Study on the relationship between vortex tilt and intensification released online in Monthly Weather Review – Hurricane Research Division

In many tropical cyclones, the center of circulation in the lower and middle levels of the atmosphere are not in the same location, what we call tilt. Using a recently developed database of wind analyses from Doppler radars on Hurricane Hunter aircraft, this study shows that the distance between the circulation centers can be a helpful predictor of future intensity changes, especially in weak systems like tropical depressions or storms. This is the first study to determine the relationship between tropical cyclone intensity change and the tilt of the tropical cyclone circulation in weak systems using a multi-case observational data set.

Previous research has shown that relatively weak tropical cyclones tend to have circulations that are vertically tilted, meaning that the center of circulation is not in the same place in the lower and middle levels of the atmosphere; however, relatively strong tropical cyclones, such as those of hurricane intensity, have more aligned circulations. This suggests that acquiring a nearly aligned circulation is a key step for tropical cyclones to become intense. Despite this knowledge, it was not clear whether vertical alignment precedes an increase in the system’s intensity or whether alignment is the result of that increase. The majority of previous work exploring the relationship between intensity change and tilt used computer models and suggests that an aligned circulation is important for rapid intensification to occur. Nevertheless, due to the lack of studies of weak tropical cyclones with data from real systems, the relationship between intensity change and tilt in nature had not been established.

This study aims to address this gap in our understanding by exploring a recently developed database of over 1,100 radar analyses of tropical cyclones collected by Hurricane Hunter Aircraft during NOAA’s annual Hurricane Field Program, referred to as the Tropical Cyclone Radar Archive of Doppler Analyses with Recentering (TC-RADAR). Because the relationship between alignment and intensification might be different for weak systems than for strong ones that have already intensified, weak tropical cyclones are placed into one of two groups (“small tilt” and “large tilt”) based on how tilted the lower and middle circulations were. The thunderstorm activity, environmental conditions (such as the background wind pattern, temperature, and humidity throughout the depth of the atmosphere, as well as the sea surface temperature), and intensity change rates of tropical cyclones in these two groups were compared (Fig. 1). This comparison allows for a better understanding of how tropical cyclone tilt is related to future intensity change.

 Important Conclusions:

• In weak tropical cyclones, intensity change is closely related to how tilted the circulation is.  Weak, small-tilt systems intensify twice as fast as large-tilt ones. This relationship does not hold in strong tropical cyclones.
• How quickly weak tropical cyclones intensify depends on both the tilt of the tropical cyclone circulation and the favorability of the tropical cyclone’s environment. The storms that intensified the most rapidly were consistently in the small-tilt group and were typically in environments favorable for intensification. Thus, aircraft observations of the tropical cyclone tilt can provide helpful information for intensity forecasts.
• The degree of tilt of the tropical cyclone circulation is strongly related to the pattern of thunderstorm activity, with the strongest thunderstorm activity found near the circulation center in the middle portion of the atmosphere (Fig. 1). Additionally, small-tilt tropical cyclones have a greater coverage of thunderstorms located closer to the center of the storm in the lower atmosphere (Fig. 1a) than large-tilt ones (Fig. 1b), which is a favorable pattern for intensification.

For more information, contact AOML.communications@noaa.gov. The study can be found at https://doi.org/10.1175/MWR-D-23-0089.1. his work was supported by Office of Naval Research award number
N00014-20-1-2057 and National Science Foundation award number 2241605.