Non-orographic gravity waves and turbulence caused by merging jet streams

Abstract

Jet streams are important sources of non-orographic internal gravity waves and clear air turbulence (CAT). We analyze nonorographic gravity waves and CAT during a merging of the polar front jet stream (PFJ) with the subtropical jet stream (STJ) above the southern Atlantic. Thereby, we use a novel combination of airborne observations covering the meso-scale and turbulent scale in combination with high-resolution deterministic short-term forecasts. Coherent phase fronts stretching along a highly sheared tropopause fold are found in the ECMWF IFS (integrated forecast system) forecasts. During the merging event, the PFJ reverses its direction from antiparallel to parallel with respect to the STJ, going along with strong wind shear and horizontal deformation. Temperature perturbations in limb-imaging and lidar observations onboard the research aircraft HALO in the framework of the SouthTRAC campaign show remarkable agreement with the IFS data. Ten hours earlier, the IFS data show a new "X-shaped" phase line pattern emanating from the sheared tropopause fold. The analysis of tendencies in the IFS wind components shows that these gravity waves are excited by a local body force as the PFJ impinges the STJ. In situ observations of temperature and wind components at 100 Hz confirm upward propagation of the probed portion of the gravity waves. They furthermore reveal embedded episodes of light-to-moderate CAT, Kelvin Helmholtz waves, and indications for partial wave reflection. Patches of low gradient Richardson numbers in the IFS data coincide with episodes where CAT was observed, suggesting that this event was well accessible to turbulence forecasting.

Non‐Orographic Gravity Waves and Turbulence Caused by Merging Jet Streams

“…The GLORIA, ALIMA and BAHAMAS observations and the ECMWF IFS data are available at DOI: https://doi.org/10.5445/IR/1000151856 (Woiwode et al., 2022). For the ECMWF data, the terms of use by the ECMWF apply.…”