理想模拟的温带气旋中冷暖输送带特征分析

Characteristic Analysis of Cold and Warm Conveyor Belts in an Idealized Extratropical Cyclone Simulation

本文利用WRF理想斜压波模式模拟了理想湿大气中温带气旋的快速发展过程,采用拉格朗日轨迹筛选方法识别了气旋内部冷、暖输送带结构,分析了沿着输送带轨迹的物理量演变特征,探究了输送带气流对气旋降水结构的影响。本文在再现前人研究结论的基础上,发现了更精细的输送带结构特征,尤其是对冷输送带特征有了进一步认识。研究表明,根据相对气旋中心运动特征可将暖输送带划分为“前倾上升”和“后倾上升”两支。两支气流均起始于对流层低层冷锋前暖区内,旋转上升到对流层中高层出流区后分别向气旋中心的下游和上游运动,并在中高层产生负位涡扰动,促进高空系统发展。同时,暖输送带向上层输送水汽,影响锋面附近降水中心的形成和维持。在对冷输送带的研究中,本文证实了前人研究描述的上升类和低层运动类特征,而且发现其可以更精细地呈现出四支气流结构。“前倾上升”和“后倾上升”两支气流的初始位置靠近暖锋,上升运动到对流层中层后分别向气旋中心的下游和上游运动,利于促进暖锋附近降水形成;而“环气旋前倾”和“环气旋后倾”两支气流始终在对流层低层运动,初始远离暖锋朝向气旋中心运动,水汽含量增加,随后环绕气旋中心缓慢上升运动到气旋西侧后分别向气旋下游和上游下沉,这两支气流导致了气旋西北侧弱降水的发生。

An idealized moist baroclinic wave simulation was performed to describe the rapid development of an extratropical cyclone. Warm and cold conveyor belts were then identified using the Lagrangian trajectory selection method, and physical quantities along their trajectories were diagnosed, followed by the analysis of their impacts on the precipitation. The study similarly presents previous conclusions of conveyor belts and finds some more refined structures,especially for cold conveyor belts. Results reveal that warm conveyor belts could generally split into “forward-sloping ascent” and “rearward-sloping ascent” branches. They originate in the warm sector ahead of the cold front and low level and move respectively forward and rearward relative to the cyclone center when they spiral upward to the upper and middle level, generating a negative potential vorticity disturbance upon the outflow region and promoting the development of the upper system. They also transport abundant moisture upward and significantly influence the formation and maintenance of the precipitation extremum around the front. Cold conveyor belts are confirmed to represent two branches of ascent and the remaining in the lower level as described by previous researches, which could then be classified finely into four branches. “Forward-sloping ascent” and “rearward-sloping ascent” branches originate near the warm front and curve separately forward and rearward relative to the cyclone center when rising upon the middle level,benefiting the rainfalls around the warm front. Meanwhile, branches of “ wrapping around the cyclone and forwardsloping” and “wrapping around the cyclone and rearward-sloping” always stay in the lower level and move from the east of the surface cyclone far from the warm front toward the cyclone center whenever their vapor content increases. They then respectively descend anticlockwise and clockwise to the downstream and upstream of the cyclone after traveling slowly upward west of the cyclone around the center, inducing the little precipitation northwest of the cyclone.