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.

Volcanic activity is a main natural climate forcing and an accurate representation of volcanic aerosols in global climate models is essential. This is, however, a complex task involving many uncertainties related to the magnitude and vertical distribution of volcanic emissions as well as in observations used for model evaluation. We analyse the performance of the aerosol-chemistry-climate model SOCOL-AERv2 for three medium-sized volcanic eruptions. We investigate the impact of differences in the volcanic plume height and SO2 content on the stratospheric aerosol burden. The influence of internal model variability and dynamics are addressed through an ensemble of free-running and nudged simulations at different vertical resolutions. Comparing the modeled evolution of the stratospheric aerosol loading to satellite measurements reveals a good performance of SOCOL-AERv2. However, the large spread in emission estimates leads to differences in the simulated aerosol burdens resulting from uncertainties in total emitted sulfur and the vertical distribution of injections. The tropopause height varies among the free-running simulations, affecting model results. Conclusive model validation is complicated by uncertainties in observations. In nudged mode, changes in convection and tropospheric clouds affect SO2 oxidation paths and cross-tropopause transport, leading to increased burdens. This effect can be reduced by leaving temperatures unconstrained. A higher vertical resolution of 90 levels increases the stratospheric residence time of sulfate aerosol by reducing the diffusion out of the tropical reservoir. We conclude that the model set-up (vertical resolution, free-running vs. nudged) as well as forcing parameters (volcanic emission strength, plume height) contribute equally to the model uncertainties.

Elevated stratopauses are typically associated with prolonged disturbed conditions in the northern hemisphere polar winter. MIPAS and MLS observed a short-lived and highly zonally asymmetric stratopause at mesospheric altitudes in November 2009, the earliest in the season reported so far. The Arctic climatological winter stratopause vanished and MIPAS and MLS measured temperatures of 260K at 82\,km and 250K at 75\,km, respectively, in a region smaller than in typical mid-winter elevated stratopause events. Planetary wave activity was initially high. Zonal mean zonal winds and the poleward temperature gradient northward of 70$^\circ$N stayed reversed during 7 days. The mesosphere did not cool during that phase. Wave activity dropped until the eastward stratospheric zonal winds resumed, a strong vortex restored in the mesosphere, and the stratopause emerged at a high altitude. An enhanced downward transport followed. It took the stratopause 9 days to move down to its typical winter altitudes.