The outer-core size and intensity of tropical cyclones may be affected by environmental helicity

It is crucial to evaluate the destructive potential of tropical cyclones (TCs), by knowing their outer-core size. It is often defined as the radius gale-force wind (R17), although it is difficult to accurately forecast R17 due to our incomplete understanding about the physical mechanism that determines it. However, it is generally agreed that the TC’s outer-core size is largely controlled by its outer spiral rain bands. Convection is a natural phenomenon that occurs in the outer-core of TCs. This makes it possible to maintain and expand their outer-core size.

Researchers from Nanjing University in China and University of Reading (UK) studied the statistical relationships between dynamic and environmental factors and TC outer core size in the South China Sea. They show strong evidence that the lowest-level environmental helicity, regardless of whether the TCs originate from the western Pacific Ocean or are local, has the greatest impact on TC outer core size.

Helicity is a predictor of intense convections by its kinematics. Positive helicity is when the rotation-axis is in the same direction that the flow. This means that the wind rotates in the direction of rotation. Positive helicity is typically produced by a clockwise veering horograph. Scientists also observe that positive helicity may be beneficial for the organization or persistence of intense convections.

This study’s composite analyses show that environmental helicity could have a significant effect on convective activities in the TC outer-core area. Largely positive environmental helicity favors the development of intense and persistent convection, strong inflow, large amounts of angular momentum import and large R17. These results were published in recent months. Letters in Oceanic and Atmospheric Science.

This study suggests that the environment’s dynamic constraints, such as environmental helicity, could be important factors in the maintenance and changing of TC outer-core sizes.

“But, whether such a conclusion can generally be applied needs to be carefully investigated in other basins using both observational as well as advanced numerical simulations. Associate Professor Chu, who is the corresponding author of this research paper, adds that the importance of the environmental helicity to the organization and intensity TC convective activity, as well as the inner-core size of TC, leads to the hypothesis of a link between the two.