Asymmetric Structure of Tropical Cyclones in the Community Atmosphere Model 5 (CAM5)

Tropical cyclones (TCs) are among the most destructive phenomena in nature. Global climate models (GCMs) are an important tool for projecting future activity. TCs are usually axisymmetric vortices to the first order, but there are environmental factors that can cause asymmetric structures. One of the most influential is vertical wind shear (VWS). The initial tilt of the vortex by VWS causes other processes to become unbalanced, leading to asymmetries in the circulation and thermodynamic structure. This is a key factor in forecasting future TC track, intensity, and impacts. However, GCMs have had biases in depiction of TCs due to a few limiting factors such as spatial resolution. Therefore, we examined the modeled structures and evolution of six storms in a variable-resolution GCM, CAM5, to assess how asymmetric structure is captured. Three of these storms were from the North Atlantic (NATL) at 0.25° grid spacing and three were from the Northwest Pacific (WPAC) at 1° grid spacing. Each analyzed storm began at a similar intensity in an environment with weak VWS, less than 5 m/s. Then, the storms encountered environments of strengthening shear where asymmetries were expected to amplify. We found that CAM5 was able to resolve many of these asymmetric structures and their evolutions in the TCs, especially at 0.25° resolution. The model was able to produce shear-relative extremes in precipitation, inflow, and vertical motion in locations largely consistent with theory, observations, and high-resolution models. This analysis supports high-resolution GCMs as viable tools for projecting future TC behavior in a changing climate.