Review paper on the impact of vertical wind shear on tropical cyclones released online in The Journal of the Atmospheric Sciences – Hurricane Research Division

Vertical wind shear, which is typically defined as the difference in the speed and direction of wind in the bottom and top of the tropical cyclone, significantly impacts the structures and intensities of tropical cyclones. This review paper summarizes the findings of previous studies that have looked at the interactions between tropical cyclones and wind shear. This paper also provides recommendations for how future work can help advance our understanding of the impacts of shear on tropical cyclones.

Summary: 

The vertical wind shear within a tropical cyclone’s environment is known to have a significant impact on the structure of the tropical cyclone’s swirling flow and thunderstorm activity. For example, vertical wind shear tends to tilt the upper portion of the tropical cyclone circulation away from the lower part of the tropical cyclone. This is significant because a tilted tropical cyclone is associated with thunderstorm activity that is not the same around the center, making the system vulnerable to dry air from the environment. Both of these characteristics are unfavorable for the tropical cyclone to strengthen.

The response of the tropical cyclone to wind shear typically depends on the strength of the shear. When the shear is weak, the tropical cyclone is often able to overcome the shear’s negative influences; however, when the shear is too strong, tropical cyclone development can cease altogether or the system may weaken, potentially to its demise. When the shear is neither weak or strong, what we call “moderate,” predicting the future strength of the tropical cyclone becomes especially challenging. This is because a diversity of outcomes are possible, ranging from the tropical cyclone weakening to strengthening rapidly. 

Previous studies have offered multiple explanations for how intensifying tropical cyclones in moderate shear are able to overcome its detrimental effects. The proposed pathways toward intensification in moderate shear are largely based on processes that reduce the tilt of the tropical cyclone’s circulation. A common thought is that the location and vigor of thunderstorm activity plays a key role. In some cases, if thunderstorm activity is strong and persistent enough, a new tropical cyclone circulation can form beneath the upper portion of the tropical cyclone’s circulation, ultimately reducing the tropical cyclone’s vertical tilt. Additionally, intense thunderstorm activity is associated with air flow that spreads outward from the tops of the thunderstorms in the upper atmosphere. This outward spreading flow can act to push back against the wind shear and create a small pocket around the tropical cyclone that contains weaker shear than the tropical cyclone originally experienced, providing a more favorable local environment for additional thunderstorm activity and tropical cyclone development. Regardless of the precise pathway in which tilt is reduced, small values of vortex tilt, humid environments, and a warm sea surface are favorable for the tropical cyclone to strengthen despite moderate shear, as these conditions promote a greater areal coverage and strength of thunderstorm activity near the tropical cyclone’s center. These characteristics are illustrated in Figure 1.

Important Conclusions:

• This review paper summarizes our understanding of how environmental vertical wind shear impacts the structure and strength of a tropical cyclone.
• This paper offers the following recommendations for how future studies can help improve our understanding of the impacts of wind shear on tropical cyclones:
  • Develop a unified definition for vertical wind shear that can be applied in both an operational and research setting.
  • Refine how we observe tropical cyclones to better see how wind shear interacts with tropical cyclones.
  • Conduct additional research to better understand how the structure and strength of a tropical cyclone influences the impacts of wind shear and the pathways through which a sheared tropical cyclone can strengthen.

For more information, contact AOML.communications@noaa.gov. The review paper can be found at https://doi.org/10.1175/JAS-D-23-0022.1.