How is the impact of model resolution on tropical cyclone development affected by varying initial conditions in AGCMs?

It has been commonly found that finer model resolution simulates stronger tropical cyclones in Atmospheric General Circulation Models (AGCMs). However, how this resolution-induced intensity change is modulated by physical parameters that control the tropical cyclone genesis has not been thoroughly studied. To understand the problem, we conducted two sets of 300-member ensemble simulations using the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM) integrated 10 model days. One set is with one-degree resolution; the other is half-degree. The 300 members are initiated with varying initial conditions including sea surface temperature (SST), atmospheric lapse rate, size and intensity of the initial vortex seed, and 500-hPa relative humidity. At day 10, each ensemble generates tropical-cyclone like vortices from tropical depression to category 5 based on the Saffir-Simpson wind scale. It is found that the intensity change is larger for stronger tropical cyclone than for weaker ones, when switching from coarse to fine resolution. The resolution-induced intensity change is mainly modulated by the size and intensity of the initial vortex seed, atmospheric stability and the SST, however, in different ways. With larger initial size and intensity, and more stable atmospheric environment, the intensity change is smaller. The relationship between intensity change and the SST is non-monotonic. For SST smaller than 28 C degree, the intensity change increases with SST. For SST larger than 28 C degree, it decreases with SST. Furthermore, these findings suggest that the spatial distribution of tropical cyclones can be changed by model resolution alone in AGCMs.