Structure Evolution of Intense Arctic Cyclones in August 2016

Arctic cyclones (ACs) are the major hazardous weather systems affecting the Arctic. Intense ACs activity induces strong near-surface wind, high ocean wave and heavy precipitation, causing dramatic environmental and socioeconomic impacts. Whether an AC can develop into an intense one is closely related to its structure evolution. Previous studies generally considered that intense summer ACs are characterized by a shift from a baroclinic structure to a barotropic structure around the maximum intensity time. This study investigates the structure evolution of two intense ACs in August 2016. Results indicate that both ACs exhibited similar structure evolution characteristics: ACs underwent two distinct structural transitions, in which a unique transitional structure is found to be between the baroclinic structure and the barotropic structure. During the developing stage, ACs featured an asymmetric baroclinic structure, with its central axis tilting towards cold anomalies. The relative vorticity was high along the warm and cold fronts, with the shallow cyclonic circulation in the lower and middle troposphere. During the mature stage, as the tropopause polar vortex (TPV) caught up to ACs, ACs experienced the first structural transition from the baroclinic structure to a transitional structure. With a tropopause fold descending down to 600hPa over the cyclone center, ACs developed into a deep cyclonic circulation extending from the troposphere to the lower stratosphere. While a warm core related to the TPV dominated in the lower stratosphere, ACs maintained the baroclinic structure in the troposphere, with the decreased tilt of the axis. During the decay stage, ACs underwent the second structural transition from the transitional structure to an axisymmetric barotropic structure, with a cold core throughout the troposphere and a warm core in the lower stratosphere. These results may have implications for advancing understanding of the structure evolution and intensification mechanism for intense summer ACs. 北极气旋（ACs）是影响北极地区的主要灾害性天气系统。强ACs活动会引起大风、大浪以及强降水，造成严重的环境和经济社会影响。一个AC能否发展为强AC，主要与其结构演变特征有关。以往的研究通常认为夏季强ACs在其最强时刻前后会出现由正压向斜压结构的转变。本文分析了2016年8月的两个强ACs个例的结构演变过程，发现两个ACs具有类似的结构演变特征：ACs经历了两次明显的结构转变，在正压结构和斜压结构之间存在一种过渡结构。在发展期，ACs为非对称的斜压结构，其中心轴线向冷异常一侧倾斜；相对涡度大值区位于暖锋和冷锋附近，在对流层中下层伴随着浅薄的气旋式环流。在成熟期，随着对流层顶极涡（TPV）追上ACs，ACs的结构开始发生第一次转变，从斜压结构转变为过渡结构。在ACs中心上空对流层顶折叠下沉至600hPa，ACs发展为一个从对流层至平流层低层的深厚气旋式环流。虽然平流层低层以与TPV有关的暖核为主，ACs在对流层依然维持斜压结构，其中心轴线倾斜程度减小。在减弱期，ACs的结构发生第二次转变，从过渡结构转变为轴对称的正压结构，伴随着对流层整层的冷核和平流层低层的暖核。本文的研究结果对理解夏季强ACs的结构及加强机制具有重要意义。