Logo Logo
Help
Contact
Switch language to German
Characterization of gravity waves in the lee of the southern Andes utilizing an autonomous Rayleigh lidar system
Characterization of gravity waves in the lee of the southern Andes utilizing an autonomous Rayleigh lidar system
The largest mountain waves worldwide are excited at the southern Andes where they subsequently propagate vertically and horizontally downwind and transfer their momentum to the mean flow in the middle atmosphere. Many questions regarding excitation, exact propagation, interaction and dissipation of these waves are still unanswered. For this reason, a Rayleigh lidar system was installed on behalf of DLR at Río Grande (53.7◦ S, 67.7◦ W), Argentina, to record vertical temperature profiles, to detect gravity wave signatures. Analysis of the lidar data set, collected in an automated manner between November 2017 and October 2020, is the core of this dissertation. What is new here is not only the measurement at this geographic location, but also the high cadence of the measurements. The measurement coverage of an average of two measurements within three nights allows to define a temperature background, which covers temporal scales from 9 days up to one year and vertical scales from 15 km. In addition, diurnal tides are extracted from nighttime lidar measurements using a new methodology that is also applied to ECMWF reanalysis data for validation. The comparison shows good agreement, although the amplitudes of the diurnal tide in the lidar data are larger in the mesosphere and vary much more than in the reanalysis data. Tidal aliasing likely results in unexpected small/large amplitudes in the annual/semi-annual oscillations. The wave energies studied are the largest ever measured in the stratosphere, reaching a saturation limit at 60 km altitude. Reaching a saturation limit at such low altitudes has not been observed before in that way and suggests that waves are already generated with very large amplitudes and also find good growth conditions during vertical propagation. Also related to saturation is an observed decrease in gravity wave intermittency in the mesosphere. The development of a new spectral tool helps in the determination of wavelengths. Here it becomes clear that about 50 % of the waves have vertical scales greater than 16,5 km. This is an important result considering that previous lidar studies have mostly focused on vertical wavelengths <15 km. In isolated cases, energy growth in the stratosphere is observed to exceed the expected exponential growth. This may indicate that the waves are propagating horizontally through the lidar’s observation volume. To investigate the propagation of the waves along with their excitation mechanism, a ray tracing study was carried out. It becomes clear, first, that measured wave energies in the middle atmosphere depend primarily on the properties of the background atmosphere and only secondarily on the strength of the forcing. Second, it has been found that the excitation defines the direction of propagation of the mountain waves. If the wind turns with altitude, lateral propagation occurs more strongly, sometimes leeward for several 100 km. A horizontal wind gradient is not able to compensate this by rotating the wave vector. This is an important result and should be considered in future parameterization schemes of climate models.
atmospheric gravity waves, southern Andes, rayleigh lidar, middle atmosphere, wavelet analysis, raytracing
Reichert, Robert
2022
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Reichert, Robert (2022): Characterization of gravity waves in the lee of the southern Andes utilizing an autonomous Rayleigh lidar system. Dissertation, LMU München: Faculty of Physics
[img]
Preview
PDF
Reichert_Robert.pdf

20MB

Abstract

The largest mountain waves worldwide are excited at the southern Andes where they subsequently propagate vertically and horizontally downwind and transfer their momentum to the mean flow in the middle atmosphere. Many questions regarding excitation, exact propagation, interaction and dissipation of these waves are still unanswered. For this reason, a Rayleigh lidar system was installed on behalf of DLR at Río Grande (53.7◦ S, 67.7◦ W), Argentina, to record vertical temperature profiles, to detect gravity wave signatures. Analysis of the lidar data set, collected in an automated manner between November 2017 and October 2020, is the core of this dissertation. What is new here is not only the measurement at this geographic location, but also the high cadence of the measurements. The measurement coverage of an average of two measurements within three nights allows to define a temperature background, which covers temporal scales from 9 days up to one year and vertical scales from 15 km. In addition, diurnal tides are extracted from nighttime lidar measurements using a new methodology that is also applied to ECMWF reanalysis data for validation. The comparison shows good agreement, although the amplitudes of the diurnal tide in the lidar data are larger in the mesosphere and vary much more than in the reanalysis data. Tidal aliasing likely results in unexpected small/large amplitudes in the annual/semi-annual oscillations. The wave energies studied are the largest ever measured in the stratosphere, reaching a saturation limit at 60 km altitude. Reaching a saturation limit at such low altitudes has not been observed before in that way and suggests that waves are already generated with very large amplitudes and also find good growth conditions during vertical propagation. Also related to saturation is an observed decrease in gravity wave intermittency in the mesosphere. The development of a new spectral tool helps in the determination of wavelengths. Here it becomes clear that about 50 % of the waves have vertical scales greater than 16,5 km. This is an important result considering that previous lidar studies have mostly focused on vertical wavelengths <15 km. In isolated cases, energy growth in the stratosphere is observed to exceed the expected exponential growth. This may indicate that the waves are propagating horizontally through the lidar’s observation volume. To investigate the propagation of the waves along with their excitation mechanism, a ray tracing study was carried out. It becomes clear, first, that measured wave energies in the middle atmosphere depend primarily on the properties of the background atmosphere and only secondarily on the strength of the forcing. Second, it has been found that the excitation defines the direction of propagation of the mountain waves. If the wind turns with altitude, lateral propagation occurs more strongly, sometimes leeward for several 100 km. A horizontal wind gradient is not able to compensate this by rotating the wave vector. This is an important result and should be considered in future parameterization schemes of climate models.