Stratosphere / Upper Atmosphere

This presentation briefly reviews our knowledge of long-term changes and trends in the upper atmosphere (mesosphere and thermosphere) and ionosphere. These changes are part of complex and comprehensive pattern of long-term trends in the Earth’s atmosphere. They also have practical impact. For example, decreasing thermospheric density causes the lifetime of orbiting space debris to increase, which is becoming a significant threat to important satellite technologies. There might be also some impact of ionospheric trends on GNSS signal propagation and their applications. Since the first paper on upper atmosphere trends was published in 1989, our knowledge has progressed considerably. Anthropogenic emissions of greenhouse gases, particularly of CO₂, affect the whole atmosphere, not only the troposphere. They cause warming in the troposphere but cooling in the upper atmosphere due to optical thinning of CO₂ layer. Greenhouse gases such as CO₂ are not the only driver of long-term changes and trends in the upper atmosphere and ionosphere. Anthropogenic changes of stratospheric ozone, long-term changes of water vapour content, long-term changes of geomagnetic and solar activity, secular change of the Earth’s magnetic field, and maybe other drivers play a role as well, although CO₂ appear to be the dominant driver of long-term trends. This makes the pattern of trends more complex, variable, and partly spatially as well as temporally unstable. Particularly the change of trend of stratospheric ozone changed trends in the mesosphere and lower thermosphere. A consistent, although incomplete, scenario of trends in the upper atmosphere and ionosphere is presented. Advances in observational and theoretical analysis have explained some previous discrepancies in this global trend scenario, therefore trends in the F2-region ionosphere parameters and in noctilucent or polar mesospheric clouds became part of the scenario in recent years. An important role in trend investigations is played by model simulations, which facilitate understanding of the mechanisms behind the observed trends. More realistic height profiles of CO₂ helped to reduce substantially (or even remove for some parameters) the quantitative difference between observational and modelled trends.

Despite some progress, various problems and tasks remain open and are to be studied in the future. The most important problems appear to be the following:

1. The key open problem is trends in atmospheric dynamics, i.e., in atmospheric circulation and particularly in atmospheric wave activity. We are only sure that these trends are of complex spatial and temporal pattern.

2. Changing role of trend drivers induces spatial and particularly temporal changes of trends in various parameters. This has to be monitored and studied.

3. Large discrepancy between trends in ion temperature in the upper thermosphere between middle and high latitudes needs to be explained.

4. For GNSS applications, more reliable quantification of trends in TEC is necessary.

5. On longer-time scale, an important task is to try to join the upper atmospheric trends with long-term changes in the stratosphere into one scenario. 

After nearly three decades of research, the role of polar stratospheric clouds (PSCs) in stratospheric ozone depletion is generally well established. However, important questions remain unanswered that limit our understanding of PSC processes and how to accurately represent them in global models, calling into question our prognostic capabilities for future ozone loss in a changing climate. A more complete picture of PSC morphology and composition on polar vortex-wide scales is emerging from a suite of recent satellite missions: the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat (2002-2012), the Microwave Limb Sounder (MLS) on Aura (2004-present), and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on CALIPSO (2006-present). These datasets have motivated numerous PSC research activities that both extend and challenge our present knowledge of PSC processes and modeling capabilities.

There is a new initiative hosted by the International Space Science Institution (ISSI) in Bern called Polar Stratospheric Cloud initiative (PSCi) to address key questions related to PSC formation and evolution and their representation in global models. This will be addressed through the following main objectives: identify the key PSC parameters which are required by global models; compare remote and in situ datasets to identify their strengths and limitations; define a methodology to obtain the key PSC properties required by models from the observational datasets; synthesize the new satellite measurements and earlier datasets into a state of the art PSC climatology; and identify remaining open science questions.

The MIPAS, Aura MLS, and CALIPSO missions comprise what could be called the ‘golden age’ for PSC observations. In combination, this suite of measurements represents the most comprehensive PSC observational data record in existence and will serve as the foundation for our state of the art PSC reference climatology.

In this paper we like to present an overview of the PSC initiative and its current status. Special emphasis will be given on the MIPAS PSC type climatology in comparison to the CALIOP type classification. The difficulties in comparisons between the very different measurements techniques of both instruments will be highlighted and potential synergy effects by combining both data sets will be discussed.

Solar, auroral, and radiation belt electrons as well as soft solar X-rays produce nitric oxide (NO) in the mesosphere and lower thermosphere (MLT, 50--150 km). Hence, the NO content in this atmospheric region reveals how solar and geomagnetic activity and variability impact the atmospheric composition. NO downward transport during polar winters then influences the lower atmosphere, in particular by catalytically depleting stratospheric ozone. This in turn changes the heating and cooling rates and eventually the atmospheric circulation down to tropospheric weather systems.

We present ten years of SCIAMACHY NO measurements in the MLT region. We combine the special MLT data (50--150 km, one day every two weeks from 07/2008 until 04/2012) and the nominal data (0--90 km, daily from 08/2002 until 04/2012) to a ten-year daily global NO density data set from 60 km to 90 km. From this data set, we extract solar and geomagnetic forcing parameters using two different statistical approaches: superposed epoch analysis and multi-linear regression. The derived parameters help to constrain how the NO content in the mesosphere is influenced by solar and geomagnetic activity. Separating other solar variability parameters, we present a simple empirical model to calculate the NO density in the middle atmosphere from charged particle precipitation.

The MesosphEO project (http://mesospheo.fmi.fi) is financed by the STSE programme of the European Space Agency (ESA), with the Finnish Meteorological Institute (FMI) as the prime contractor. The aim is to provide the atmospheric science community a comprehensive data set of Level 2-4 mesospheric data products covering a time period of at least 10 years from satellite instruments on Envisat (GOMOS, MIPAS, SCIAMACHY), on Odin (OSIRIS, SMR), and on SCISAT (ACE-FTS). MesosphEO is a science-driven project, it serves as preparation and support for possible future mesospheric work, e.g., in the lines of the ESA Climate Change Initiative (CCI) projects on Essential Climate Variables (ECV). The idea is to start from the science questions and define the requirements for satellite observations from thereon, build the data sets from that basis as much as possible, learn as much as possible about what can be done in practice, and demonstrate the usability of the resulting data products. This preparation is important because the task of creating merged data sets for mesospheric variables is expected to be significantly more difficult compared to the stratosphere. Here we give a general overview of the project and its aims. We also describe the current status of the project, the expected results, and the plans to distribute the generated data sets openly to the atmospheric science community.

The forcing of gravity waves (GWs) is an important coupling mechanism in the atmosphere. GWs are a major driver of the middle atmosphere circulation and may play a role, for instance, in sudden stratospheric warmings and the rebuilding of the stratopause afterwards. The GW-LCycle (Gravity Wave Life Cycle) campaign, taking place in Scandinavia in winter 2015/2016, will help to get a better understanding of GWs. This campaign aims on studying the life cycle of GWs, their excitation, propagation, and dissipation, with different airborne and ground based instruments. During this campaign the first 3D tomographic measurements of GWs are performed with the infrared limb imager GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) aboard the German research aircraft HALO.

GLORIA is a joint development of the Helmholtz Research Facilities Karlsruher Institut für Technologie (KIT) and Forschungszentrum Jülich (FZJ) and combines a classical Fourier Transform Spectrometer with a 2D detector array. GLORIA is a demonstrator for a future infrared limb imager satellite as a follow up concept to PREMIER (EE7 candidate). The capability to image the atmosphere and thereby take several thousand spectra simultaneously improves the spatial sampling of conventional limb sounders by an order of magnitude. Furthermore GLORIA is able to pan the horizontal viewing direction and therefore measure the same volume of air under different angles. Due to these properties tomographic methods can be used to derive 3D temperature and tracer fields with spatial resolutions of better than 30km x 30km x 300m from measurements taken during circular flight patterns.

These temperature fields allow for the first time to retrieve the full wave vector from measurements from one single instrument. First results of GLORIA measurements during the GW-LCycle campaign will be presented.