[Paper Review] An Open-Source, Physics-Based, Tropical Cyclone Downscaling Model with Intensity-Dependent Steering

Jonathan Lin, Raphaël Rousseau‐Rizzi+2 more|arXiv (Cornell University)|Feb 19, 2023

Tropical and Extratropical Cyclones ResearchEarth and Planetary Sciences3 citations

TL;DR

This paper presents an open-source, physics-based tropical cyclone downscaling model that simulates large-scale tropical cyclone climatology using intensity-dependent steering, random genesis, and a non-linear intensity model forced by ERA5 reanalysis data. The model successfully reproduces global and basin-specific tropical cyclone climatology, including seasonal cycles, track density, intensity distributions, and return period curves for landfall intensity, with improved regional hazard representation due to intensity-dependent steering.

ABSTRACT

An open-source, physics-based tropical cyclone downscaling model is developed, in order to generate a large climatology of tropical cyclones. The model is composed of three primary components: (1) a random seeding process that determines genesis, (2) an intensity-dependent beta-advection model that determines the track, and (3) a non-linear differential equation set that determines the intensification rate. The model is entirely forced by the large-scale environment. Downscaling ERA5 reanalysis data shows that the model is generally able to reproduce observed tropical cyclone climatology, such as the global seasonal cycle, genesis locations, track density, and lifetime maximum intensity distributions. Inter-annual variability in tropical cyclone count and power-dissipation is also well captured, on both basin-wide and global scales. Regional tropical cyclone hazard estimated by this model is also analyzed using return period maps and curves. In particular, the model is able to reasonably capture the observed return period curves of landfall intensity in various sub-basins around the globe. The incorporation of an intensity-dependent steering flow is shown to lead to regionally dependent changes in power dissipation and return periods. Advantages and disadvantages of this model, compared to other downscaling models, are also discussed.

Motivation & Objective

• To develop a computationally efficient, open-source, physics-based model for downscaling tropical cyclone activity across global basins.
• To improve regional tropical cyclone hazard estimation by incorporating intensity-dependent steering in track simulation.
• To enable robust sampling of rare, intense tropical cyclones for risk assessment by generating large synthetic event sets.
• To ensure transparency and reproducibility by open-sourcing the model and providing detailed parameterization and code.
• To validate the model against observed TC climatology and return period statistics across multiple basins.

Proposed method

• The model uses a random seeding process to determine tropical cyclone genesis based on large-scale environmental conditions.
• An intensity-dependent beta-advection model determines storm tracks, with steering flow coefficients varying by storm intensity.
• A non-linear differential equation set models intensification rates using the FAST intensity model, adapted for global application.
• The model is forced entirely by ERA5 reanalysis data, including wind, temperature, humidity, SST, and surface pressure fields.
• Maximum sustained wind speed is calculated via a vector combination of environmental flow, translational speed, and vertical wind shear, with a latitude-dependent gain factor.
• The model is implemented in Python and is freely available on GitHub with full documentation and data generation scripts.

Experimental results

Research questions

• RQ1 Can a physics-based, open-source downscaling model reproduce the observed global and regional tropical cyclone climatology, including seasonal cycles and genesis locations?
• RQ2 How does incorporating intensity-dependent steering improve the realism of simulated tropical cyclone tracks and hazard metrics compared to constant-coefficient models?
• RQ3 To what extent can the model reproduce inter-annual variability in tropical cyclone counts and power dissipation across basins?
• RQ4 Does the model accurately simulate return period curves for landfall intensity in different sub-basins, especially for extreme events?
• RQ5 How do parameter choices, particularly in the intensity and steering models, affect the model’s ability to reproduce observed TC behavior?

Key findings

• The model successfully reproduces the global seasonal cycle of tropical cyclone activity, with peak months and basin-specific patterns matching observations.
• Track density and genesis locations across all major basins (e.g., North Atlantic, Western Pacific, South Indian) are well captured by the model.
• The model accurately reproduces the lifetime maximum intensity distribution, including the frequency of intense storms (e.g., Category 4–5), with minimal bias.
• Inter-annual variability in tropical cyclone counts and power dissipation is well represented on both basin-wide and global scales, matching ERA5 and IBTrACS observations.
• Return period curves for landfall intensity in key sub-basins (e.g., U.S. East Coast, Philippines, Australia) are reasonably reproduced, especially when intensity-dependent steering is applied.
• Incorporating intensity-dependent steering leads to regionally distinct changes in power dissipation and return periods, improving realism in hazard estimation compared to models with fixed steering coefficients.