Revealing the role of missing scales in boundary layer observations in gravity wave propagation using the Flying Fiber Optic eXperiment (FlyFOX)

The stable boundary layer, especially the very stable boundary layer, (vSBL) is a fundamental challenge for boundary layer meteorology as assumptions such as ergodicity and local scaling do not apply. The violation of these commonly-employed theories is associated with the presence of submeso-scale structures, which span spatial scales between tens of meters and kilometers and temporal scales from tens of seconds up to an hour. The nature of these structures is largely unknown but they are suspected to encompass a wide-range of flow modes, including meandering of the horizontal wind direction, thermal submeso fronts, complex and unknown non-stationary modes, and relevant to this work, various wave modes. Progress on submeso-turbulence interactions requires distributed observations with fine enough resolution to separate between the submeso and turbulent scales.

 

To that end we present results from FlyFOX in which fiber optic distributed sensing (FODS) was deployed along a tethered balloon. FODS yields spatially continuous observations of air temperature with fine spatial (0.25m – 0.5m) and temporal (1s-10s) resolutions along fiber optic cables that can span kilometers. In this case FlyFOX spanned between 0.5m and 200m height. FlyFOX was deployed in a broad mountain valley in the Ficthelgebirge mountains, Germany in which intense cold air pooling commonly occurs.

 

Using FlyFOX we simultaneously characterize the spatial and temporal spectra of the boundary layer through morning transitions, revealing that the vSBL has a unique spectral enhancement between 80s-640s and 8m-64m relative to weakly-stable and neutral conditions. These scales correspond to a gap in the observational capabilities of existing methods, which FlyFOX fills.

 

Corresponding to this observational gap, we demonstrate the existence of “sublayer striations”, thin (5m-20m) but persistent layers (duration up to an hour) of exceptionally stable air separated by layers of near-neutral stability. Using wavelet coherence for different time scales, gravity waves were found to be unable to penetrate into the sublayer striations and instead ducted in the neutral air between striations. During periods with overall lower static stability, these sublayer striations did not occur and waves acted across the entire depth of the SBL from ~120m down to ~0.5m and can be tracked propagating along the surface at 1m height using a near surface DTS array. These sublayer striations thereby acted to decouple the upper boundary layer from the surface layer in this mountain valley. FlyFOX and FODS provide an observational breakthrough for the study of vertical coupling and wave activity in the vSBL by closing an observational gap and facilitating observations of atmospheric properties from the turbulent to submeso scales.