Observation and Large-Eddy Simulation of an Offshore Atmospheric Undular Bore During Sea-Breeze Initiation

Abstract

A sea-breeze (SB) initiation under land synoptic wind near a peninsula is analyzed by means of LiDAR measurements and large-eddy simulations (LES) using the Weather Research and Forecasting (WRF) framework. In the simulation results, local SBs initiated over several coast segments converge in the morning and form a front near the peninsula. As the marine atmospheric boundary layer is stably stratified, the front generates an undular bore featuring gravity waves (GW). Despite the absence of cloud signatures, the GW are detected in the LiDAR horizontal scans, providing a direct observation. The GW have a low propagation speed, a small wavelength, and their amplitude decreases with increasing distance from the coastline. The GW amplitude increases with the strengthening of the local convergence and then decreases when the local SBs merge into a regional SB. The turbulent kinetic energy (TKE) profile in the SB without GW forms a peak in the center of the SB cell. Within the GW, a significant part of the TKE calculated from simulation results is related to the GW horizontal motion. A method is proposed to extract only the part originating from the turbulent field. A peak in the TKE profile is also observed, and its intensity is 50%–100% higher near the crests than at the troughs. Analysis of the TKE budget demonstrates that, at the peak height, the shear production is positive only in the ascending phase of the GW, and that the TKE maximum at the crests mainly results from the advection process.