Turbulent Kinetic Energy and Coherent Structures Past the Land-Sea Transition During a Low-Level Jet Event

A low-level jet (LLJ) is a phenomenon characterized by a local maximum in the wind-speed profile and often associated with strong wind shear. LLJs are generally observed under stable conditions, both inland and offshore. Using a realistic mesoscale-to-microscale simulation performed with the Weather Research and Forecasting code, this paper presents an in-depth analysis of the offshore characteristics of a LLJ generated inland and the turbulence in the marine atmospheric boundary layer. Past the coastline, the LLJ core speed decreases and its height increases due to the mixing induced by the development of a convective thermal internal boundary layer (TIBL). Turbulence profiles are different than in a nocturnal LLJ over land and are divided into three layers. In the lower layer corresponding to the TIBL, the buoyancy production induces horizontal convective rolls (HCR) and the turbulent kinetic energy (TKE) increases with distance. This TKE is anisotropic as the variance of the fluctuations in the direction of the mean wind is larger than the two other components. In the intermediate layer between the TIBL and the jet core, the shear production prevails, the TKE also increases with distance, and Kelvin-Helmholtz waves (KHW) are triggered by the underlying HCR updrafts. In the upper layer, the TKE is very low and a few intermittent patches of KHW are observed.