Observed large-scale trapping of gravity waves within the free troposphere

Abstract. Gravity waves are a key dynamical feature of atmospheric flows, yet remain difficult to simulate and observe. However, on 15 August 2024, widespread wave signatures were observed over a broad region of the northeastern tropical Atlantic, with remarkable spatio-temporal coherence. One such packet was sampled in situ by the research aircraft operated within the MAESTRO field campaign, providing an ideal case study.

We perform a comprehensive characterization of the wave packet through a combination of in situ, remote sensing and radiosonde observations, using linear wave theory. Some incongruous features, notably the lack of coherence between horizontal and vertical wind perturbations, are additionally explained by an idealized cavity model. We also investigate the synoptic-scale environmental conditions: a well-mixed Saharan Air Layer in the lower-troposphere and significant upper-level shear delimited a clear free-tropospheric waveguide, allowing for long-lasting and long-range horizontal wave propagation, and a selection of specific wavelengths.

Building on this case study, we introduce a Trapping Likelihood Index, referred to as TraLI, which captures the environmental conditions conducive to gravity wave trapping at synoptic scales. This index can be computed over any chosen spatio-temporal domain using reanalysis data. For illustration, we analyze its variability over the region of interest during the year 2024, and show that its main variations are consistent with local climatology and previous studies of gravity wave trapping.