Asymmetric Tropical Cyclone Structures and Processes in Reanalyses and Climate Models

While tropical cyclones (TCs) are axisymmetric vortices to first order, they often exhibit noteworthy structural asymmetries. These often result from a response to environmental vertical wind shear, which modulates the TC’s circulation, convection, and precipitation in a wavenumber-one pattern. Prior work has leveraged aircraft and radar observations, along with idealized modeling, to better understand the TC-wind shear relationship and its consequences for TC hazards. Concurrently, computational advances have allowed reanalyses and global climate models (GCMs) to represent TC structure, intensity, and climatology with higher fidelity.

Obstacles remain, however, particularly in climate modeling for TCs. Small-scale processes remain unresolved, and TC simulations depend strongly on model characteristics such as the dynamical core, resolution, physical parameterizations, and in the case of reanalyses, data assimilation techniques. In addition, prior work in this area has largely examined TCs from an axisymmetric perspective, neglecting structural asymmetries and the effects of wind shear. This work uses a blend of reanalyses and a high-resolution GCM, with grid spacings ranging from 0.25-0.5˚, to assess TC structures and processes relative to the deep-layer wind shear.

Individual TC snapshots are extracted from geopotential thickness and surface pressure fields using the TempestExtremes algorithm, then composited by TC intensity and wind shear magnitude. In qualitative agreement with observations and high-resolution modeling, the strongest tangential winds are usually found left of shear, while inner-core rainfall, ascent, vortex tilt, and low-level inflow are favored either directly downshear or in the downshear left quadrant. Thermodynamic asymmetries are also apparent, with anomalous low-level moisture right of shear, low-level warm temperatures in the upshear right quadrant (uptilt), and cloud properties suggestive of a realistic precipitation life cycle from growth to fallout. Quantitative diagnostic tools are then used to examine asymmetric processes in the inner core and outer rainband regions.