Accurately forecasting whether and when a tropical cyclone (TC) will undergo rapid intensification is challenging. This study shows for the first time that knowing how quickly the wind speed of the TC circulation, or vortex, decays with height helps predict rapid intensification.
Figure 1. (a) Tangential wind-speed (Vt or wind swirling around the TC center) analysis at a height (z) of 3 km from TC-RADAR in Hurricane Florence at Category 4 intensity and (b) azimuthally averaged tangential wind speed at the same time. The radius of maximum wind speed (RMW) from the TC center is denoted by the cyan contour. Black contours (Vt decay) are tangential wind speed divided by its value at the RMW at 2 km above the surface. Both HOV24 and Dynamic HOV are shown.
Rapid intensification of TCs continues to be one of the most difficult problems for hurricane forecasters, with enormous implications if it occurs just before landfall. To gather clues to improve forecasts of rapid intensification, a large dataset of radar observations from Hurricane Hunter aircraft in TCs (TC-RADAR) developed by scientists at HRD was used to investigate relationships between how high the TC circulation reaches, intensity, and intensity change.
As TCs intensify, their circulations reach higher into the atmosphere. The height-intensity relationship is known to be present from the beginning of a TC’s lifetime until its peak strength. However, previous research had yet to determine if the height of the vortex (HOV), defined as the greatest height at which a threshold wind speed occurs, is important to future changes in TC intensity. Analyses of TC winds from Doppler radars on NOAA Hurricane Hunter aircraft were azimuthally averaged, a process in which the average wind velocity at each radius from the center of the TC and height (Fig. 1b) is calculated around the storm (Fig. 1a). Dynamic HOV (the height at which the wind speed decays to 40% of its value at 2 km height) was compared to intensity change between four TC intensity change groups (Fig. 2).
Figure 2. Scatter plot of the dynamic HOV (determined by 60% decay of Vt with respect to its value 2 km above the surface) and TC intensity change. The intensity at the analysis time (colorbar) as well as four intensity change groups (shaded background) are shown. Intensity change groups are rapidly intensifying (red), intensifying (orange), steady-state (gray), and weakening (blue) which are determined by changes in minimum sea level pressure during the following 24 hours. All of the TCs that intensified most rapidly (largest decrease in Pmin) have large Dynamic HOV. A hollow X surrounds the case of extreme rapid intensification in Hurricane Eta (2020).
Important Conclusions:
- All strong TCs are tall (orange and yellow dots in Fig. 2), but some weak ones (blue and purple dots in Fig. 2) also have tall circulations, which may help them grow strong.
- HOV as determined by the tallest observed 24 m s-1 (or about 54 mph) wind speed (HOV24; Fig. 1b) is strongly related to the present intensity of the TC.
- The favorability for a tall TC to intensify more quickly than a shallow TC is more evident in weak TCs. All of the TCs that intensified most rapidly have large Dynamic HOV (greater than 10 km in Fig. 2), meaning that a deep wind field was always present before a period of rapid intensification occurred.
- Vortex height decreases with increasing environmental vertical wind shear (VWS; the difference in the wind velocity at the bottom and top of a TC). The favorability for a tall vortex to strengthen more quickly than a shallow vortex was particularly apparent when the VWS was moderate (between about 9 and 21 kt), where the VWS is not weak enough to benefit TC development, nor strong enough to halt it.
The study can be found at https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2022GL101877. For more information, contact aoml.communications@noaa.gov. The research was supported by the Office of Naval Research Award award N000142012069.