Multi-case Evaluation of the Met Office Unified Model in Simulating the Tropical Cyclone Boundary Layer

The Met Office Unified Model (MetUM) is used by several operational forecasting agencies to forecast tropical cyclone activity. Until now, there has been no detailed evaluation of the performance of the MetUM in representing the tropical cyclone boundary layer, despite the major theories of tropical cyclone intensification giving credit to the importance of the boundary layer, particularly due to the inflow layer's provision of heat, moisture and momentum to the inner core of the storm. In favourable low-level environments, the boundary layer can provide enthalpy, entropy and inertial stability which resists the detrimental effects large-scale disruptions such as vertical wind shear. Alternatively, the boundary layer can contribute to weakening over cooler water or land via a reduction of the surface enthalpy fluxes.

A good boundary layer representation is also important to quantify risk for societies; the highest tangential wind speeds tend to be within this region and the distribution of rainfall can be tightly linked to the surface features.

This evaluation contains a collection of post-2015 hurricanes from the Atlantic basin which translated over the open ocean. The cases cover a range of storm sizes, translation speeds, intensities (and intensity fluctuations), and environmental conditions. The full-physics, convection-permitting MetUM data includes 18-member ensemble simulations with various initialisation times and a series of horizontal grid resolutions (largest 4.4 km). Dropsonde and in-flight radar data from the NOAA reconnaissance flight missions is used to provide a novel insight into how well the MetUM is representing the low-level thermodynamic properties and wind structure of tropical cyclones.

By providing a comprehensive analysis of the strengths and weaknesses of the MetUM's tropical cyclone boundary layer representation, this work aims to highlight areas for model development and provide a richer level of context for researchers and operational forecasters when interpreting the MetUM tropical cyclone simulations.