rainfall extremes and their future changes. Further work is needed to understand the uncertainty in the future response by sampling additional climate models.

中文翻译：

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复合风雨极端事件：英国和爱尔兰的驱动因素和未来变化


风和降雨极端事件同时发生比任一灾害单独发生时产生的影响更大。本研究从两个角度评估冬季温带气旋（ETC）产生的复合极端事件。首先，我们评估具有极端风和降雨足迹的 ETC；足迹严重程度是使用风严重指数 (WSI) 和雨严重指数 (RSI) 来衡量的，该指数考虑了任一危险的强度、持续时间和面积。其次，我们评估 ETC 内 6 小时风雨极端事件的局部共现情况。我们从这两个角度量化了复合极端事件的可能性，并从控制（1981-2000）和未来（2060-2081，RCP8.5）气候模拟中描述了它们的许多驱动因素（急流、气旋轨迹和锋面）。由 12 名成员组成的集合，对英国和爱尔兰当地允许对流的 2.2 公里气候进行预测。模拟表明，未来 ETC 在同一场风暴中产生极其严重的 WSI 和 RSI 的可能性会增加，发生频率增加 3.6 倍（每 5 年一次，而对照中每 18 年一次）。这种频率的增加主要是由降雨强度增加驱动的，这表明主要是热力学驱动因素。然而，未来的风也会随着急流的增强而增加，而在这些事件中向南移动的急流和气旋路径会导致温度动态增强。这会加剧降雨，符合克劳修斯-克拉佩龙定律，并且由于额外的潜热能而可能导致风速增强。 未来的模拟还表明，局部同时发生的风雨极端天气的陆地面积将会增加；主要原因是暖锋和冷锋内降雨量增加，尽管冷锋附近的相对增幅最高，表明对流活动增加。这些局部同时发生的极端事件更有可能出现在具有严重 WSI 和 RSI 的风暴中，但并非完全如此，因为局部同时发生需要 ETC 内不同驱动因素的一致。总体而言，我们的结果揭示了造成复合风雨极端事件及其未来变化的许多因素。需要进一步开展工作，通过对其他气候模型进行采样来了解未来响应的不确定性。