Resolving Offshore Mesoscale Convective Circulations Using a Three-dimensional Planetary Boundary Layer Parameterization

Cold air outbreaks (CAOs), excursions of cold polar air toward the equator, are a common occurrence along the East Coast of the United States in winter and spring. CAOs are characterized by high wind speeds that therefore can represent significant renewable energy resource. Under high winds, as the cold air flows over a warmer ocean, convection organizes into helical streamwise rolls that transition to convective cells as the wind speed decreases. Resolving these mesoscale convective circulations (MCCs) requires grid cell sizes in the “gray zone” or Terra Incognita (Wyngaard, 2004), between 100 m and 1 km. However, MCCs cannot be resolved accurately using one-dimensional planetary boundary layer (1D PBL) parameterizations commonly implemented in numerical weather prediction models (Ching et al. 2014, Zhou et al 2014). Resolving MMCs requires accounting for three-dimensional (3D) effects of shear and turbulence in a boundary layer. We therefore implemented a 3D PBL parameterization in the Weather Research and Forecasting (WRF) model (Kosovic et al. 2020, Juliano et al. 2022) to achieve better representation of the impact of MMCs on hub-height wind speed and turbulence. We assessed the performance of the 3D PBL parameterization by conducting high-resolution simulations of several CAOs observed since 2011 using the WRF model and comparing them to simulations with 1D PBL parameterizations. Simulations with the 3D PBL parameterization result in smaller MMC length scales and higher levels of turbulence compared to a 1D PBL parameterization.