An Exploration of Tropical Cyclone Simulations in NCAR's Community Atmosphere Model.

Abstract

Using General Circulation Models (GCMs) for tropical cyclone studies is challenging due to the relatively small size of the storms, the intense convection and a host of scale interactions. However, with the advancement of computer architectures, GCMs are becoming capable of running at high horizontal resolutions with grid spacings of less than 60 km. As a result, high-resolution GCMs are becoming a tool of choice to evaluate tropical cyclones in current and future climate conditions. This raises questions concerning the fidelity of GCMs for tropical cyclone assessments. The physical and dynamical components of GCMs need to be evaluated to assess their reliability for tropical cyclone studies.

An idealized tropical cyclone test case for high-resolution GCMs is developed and implemented in aqua-planet mode with constant sea surface temperatures. The initial conditions are based on an analytic initial vortex seed that is in gradient-wind and hydrostatic balance and intensifies over a 10-day period. The influence of the model parameterization package on the development of the tropical cyclone is assessed. In particular, different physics parameterization suites are investigated within the National Center for Atmospheric Research's Community Atmosphere Model CAM, including physics versions 3.1, 4 and 5. The choice of the CAM physics suite has a significant impact on the evolution of the idealized vortex into a tropical cyclone.

In addition, a test case of intermediate complexity is introduced. Therein it is suggested that a GCM dynamical core be paired with simple moist physics to test the evolution of the test vortex. This simple-physics configuration includes important driving mechanisms for tropical cyclones, including surface fluxes, boundary layer diffusion and large-scale condensation. The impact of the CAM dynamical core (the resolved fluid flow component) on the tropical cyclone intensity and size is evaluated. In particular, the finite-volume, spectral element, Eulerian spectral transform and semi-Lagrangian spectral transform dynamical cores are utilized. The simple-physics simulations capture the dominant characteristics of tropical cyclones and are compared to the CAM 5 full physics results for each dynamical core. The research isolates the impact of the physical parameterizations, numerical schemes and uncertainties on the evolution of the cyclone in CAM.