2006, Blum, U., Baumgarten, G., Schöch, A., Kirkwood, S., Naujokat, B., Fricke, K.H.

The atmosphere background wind field controls the propagation of gravity waves from the troposphere through the stratosphere into the mesosphere. During January 2003 the MaCWAVE campaign took place at Esrange, with the purpose of observing vertically ascending waves induced by orography. Temperature data from the U. Bonn lidar at Esrange (68° N/21° E) and the ALOMAR RMR lidar (69° N/16° E), wind data from Esrange MST radar ESRAD, as well as wind data from the ECMWF T106 model, are used to analyse the atmospheric background situation and its effect on mountain wave propagation during January/February 2003. Critical levels lead to dissipation of vertically ascending waves, thus mountain waves are not observable above those levels. In the first half of January a minor as well as a major stratospheric warming dominated the meteorological background situation. These warmings led to a wind reversal, thus to critical level filtering and consequently prevented gravity waves from propagating to high altitudes. While the troposphere was not transparent for stationary gravity waves most of the time, there was a period of eight days following the major warming with a transparent stratosphere, with conditions allowing gravity waves generated in the lower troposphere to penetrate the stratosphere up to the stratopause and sometimes even into the lower mesosphere. In the middle of February a minor stratospheric warming occurred, which again led to critical levels such that gravity waves were not able to ascend above the middle stratosphere. Due to the unfavourable troposphere and lower stratosphere conditions for gravity wave excitation and propagation, the source of the observed waves in the middle atmosphere is probably different from orography.

2018, Peters, D.H.W., Schneidereit, A., Karpechko, A.Y.

Extreme warm stratospheric events during polar winters from ERA-Interim reanalysis and CMIP5-ESM-LR runs were separated by duration and strength of the polar-night jet oscillation (PJO) using a high statistical confidence level of three standard deviations (strong-PJO events). With a composite analysis, we demonstrate that strong-PJO events show a significantly stronger downward propagating signal in both, northern annular mode (NAM) and zonal mean zonal wind anomaly in the stratosphere in comparison with non-PJO events. The lower stratospheric EP-flux-divergence difference in ERA-Interim was stronger in comparison to long-term CMIP5-ESM-LR runs (by a factor of four). This suggests that stratosphere-troposphere coupling is stronger in ERA-Interim than in CMIP5-ESM-LR. During the 60 days following the central date (CD), the Arctic oscillation signal was more intense during strong-PJO events than during non-PJO events in ERA-Interim data in comparison to CMIP5-ESM-LR runs. During the 15-day phase after CD, strong PJO events had a significant increase in stratospheric ozone, upper tropospheric zonally asymmetric impact, and a regional surface impact in ERA-Interim. Finally, we conclude that the applied high statistical threshold gives a clearer separation of extreme warm stratospheric events into strong-PJO events and non-PJO events including their different downward propagating NAM signal and tropospheric impacts. © 2018 by the authors.

2006, Serafimovich, A., Zülicke, Ch., Hoffmann, P., Peters, D., Dalin, P., Singer, W.

We present an experimental and modelling study of a strong gravity wave event in the upper troposphere/lower stratosphere near the Scandinavian mountain ridge. Continuous VHF radar measurements during the MaCWAVE rocket and ground-based measurement campaign were performed at the Norwegian Andoya Rocket Range (ARR) near Andenes (69.3° N, 16° E) in January 2003. Detailed gravity wave investigations based on PSU/NCAR Fifth-Generation Mesoscale Model (MM5) data have been used for comparison with experimentally obtained results. The model data show the presence of a mountain wave and of an inertia gravity wave generated by a jet streak near the tropopause region. Temporal and spatial dependencies of jet induced inertia gravity waves with dominant observed periods of about 13 h and vertical wavelengths of ~4.5–5 km are investigated with wavelet transform applied on radar measurements and model data. The jet induced wave packet is observed to move upstream and downward in the upper troposphere. The model data agree with the experimentally obtained results fairly well. Possible reasons for the observed differences, e.g. in the time of maximum of the wave activity, are discussed. Finally, the vertical fluxes of horizontal momentum are estimated with different methods and provide similar amplitudes. We found indications that the derived positive vertical flux of the horizontal momentum corresponds to the obtained parameters of the jet-induced inertia gravity wave, but only at the periods and heights of the strongest wave activity.

2007, Luderer, G., Trentmann, J., Hungershöfer, K., Herzog, M., Fromm, M., Andreae, M.O.

Deep convection induced by large forest fires is an efficient mechanism for transport of aerosol particles and trace gases into the upper troposphere and lower stratosphere (UT/LS). For many pyro-cumulonimbus clouds (pyroCbs) as well as other cases of severe convection without fire forcing, radiometric observations of cloud tops in the thermal infrared (IR) reveal characteristic structures, featuring a region of relatively high brightness temperatures (warm center) surrounded by a U-shaped region of low brightness temperatures. We performed a numerical simulation of a specific case study of pyroCb using a non-hydrostatic cloud resolving model with a two-moment cloud microphysics parameterization and a prognostic turbulence scheme. The model is able to reproduce the thermal IR structure as observed from satellite radiometry. Our findings establish a close link between the observed temperature pattern and small-scale mixing processes atop and downwind of the overshooting dome of the pyroCb. Such small-scale mixing processes are strongly enhanced by the formation and breaking of a stationary gravity wave induced by the overshoot. They are found to increase the stratospheric penetration of the smoke by up to almost 30 K and thus are of major significance for irreversible transport of forest fire smoke into the lower stratosphere.