A Simplified Approach to Understanding Boundary Layer Structure Impacts on Tropical Cyclone Intensity

The tropical cyclone boundary layer (TCBL) plays an important role in storm structure and intensity change, but the impacts are difficult to understand due to complex non-linear interactions with the mean vortex above the boundary layer and limited observations. In this study, we derive a new, simple logistic growth equation (LGE) for the maximum axisymmetric tangential wind speed from first principles that uses TCBL structural characteristics and the surface drag coefficient to determine potential TC intensification rates. The equation bears some similarities to the LGE model used operationally for TC forecasting with relatively few parameters (LGEM; DeMaria 2009), but replaces the growth-limiting maximum potential intensity (MPI) factor with a term related to TCBL structure and drag. The ratio of coefficients related to radial inflow and TCBL structure in this LGE yields the carrying capacity, which represents the axisymmetric tangential potential intensity (PI) in this case. While the equation is not designed to yield an a priori MPI estimate, the simplified LGE elucidates a conceptual framework that highlights relationships between TCBL structure, surface drag, and intensity change.

The validity of the LGE will be explored in idealized numerical modeling using the axisymmetric Cloud Model 1 and with two observational datasets—Hurricane Joaquin (2015) dropsondes from the Tropical Cyclone Intensity Experiment, and Hurricane Research Division airborne radar vertical cross-sections. The results suggest that the simplified framework has limitations, but can provide some insights into TCBL impacts on intensity change and could potentially be used for forecasting or to estimate the drag coefficient from observations.