Composite Sharpening by Vortex Symmetrization and Normalization of Tropical Cyclones

Abstract. Cyclone composites are a powerful tool for investigating the mean characteristics of tropical and extratropical cyclones, offering insights into the mechanisms driving storm development. Traditional composite methods align cyclone centers to capture persistent patterns but they tend to smooth out small-scale features.

We introduce a novel compositing framework, the SYmmetrized-Normalized Cyclone (SyNC) compositing, designed to address the structural variety of tropical cyclones (TCs). This method symmetrizes storms to axisymmetric vortices and normalizes them according to their eyewall location and the size of the TC. By accurately detecting the eyewalls and the horizontal extents of TCs, the SyNC method enables detailed storm structural analysis. The method is applied to simulated TCs with the weather and climate model ICON, which show strong agreement with the observed wind–pressure relationships. ICON reveals the ability to simulate even most intense storms, while overestimating the frequency of major hurricanes. A large structural variability and asymmetries are found across all simulated storm intensities, agreeing with observations and emphasizing the importance of SyNC composites.

The vortex alignment of the SyNC framework successfully sharpens composite fields, preserving small-scale features such as super-gradient winds, subsidence within the eye, eyewall updrafts, and localized diabatic heating and cooling related to cloud microphysics. It also reduces within-group variance, thereby increasing statistical power and enabling the detection of differences between TC groups that would be missed using traditional center-based compositing. Limitations of the SyNC composites include reduced applicability during early storm stages, when tangential winds have not yet formed a Rankine-like vortex, and potential data extrapolation during normalization in small storms. Nonetheless, the method proves robust for weakly organized storms and is particularly beneficial for analyzing circulation-related and cloud microphysics-related fields. Overall, the SyNC compositing method provides a cyclone-relative framework that improves the accuracy of TC composite analysis, thereby facilitating the investigation and understanding of storm development.