Model Physics Influences on Tropical Cyclone Size

Tropical cyclone (TC) size is an important factor directly and indirectly influencing track, intensity, and related hazards such as storm surge.  Using a semi-idealized version of the operational Hurricane WRF (HWRF) 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 (Bu et al. 2014, Fovell et al. 2015), eddy mixing moistens the boundary layer from below, both making peripheral convection more likely.  Thus, these two processes can cooperate and compete, making these 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.

We find that deficiencies in how CRF was handled in the operational HWRF model was at least partially mitigated by permitting excessive mixing in the TC's inner core boundary layer as compared to available observations, as we demonstrate how and why PBL mixing influences storm size. This led us to develop a new version of the GFS PBL scheme for HWRF that can generate realistic inner core mixing without excessively limiting PBL activity elsewhere in the storm and domain.  The modified GFS scheme is being used in the 2015 operational model.