Asymmetric structure is common in tropical cyclones (TCS), often significantly influencing the distribution of TC-related hazards including wind, rainfall, and storm surge relative to the storm's…

Asymmetric structure is common in tropical cyclones (TCS), often significantly influencing the distribution of TC-related hazards including wind, rainfall, and storm surge relative to the storm's center. Climate models have improved substantially in recent years in capturing TC climatology and some aspects of axisymmetric TC structure. However, a fully reliable simulation of TC structure is dependent on resolving storm asymmetries, such as the vortex tilt associated with environmental vertical wind shear (VWS), and convective processes in TC rainbands. Existing limitations in model resolution, along with dependence on parameterized boundary layer and precipitation processes, cast doubt on the ability of climate models to reliably simulate these structural asymmetries. Observations and mesoscale modeling have aided our process-level understanding of TC asymmetry in recent years, which now offers an opportunity to assess and improve upon these processes in climate models. This project addresses the following scientific questions: 1) how accurately do climate models simulate asymmetries in TC kinematic and thermodynamic structure compared to observations? 2) how do model factors such as boundary layer, convection, and microphysics parameterizations, or resolution, contribute to these biases in TC structure? 3) will TC structure, including asymmetry and interactions with VWS, change in a warming climate? We will first produce process-oriented diagnostics (PODS) to describe TC asymmetries relative to both the VWS and storm motion, and apply them to the highresmip models within CMIP6. The project will build upon PODS recently developed by the PIS to describe vwsrelative asymmetries in precipitation, kinematic, and thermodynamic fields in global reanalyses. We will leverage airborne observations of TC structures, primarily from noaas hurricane field program, for comparison. Following this initial evaluation of TC asymmetry in a broad set of models, we will work with model data produced by gfdl and other groups to evaluate the relative importance of various model characteristics on the simulated TC structural bias. This includes resolution and parameterized convective and boundary layer physics. The analysis will provide a more controlled view of dynamic and moist convection processes simulated by these models, which may also inform parameterizations for other types of convective systems. Our diagnostics will then be applied to warming scenario simulations from highresmip to reveal possible changes in TC structure in a warming climate. Through this project, we will provide developers with paths for improving model fidelity and credibility. Our PODS can then be applied to future generations of climate models, such as the upcoming HIGHRESMIPV2 ensemble. This project addresses two key issues prioritized by the mapp program. First, IT will advance our ability to simulate tropical cyclones, both in terms of mesoscale processes relevant to TC structure and the spatial distribution of extreme wind and rainfall, and more broadly, for climate prediction related to TC behavior. Second, our detailed analysis of TC precipitation will help to address and improve biases in extreme rainfall related to convective and cloud parameterizations. In performing this research, we aim to better constrain future projections of TC activity and their associated hazards, informing decision-makers responsible for climate adaptation, resilience, and mitigation as coastal and inland population and infrastructure continue to expand.