Evolution of Dynamic and Thermodynamic Structures before and during Rapid Intensification of Tropical Cyclones: Sensitivity to Vertical Wind Shear

This study explores the spatial and temporal changes in tropical cyclone (TC) thermodynamic and dynamic structures before, near, and during rapid intensification (RI) under different vertical wind shear conditions through four sets of convection-permitting ensemble simulations. A composite analysis of TC structural evolution is performed by matching the RI onset time of each member. Without background flow, the axisymmetric TC undergoes a gradual strengthening of the inner-core vorticity and warm core throughout the simulation. In the presence of moderate environmental shear (5–6 m s−1), both the location and magnitude of the asymmetries in boundary layer radial flow, relative humidity, and vertical motion evolve with the tilt vector throughout the simulation. A budget analysis indicates that tilting is crucial to maintaining the midlevel vortex while stretching and vertical advection are responsible for the upper-level vorticity generation before RI when strong asymmetries arise. Two warm anomalies are observed before the RI onset when the vortex column is tilted. When approaching the RI onset, these two warm anomalies gradually merge into one. Overall, the most symmetric vortex structure is found near the RI onset. Moderately sheared TCs experience an adjustment period from a highly asymmetric structure with updrafts concentrated at the down-tilt side before RI to a more axisymmetric structure during RI as the eyewall updrafts develop. This adjustment period near the RI onset, however, is found to be the least active period for deep convection. TC development under a smaller environmental shear (2.5 m s−1) condition displays an intermediate evolution between ensemble experiments with no background flow and with moderate shear (5–6 m s−1).

本研究通过四组允许对流的集合模拟，探讨了在不同垂直风切变条件下，热带气旋（TC）在快速增强（RI）之前、临近和期间的热力和动力结构的时空变化。通过匹配每个成员的RI起始时间，对TC结构演变进行了合成分析。在无背景气流的情况下，轴对称的TC在整个模拟过程中内核涡度和暖心逐渐增强。在中等环境切变（5 - 6米/秒）存在的情况下，边界层径向流、相对湿度和垂直运动的不对称性的位置和强度在整个模拟过程中随着倾斜矢量而演变。收支分析表明，当出现强烈不对称性时，倾斜对于维持中层涡旋至关重要，而拉伸和垂直平流则是RI之前高层涡旋产生的原因。当涡旋柱倾斜时，在RI开始之前观测到两个暖异常。当接近RI开始时，这两个暖异常逐渐合并为一个。总体而言，在RI开始附近发现最对称的涡旋结构。中等切变的TC经历一个调整期，从RI之前上升气流集中在向下倾斜一侧的高度不对称结构，到RI期间随着眼壁上升气流发展而变为更轴对称的结构。然而，在RI开始附近的这个调整期被发现是深对流最不活跃的时期。在较小环境切变（2.5米/秒）条件下的TC发展，在无背景气流和中等切变（5 - 6米/秒）的集合实验之间呈现出一种中间演变。