Using Variable Resolution Community Atmosphere Model to Simulate Regional Climate and Hurricanes
High-resolution climate modeling can reveal new insights into the climate system’s many multiscale interactions. At grid resolution around 30 km and finer, mesoscale phenomena like tropical cyclones (TCs), topographically forced local wind patterns, or mesoscale convective systems start to become resolved and their climatology can be investigated in global climate models (GCMs). This provides an in-depth look at the model skill at fine resolutions and potential deficiencies in the physical parameterization packages. However, high-resolution climate modeling is costly from a computational viewpoint, and so far only very few global modeling studies with grid spacing of 30 km and below have been conducted. Therefore, new efforts are underway to use variable-resolution (VR) grids that lower the computational demand while providing high-resolution regional climate information over areas of interests.
In a recent study, scientists from the University of Michigan demonstrated that the regionally refined Community Atmosphere Model (CAM version 5) with its spectral element (SE) dynamical core reproduces many of the Atlantic hurricane statistics. In particular, they zoomed into the North Atlantic Ocean basin with a 0.25-degree (28-km) mesh, which was embedded within a 1-degree (111-km) global grid. Two 23-year simulations with prescribed sea surface temperatures and sea ice were conducted (with and without the refined nest) to investigate hurricane climatologies and impact of the enhanced resolution on TCs. The VR simulation contains significantly more TCs than the unrefined simulation. Its increased resolution in the Atlantic region enables it to resolve much more intense storms, with multiple storms strengthening to Saffir-Simpson category 3 intensity or higher. Both count and spatial distribution of TC genesis and tracks in the VR simulation are well matched to observations and represent significant improvements over the unrefined simulation. Some degree of interannual skill also is noted, with the VR grid able to reproduce the observed connection between Atlantic TCs and the El Nino Southern Oscillation (ENSO). Potential ‘upscale’ effects are noted in the VR simulation, suggesting stronger TCs in refined nests may play a role in meridional transport of momentum, heat, and moisture. These sorts of resolution-change influences were further explored in a subsequent publication. Both studies indicate that VR modeling with CAM-SE is free of numerical artifacts and has become a mature technique for regional climate studies.
Zarzycki, C. M., and C. Jablonowski. 2014. “A Multidecadal Simulation of Atlantic Tropical Cyclones Using a Variable-Resolution Global Atmospheric General Circulation Model,” Journal of Advances in Modeling Earth Systems 6(3), 805–28. DOI: 10.1002/2014MS000352.
Zarzycki, C. M., C. Jablonowski, D. R. Thatcher, and M. A. Taylor. 2015. “Effects of Localized Grid Refinement on the General Circulation and Climatology in the Community Atmosphere Model,” Journal of Climate 28, 2777–803. DOI: 10.1175/JCLI-D-14-00599.1.