The accurate determination of gravity wave momentum flux in the mesosphere lower thermosphere over the northern Scandes (67°N) and the southern Andes (54°S)

• Applicant: Dr. Robert Reichert
• Subject Area: Atmospheric Science
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 526119858

Atmospheric gravity waves (GW) are one of the main drivers of the global circulation in the middle atmosphere. Once excited, e.g. over mountains, they transport horizontal momentum vertically across various atmospheric layers while their amplitudes grow strongly due to the decrease of air’s density and conservation of energy. These growing amplitudes lead usually at mesospheric altitudes to convective instability and turbulent wave breaking where the GW’s momentum is transferred into the mean-flow. This momentum deposition acts to accelerate the mean-flow in the direction of wave propagation and secondary GWs (SGW) are excited. State-of-the-art general circulation models do not consider the existence of SGWs which leads to wind biases in the mesosphere lower thermosphere (MLT) of up to 60ms-1 and an incorrect temperature and location of the mesopause. It is of high priority to gain knowledge about SGWs in the MLT to make the right adjustments in parametrization schemes. Observations with great accuracy are needed. In fact, they are available and wait to be analysed. Today, the most accurate way to determine GW momentum flux (GWMF) in the MLT is to combine lidar and airglow temperature measurements with meteor wind radar (MWR) wind measurements. The MWR and the lidar provide well defined atmospheric background conditions such as thermal stability and vertical wind shear while the airglow imager detects GWs with high temporal (~30s) and spatial (~0.7km) resolution. Such combined observations were done in northern Finland (2015/16) and southern Argentina (2017 until today) utilizing a Rayleigh lidar, an OH-temperature mapper, and MWRs. The major challenge though is the GW identification and subsequent GWMF calculation in an automated way such that statistically robust results are derived. In order to approach this problem, I will develop a novel GW field decomposition tool that is based on multi-dimensional continuous wavelet transforms and a density based spatial clustering algorithm. This tool will identify independent GW packets that are localized in time and space and explicitly not monochromatic but have spectral properties also as function of time and space. This tool could also be of great use in many other applications involving the decomposition of wave fields. In an iterative manner, identified GW packets are analysed from largest to smallest scales and their scale-dependent GWMFs are computed. With the identified GW packets at hand, it is envisaged to look for signatures of SGWs, i.e. so called fishbone structures, and derive their momentum fluxes and drag in the MLT. Furthermore, the derived GWMFs are compared with traditionally derived fluxes from MWRs which generally consider only large-scale GWs. The statistics collected over one GW hot spot, Río Grande, and one rather wave inactive region, Sodankylä, will set a new benchmark for models and will help to guide GW parameterization schemes in weather and climate models.

• DFG Programme: Research Grants
• International Connection: Finland, United Kingdom, USA
• Cooperation Partners: Dr. Diego Janches; Dr. Alexander Kozlovsky; Professor Dr. Mark Lester; Dr. Pierre-Dominique Pautet