Investigation of nocturnal low-level jet-generated gravity waves over Oklahoma City during morning boundary layer transition period using Doppler wind lidar data

摘要

Low-level jet (LLJ)-generated gravity waves were observed over Oklahoma City by a scanning Doppler wind lidar during the transition periods of atmospheric boundary layer (ABL) from stable to convective conditions in the mornings after sunrise. The temperature profiles had a multilayer structure with a shallow neutral layer immediately above the ground and a stable cap layer above the neutral layer. The wind profiles exhibited a typical shape of a LLJ with nearly linear growth of wind speed with respect to height, and maximum wind speed occurred at the top of the stable cap layer. The average wavelength and its relation with mean wind and temperature profiles are characterized with data from Doppler wind lidar, radiosonde, and wind profiler. A linear stability analysis was performed to check the stratification conditions for wave occurrence. The wind signals from sonic anemometers near the ground were separated into waves and turbulence parts using a wavelet decomposition method, and the momentum fluxes due to the wave motions and turbulence were computed. The downward gravity wave momentum flux was 1.5 to 3.0 times of turbulent momentum flux. The analysis indicated that gravity wave momentum transport from the stable cap layer downward is one of the mechanisms of stable-to-convective transition in the LLJ-dominated ABL.