Improving Tropical Cyclone Track and Intensity in a Global Model with Local Mesh Refinement

Abstract:

Even with recent improvements in general circulation model (GCM) resolution, tropical cyclones (TCs) are typically underresolved, resulting in fewer or weaker storms than observed. In an effort to alleviate these issues, the use of limited area models (LAMs) allowing for higher resolutions has become popular. However, LAMs require lateral boundary conditions and typically lack two-way communication with the exterior domain. Variable-resolution GCMs can serve as the bridge between traditional global models and high-resolution LAMs. These models can reach 10 km or finer resolution in low-latitude ocean basins where TCs are prevalent. They do so while maintaining global continuity, therefore eliminating the need for externally-forced and possibly numerically and physically inconsistent boundary conditions required by LAMs.

Recent developments allow the Community Atmosphere Model's (CAM) Spectral Element (SE) dynamical core to be run on unstructured, statically-nested, variable-resolution grids. We present deterministic CAM-SE model simulations of TCs during recent summers and compare the model's prediction of storm track and intensity to other global and regional models as well as observations. The simulations are run on a 55 km global cubed-sphere grid with additional refinement to 13 km over the Atlantic and Eastern Pacific Oceans. Forecasts are integrated for eight days and the period of analysis spans three months (August, September, and October) during 2012 and 2013. We compare these simulations to identically initialized model runs without mesh refinement to demonstrate the impact of high resolution on TC behavior in CAM. We also investigate cyclone genesis and whether locally high resolution in a global model leads to improved forecast skill at longer lead times. In addition, the impact of the localized refined patch on the remainder of the coarser global solution during the simulation period is discussed.