My DFG Proposal 2023 - Accepted!

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)

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I propose to derive the gravity wave momentum flux (GWMF) in the mesosphere / lower thermosphere (MLT) by combining airglow with lidar and radar observations. The goal is to study the momentum transport and intermittency of atmospheric gravity waves (GW) and secondary GWs (SGW) over two stratospheric GW hot spots in the northern and southern hemispheres.

Atmospheric GWs are one of the main drivers of the global circulation in the middle atmosphere. Once excited, they transport horizontal pseudo 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 eventually 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 higher-order GWs are excited. State-of-the-art general circulation models (GCM) do not consider the existence of SGWs which leads to wind biases in the MLT of up to 60m/s 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.

Most studies inferring GWMFs from wind measurements and/or airglow intensity measurements show large uncertainties due to insufficient statistics or too many assumptions, i.e. no complementary measurements. Today, the most accurate way to determine GWMFs 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 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 data is waiting to be analysed. 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 (CWT) and a density based spatial clustering algorithm. This tool will identify independent contiguous 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.

This figure illustrates the coinciding measurements in the mesosphere lower thermosphere region. The AMTM observes the horizontal propagation of GWs while CORAL and SAAMER provide valuable information on the atmospheric background such as wind and thermal stability.