How and why model physics controls tropical cyclone structure

Tropical cyclone (TC) size is an important factor directly and indirectly influencing track, intensity, and related hazards, such as storm surge. Using the operational Hurricane WRF (HWRF) and an axisymmetric model, we show that enabling cloud-radiative forcing (CRF) and enhancing planetary boundary layer (PBL) vertical mixing can both encourage wider storms by enhancing TC outer core convective activity. While CRF acts primarily above the PBL, eddy mixing moistens the boundary layer from below, both making peripheral convection more likely. Thus, these two processes can cooperate and compete, making their influences difficult to deconvolve and complicating the evaluation of model physics improvements, especially since the sensitivity to both decreases as the environment becomes less favorable. Further study shows not only the magnitude, but also the vertical distribution, of the eddy mixing coefficient can strongly impact the TC size and structure.