Typhoon landing is often accompanied by strong wind, rainstorm and storm surge, causing great disasters to coastal areas (as shown in Figure 1), and strong wind, rainstorm and typhoon spiral rain are closely related. At present, typhoon intensity and precipitation are still difficult to predict in the world. An important reason is that the structure and evolution characteristics of spiral rain belt are not well understood. With the rapid development of computer and the continuous improvement of numerical model, using numerical test to understand typhoon structure has become an important research means, which effectively makes up for the serious shortage of observation data. In recent years, in order to deeply understand the convective scale structure and evolution characteristics of the Taiwan storm zone, using the Tianhe 2 supercomputer of Guangzhou Supercomputing Center and large eddy simulation technology, the typhoon team of Professor Wu Liguang of the Academy of Atmospheric Sciences of Fudan University has completed a series of typhoon numerical simulation tests. The following mainly introduces some characteristics of convective scale structure in typhoon main rain belt found in large eddy simulation test.
Figure 1 street view of super typhoon “weimasson” landing in Hainan and its attack on Hainan (picture from the Internet)
Fig. 2 is a conceptual diagram of the convective structure in the middle reaches of the main rain belt obtained from the numerical test data. A new discovery is that there is a high-level inflow flow in the middle and downstream areas of the main rain belt, which carries dry air between the eye wall and the overturned updraft. As shown in Figure 2, there is strong inflow from the outside 10 km above the rain belt and sinks outside the eye wall (blue curve arrow), which is combined and strengthened with the downdraft induced by eye wall convection to form a strong downdraft at the inner boundary of the rain belt. The intrusion of this air flow not only affects the height of the main rain belt and the development of convection, but also may further affect the convective structure of the typhoon eyewall.
Another finding is that there is a small-scale disturbance structure in the overturned updraft of the main rain belt, which is mainly located inside the outward inclined high radar echo. By removing the average vertical wind field and horizontal wind field in the 3km region, we obtain the small-scale disturbed circulation in the convective structure (Fig. 3), in which the warm (cold) color represents the ascending (descending) airflow and the streamline represents the small-scale disturbed airflow. It can be seen that there are three small-scale disturbance circulations with different heights in the average updraft, which are arranged radially outward. Such small-scale disturbance circulations can fully mix the heat, momentum and water vapor in the rain belt.
The paper has been accepted and published by advances in Atmospheric Sciences in 2020. The research has been jointly funded by the National Natural Science Foundation of China and the national key basic research and development plan (973 Plan). Jiang Yue is the first author and Professor Wu ligung is the corresponding author.
Fig. 2 Schematic diagram of convection structure in the middle reaches of the main rain belt. The dotted line represents the outward inclined high radar echo area, the red ellipse represents the large value of secondary horizontal wind speed, the blue curve arrow represents the inner downdraft and outer low-level downdraft, the red curve arrow represents the boundary layer inflow and overturned updraft, and the solid and hollow arrows represent the small-scale disturbed circulation existing in the overturned updraft and boundary layer inflow.
Fig. 3 three dimensional disturbed wind field (streamline, unit: m s-1) in convection in the middle reaches of the main rain belt. Among them, the vertical section is the sliding average vertical movement through the 3km area, the warm (cold) color represents the disturbance ascending (sinking) movement, and the background warm color represents the strong updraft of the rain belt.
Jiang, Y., L. G. Wu, H. K. Zhao, X. Y. Zhou, and Q.Y. Liu, 2020: Azimuthal variations of the convective-scale structure in a simulated tropical cyclone principalrainband. Adv. Atmos.Sci., 37(11), https://doi.org/10.1007/s00376-020-9248-x.
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