The developed typhoons include the cloudless "eye" near the center and the ring-shaped cloud "eye wall cloud" that surrounds it.In a strong typhoon, another ring-shaped cloud "outer wall cloud" may form outside the center.In this case, the structure of the typhoon changes rapidly and the intensity changes dramatically, which makes it very difficult to predict the intensity of the typhoon.However, its formation mechanism has been regarded as one of the biggest mysteries in typhoon research.

　The research team developed the existing theory (Ekman theory) and discovered that there is a solution that a ring-shaped updraft (outer wall cloud) is formed when the wind field of a typhoon meets certain conditions. bottom.This ascending current is generated by the interaction between the flow near the ground (altitude several 100 m) driven by the friction of the ground surface and the flow in the lower troposphere (altitude several km).In Ekman theory, it is important that the ascending current and the horizontal flow distribution have a positive relationship, but in this study, we found that this relationship can be reversed in a developed typhoon.Once this relationship is reversed, the mechanism that worked to weaken the ascending current in Ekman theory changed completely and rather worked to strengthen the ascending current, creating a very strong ascending current in a short time, and a cloud was generated. Turns out to grow into a wall cloud.The predictions made by this theory are in agreement with past observations, and their usefulness was confirmed by simulations of multiple typhoons.

　With this result, there is a possibility that the accuracy of typhoon intensity forecast can be dramatically improved.Furthermore, it may be applicable to phenomena other than typhoons.

Paper information:[Journal of the Atmospheric Sciences] A Dynamical Mechanism for Secondary Eyewall Formation in Tropical Cyclones