Mountain Wave Momentum Flux Estimates From Airborne Lidar Measurements in the Middle Atmosphere Above the Southern Andes

Abstract

Cross-mountain flow over the Southern Andes and strong zonal winds extending higher up in the atmosphere allowed for mountain waves to penetrate into the mesosphere on 11/12 September 2019 during the Southern Hemisphere Transport, Dynamics, and Chemistry–Gravity Waves (SOUTHTRAC-GW) campaign. The middle atmosphere responses above and in the lee of the mountain ridge were observed by the Airborne Lidar for Middle Atmosphere Research (ALIMA) onboard the German High-Altitude and Long-Range research aircraft (HALO), which provided temperature measurements with both high horizontal (approx. 10 km) and high vertical (1.5 km) resolution. The observations reveal a complex wave field with multiple superimposed wave packets with horizontal scales $\lambda_h$ ranging from about 33 to 395 km. This paper employs spectral analysis of observational data and results of Fourier ray modeling to decompose the wave field and analyze the scales and properties of gravity wave packets. Profiles of the vertical flux of horizontal mountain wave momentum reveal contributions of each wave packet to the total momentum flux and gravity wave drag. The derived momentum flux spectrum suggests a spectral response of the form $\lambda_h^x$ with the exponent in the range -1.0 to -1.2 and peak momentum flux occurring at approximately $\lamba_h$ = 45 km. Results show that > 80% of the total mountain wave momentum flux is carried by waves with horizontal wavelengths shorter than 100 km.