TIMA: Trends In the Middle Atmosphere (Phase 2)

Prof. Dr. Franz-Josef Lübken
Dr. Gerd Baumgarten,
Prof. Dr. Erich Becker,
Dr. Ralph Latteck,
Dr. Toralf Renkwitz

Leibniz-Institute of Atmospheric Physics
Schloss-Str. 6
18225 Ostseebad Kühlungsborn
Email: luebken@iap-kborn.de
Telephone: +49-38293-68100

Summary of goals

The prime objective of this proposal is to study trends and solar cycle variations in the middle atmosphere and their potential relevance for climate. This includes anthropogenic changes in the mesosphere as well as potential links to the troposphere. This proposal is based on our results from the first phase of TIMA. As has been shown in TIMA-1, temperature trends in the mesosphere are much larger than at lower altitudes. In TIMA-2 we want to broaden these studies. In particular, we will investigate the link of trends around the summer polar mesopause in the northern hemisphere to tropospheric climate change. To this end we will determine the changes in tropospheric gravity-wave sources and the consequences for the mesosphere. Furthermore, we want to investigate consequences of future climate scenarios on trends in the middle  atmosphere. This includes the shielding of solar radiation by mesospheric ice particles and its potential effects on regional climate at middle and polar latitudes, as well as on temperatures and winds around the summer stratopause, which alter the transmission of gravity waves (GWs). First estimates show that this new albedo effect will indeed be relevant in the future.

Main goals of TIMA-2

Study trends in the mesosphere

As is known by now, temperature trends in the mesosphere are much stronger compared to the troposphere and stratosphere. We aim at characterizing trends in temperatures, winds, gravity waves, and ice clouds (NLC, noctilucent clouds, PMSE, PMSE=polar mesosphere summer echoes) in more detail, and at understanding their main driving mechanisms, e.g., forcing by increased greenhouse gases or by GW and circulation changes. Because of low natural variability we will concentrate on summer at middle and high latitudes (NH and SH), but will also consider winter and spring/fall transitions. We intent to study past and current situation, and investigate effects of future climate scenarios. This includes trends in dynamics, i.e., in mean winds and the consequences for GW propagation and feedback on temperatures (note that meridional differences in temperature trends causes trends in zonal winds). We have demonstrated that changes in the radiation budget in the entire middle atmosphere impact pressures and temperatures around the summer mesopause. In TIMA-2 we therefore also want to analyze the role of future scenarios of the ozone layer. Furthermore, we have improved our KMCM (Kühlungsborn Mechanistic Climate Model) global model which now explicitly covers resolved gravity waves from the surface to the upper mesosphere. This enables us to study the link of climate change in the mesosphere to the troposphere.  We will study changes which result from trends in tropospheric gravity wave sources. This includes the southern winter troposphere, since recent studies have shown that gravity waves in the southern winter mesosphere affect the northern mesosphere through inter-hemispheric coupling.

Examine solar variability effects

Solar EUV is entirely absorbed in the middle atmosphere. What are the effects of solar activity (11y solar cycle and long-term ; new Maunder Minimum) on composition, temperatures, winds etc.? Some quasi-continuous
observations (lidar, radar) are now available for more than a solar cycle and allow for comparison with models. NLC are very sensitive to temperatures and water vapor, both of which are affected by solar EUV. Why is the modulation with solar cycle in models significantly larger compared to observations (SBUV, NLC/ALOMAR). How does solar cycle modulation of other parameters (winds) compare with models. The underlying idea is to study solar cycle effects in the mesosphere and then extrapolate to long-term variations.

Investigate impact of the mesosphere on the troposphere

We want to study the impact of mesospheric ice particles on Earth's albedo, in particular at middle and polar latitudes in summer.  First rough estimates show that this effect can presumably no longer be neglected in the future.
We will investigate the relevance for radiative processes in the stratosphere (heating in ozone layer) and troposphere, as well as regional climate effects and secondary mechanisms (changes of circulation and related modifications of propagation conditions for GWs). It is well known that the stratosphere couples to the troposphere and that chemical active species from the thermosphere are transported (in winter) through the mesosphere to the stratosphere. However, the corresponding transport by dynamics is poorly represented in GCMs. We want to study the physics of this transport in the mesosphere and the impact of GW and circulation by KMCM.