Abstract
Jet streams are important sources of non-orographic internal gravity
waves and clear air turbulence (CAT). We analyze non-orographic 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.