The Influences of Boundary Layer Mixing and Cloud-Radiative Forcing on Tropical Cyclone Size

Tropical cyclone (TC) size is an important factor directly and indirectly influencing track, intensity, and related hazards, such as stormsurge. Using a semi-idealized version of the operational Hurricane Weather Research and Forecasting Model (HWRF), the authors show that both enabling cloud-radiative forcing (CRF) and enhancing planetary boundary layer (PBL) vertical mixing can 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 of the eddy mixing coefficient but also the shape of it can determine the TC size and structure.