When a Hurricane Falls Apart: Identifying Factors That Lead to Rapid Weakening

Extensive work in recent decades has advanced our ability to forecast tropical cyclone (TC) track and intensity, yet the rate of improvement for intensity has been slower than that for track. Both external factors and internal processes affect TC intensity change, and more intense TCs often cause greater impacts, thus considerable effort has been directed toward studying TC intensification. Less research has examined the other end of the intensity change spectrum, that of weakening, as typical contributors include lower sea surface temperatures (SSTs), dry air, and increasing vertical wind shear. However, TCs can intensify in the presence of moderate shear and/or marginal SSTs, and TCs can weaken even in low shear environments.

To capture TC evolution during weakening over open ocean, this study explores spatial characteristics of satellite-derived cloud-top temperature and rainfall and evaluate how those characteristics evolve over time. In the North Atlantic (ATL), convection tends to become increasingly asymmetric as intensity decreases, yet eastern North Pacific (ENP) TCs are more likely to retain symmetry as overall convective strength wanes. These contrasting outcomes imply that there may be stronger influence from vertical wind shear in the ATL and stronger influence from thermodynamic factors such as stability in the ENP. To test this possibility, potential intensity (via tcpyPI, a Python package) as well as vertical wind shear are computed from storm-centered ERA5 reanalysis fields to evaluate the spatial patterns associated with weakening cases. These results should inform an improved understanding of the factors that contribute to TC weakening.