Extratropical Transition Using 23 Years of Tropical Cyclones in a Variable-Resolution Global GCM

Abstract:

The impact of increased resolution on extratropical transition of tropical cyclones is analyzed using the Community Atmosphere Model version 5 (CAM5) with the variable-resolution Spectral Element (SE) dynamical core. Over the North Atlantic tropics, a high-resolution 28 km grid spacing allows the model to sufficiently resolve the structure of tropical cyclones, including sharp gradients in the eye wall. As the tropical cyclone travels beyond the high-resolution nest, the grid spacing transitions to 110 km. This approach is computationally efficient, while the resolution is high enough for the study of the extratropical transition of cyclones in the Atlantic Ocean.

We compare 23-year simulations with prescribed sea surface temperatures and sea ice according to the Atmospheric Model Intercomparison Project (AMIP) protocol using both the variable-resolution 28-110 km grid and a uniform-mesh 110 km simulation. In particular, we evaluate the climatology of extratropical transition throughout the simulations and compare to reanalysis data. Phase space analysis follows the structural evolution of storms as they transition from symmetric, warm-core tropical cyclones to asymmetric, cold-core extratropical storms. The precipitation rate and the distribution about the storm change throughout the storms’ lifecycles. An individual analysis of selected storms demonstrates the development of cyclone asymmetries, a cold-core structure, and precipitation extremes as the storm undergoes extratropical transition.