理想热带气旋增强过程中的多尺度涡度收支特征分析

ANALYSIS OF MULTI-SCALE VORTICITY BUDGET CHARACTERISTICS DURING THE INTENSIFICATION OF AN IDEAL TROPICAL CYCLONE

关键词:  WRF模式  理想模型  热带气旋增强  多尺度  涡度收支

Key words:  WRF model  ideal model  tropical cyclone intensification  multi-scale  vorticity budget

摘要:

基于中尺度WRF模式，研究了无背景气流环境假设条件下理想热带气旋中低层大气多尺度涡旋运动的发展演变特征。精确的尺度分离是基于傅里叶变换实现的，且原始涡度场被划分为三个尺度范围：系统尺度(大于150 km)、中间尺度(50～150 km)、对流尺度(小于50 km)。研究结果表明：热带气旋的非轴对称本质主要是由于中间尺度和对流尺度上的运动造成的，且中间尺度涡度演变特征与热带气旋增强的阶段性有很好的对应关系，尤其是其快速增强阶段；全尺度涡度收支特征主要表现为两两抵消效应：STR/HAD和TIL/VAD，且前者的净贡献明显强于后者；系统尺度涡度收支特征与全尺度基本一致，但中间尺度涡度收支表现出明显不同特征：积分70 h之前，各收支项均表现出了与系统尺度相反的贡献，之后，各收支项的符号转变与系统尺度相同，但收支项净贡献明显大于系统尺度。总的来说，水平分辨率5 km下模拟的理想热带气旋的快速增强主要与中间尺度上STR/HAD净贡献的快速增长有关。此外，进一步研究了特定时段中间尺度涡度收支项的空间演变，结果表明：在热带气旋增强阶段，各收支项均在涡旋内核的轴对称化中有不可忽视的作用，且TIL在中心负涡度异常衰退、最终变为正涡度过程中起主导作用。

Abstract:

Based on the mesoscale WRF model, the present study analyzes the development and evolution characteristics of multi-scale vortices in the middle and lower atmospheres of an ideal tropical cyclone under the assumption of no background airflow environment. The precise scale separation is based on Fourier transform, and the original vorticity field is divided into three scale ranges including system scale (greater than 150 km), intermediate scale (50～150 km), and convective scale (less than 50 km). The results show that the non-axisymmetric structure of the tropical cyclone is mainly caused by the movement on intermediate and convective scales. Moreover, the evolution characteristics of the intermediate-scale vorticity correspond well with the phases of tropical cyclone intensification, especially the rapid intensification phase. The full-scale vorticity budget feature is mainly manifested as offset effects, i.e., STR/HAD and TIL/VAD, and the net contribution of the former is significantly stronger than that of the latter. The features of the system-scale vorticity budget and the full-scale vorticity budget are basically the same, but the intermediate-scale vorticity budget shows distinctive characteristics: before the integration of 70h, the contribution of each budget item is opposite to that on the system scale, and afterwards, the sign transition of the items is the same as that on the system scale; however, the net contribution of the income and expenditure items is significantly larger than that on the system scale. In general, the rapid increase of the simulated ideal tropical cyclone with the horizontal resolution lower than 5km is mainly related to the rapid increase of the net contribution of STR/HAD on the intermediate scale. Furthermore, we further study the spatial evolution of the intermediate-scale vorticity budget items during a specific period. The results show that during the intensification stage of the tropical cyclone, each budget term plays a non-negligible role in the axisymmetricalization of the vortex core, and TIL plays a leading role when the abnormally negative vorticity in the center decays and finally becomes a positive vorticity.