A Framework for Simulating the Tropical-Cyclone Boundary Layer Using Large-Eddy Simulation and Its Use in Evaluating PBL Parameterizations Journal of the Atmospheric Sciences

Boundary layer turbulent processes affect tropical cyclone (TC) structure and intensity change. However,uncertainties in the parameterization of the planetary boundary layer (PBL) under high-wind conditions remain chal-lenging, mostly due to limited observations. This study presents and evaluates a framework of numerical simulation that canbe used for a small-domain [O(5)-km] large-eddy simulation (LES) and single-column modeling (SCM) to study the TCboundary layer. The framework builds upon a previous study that uses a few input parameters to represent the TC vortexand adds a simple nudging term for temperature and moisture to account for the complex thermodynamic processes in TCs.The reference thermodynamic profiles at different wind speeds are retrieved from a composite analysis of dropsondeobservations of mature hurricanes. Results from LES show that most of the turbulence kinetic energy and vertical mo-mentum flux is associated with resolved processes when horizontal grid spacing isO(10) m. Comparison to observations ofturbulence variables such as momentum flux, effective eddy viscosity, and turbulence length scale show that LES producesreasonable results but highlight areas where further observations are necessary. LES results also demonstrate that com-pared to a classic Ekman-type boundary layer, the TC boundary layer is shallower, develops steady conditions muchquicker, and exhibits stronger wind speed near the surface. The utility of this framework is further highlighted byevaluating a first-order PBL parameterization, suggesting that an asymptotic turbulence length scale of 40 m produces agood match to LES results Grant no. NA21OAR4320190