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

<title xmlns:mml="http://www.w3.org/1998/Math/MathML">Abstract</title><p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en">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 merger 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 lines of temperature perturbations by gravity waves stretching along a highly sheared tropopause fold are simulated by the ECMWF IFS (integrated forecast system) forecasts. During the merging event, the PFJ reverses its direction from approximately 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 during the SouthTRAC campaign show remarkable agreement with the IFS data. Ten hours earlier, the IFS data show an “X‐shaped” pattern in the temperature perturbations emanating from the sheared tropopause fold. Tendencies of the IFS wind components show that these gravity waves are excited by spontaneous emission adjusting the strongly divergent flow when 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 Richardson numbers in the IFS data coincide with the CAT observations, suggesting that this event was accessible to turbulence forecasting.</p>

Plain Language Summary: Gravity waves play an in important role in vertical and horizontal energy transport in the atmosphere and are significant factors in wheather forecasting and climate projections. Among other processes, tropospheric jet streams are known to be sources of gravity waves. They furthermore can be accompanied by tropopause folds (i.e., local tropopause depressions, where stratospheric air can reach deeply into the troposphere) and turbulence, which is relevant for aviation safety. Using a novel combination of airborne observations and data by a state‐of‐the‐art forecasting system, we analyze gravity waves and turbulence during a merger of tropospheric jet streams above the southern Atlantic. The observations show a high degree of agreement with the forecast data from the troposphere to the stratosphere. Ten hours earlier, the forcast data show an “X‐shaped” gravity wave structure that emerges from a highly sheared tropopause fold between the merging jet streams. Fast in situ observations at the flight level provide information on the characteristics of the observed waves and show light‐to‐moderate turbulence, small‐scale waves and indications for partial wave reflection. The observed turbulence events are consistently located in regions where the forecast data suggest potential for turbulence.</p>

Key Points: <list list-type="bullet"> <list-item> <p xml:lang="en">Non‐orographic internal gravity waves and clear air turbulence are observed in merging jet streams</p></list-item> <list-item> <p xml:lang="en">State‐of‐the art high resolution forecast agrees with novel combination of airborne sensors</p></list-item> <list-item> <p xml:lang="en">“X‐shaped” gravity wave feature resulting from merging jet streams at a highly sheared tropopause fold</p></list-item> </list> </p>

Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

https://doi.org/10.5445/IR/1000151856

https://www.ecmwf.int/en/forecasts

https://www.ready.noaa.gov/