The future increase of methane concentration leads to a raise in water vapor abundance in the middle atmosphere. This will enhance the brightness of noctilucent clouds (NLC). We use an atmospheric background model and a microphysical model to study the associated absorption of solar radiation in the period 1950 to 2100. At 69°N mean absorptions at λ=126nm will increase from ~3% to ~7% from 1950 to 2100, respectively. Locally, the absorption can increase to ~30% in the year 2100. In the visible we find an increase from 0.0030% (1950) to 0,020% (2100), i.e., by a factor of ~7, and local maxima up to 0.35% in 2100. The results are similar for polar latitudes (79°N) but are smaller at middle latitudes (58°N). Future mean absorptions are comparable to solar cycle variations, but much larger locally. The ice mass bound in NLC increases from 677 to 1871 tons in 1950 and 2100.

An international joint research project, entitled Interhemispheric Coupling Study by Observations and Modelling (ICSOM), is ongoing. In the late 2000s, an interesting form of interhemispheric coupling (IHC) was discovered: when warming occurs in the winter polar stratosphere, the upper mesosphere in the summer hemisphere also becomes warmer with a time lag of days. This IHC phenomenon is considered to be a coupling through processes in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several plausible mechanisms have been proposed so far, but they are still controversial. This is mainly because of the difficulty in observing and simulating gravity waves (GWs) at small scales, despite the important role they are known to play in middle atmosphere dynamics. In this project, by networking sparsely but globally distributed radars, mesospheric GWs have been simultaneously observed in seven boreal winters since 2015/16. We have succeeded in capturing five stratospheric sudden warming events and two polar vortex intensification events. This project also includes the development of a new data assimilation system to generate long-term reanalysis data for the whole middle atmosphere, and simulations by a state-of-art GW-permitting general circulation model using reanalysis data as initial values. By analyzing data from these observations, data assimilation, and model simulation, comprehensive studies to investigate the mechanism of IHC are planned. This paper provides an overview of ICSOM, but even initial results suggest that not only gravity waves but also large-scale waves are important for the mechanism of the IHC.

The polar summer mesosphere is the Earthâ\euro™s coldest region, allowing the formation of mesospheric ice clouds. These ice clouds produce strong polar mesospheric summer echoes (PMSE) that are used as tracers of mesospheric dynamics. Here we report the first observations of extreme vertical drafts ($\pm$50 ms$^{-1}$) in the mesosphere obtained from PMSE, characterized by velocities more than five standard deviations larger than the observed vertical wind variability. Using aperture synthesis radar imaging, the observed PMSE morphology resembles a solitary wave in a varicose mode, narrow along propagation (3–4 km) and elongated ($>10$ km) transverse to propagation direction, with a relatively large vertical extent ($\sim$13 km). These spatial features are similar to previously observed mesospheric bores, but we observe only one crest with much larger vertical extent and higher vertical velocities.

The polar summer mesosphere is the Earth’s coldest region, allowing the formation of mesospheric ice clouds. These clouds produce strong polar mesospheric summer echoes (PMSE) that are used as tracers of mesospheric dynamics. Here we report the first observations of extreme vertical drafts (±50~m/s) in the mesosphere obtained from PMSE, characterized by velocities more than five standard deviations larger than the observed vertical wind variability. Using aperture synthesis radar imaging, the observed PMSE morphology resembles mesospheric bores, i.e., narrow along propagation (3–4~km) and elongated (>10~km) transverse to propagation direction. Additionally, our event presents a large vertical extent (± 3–4~km), resembling a “super bore”. Powerful vertical drafts, intermittent in space and time, emerge also in direct numerical simulations of stratified flows, predicting non-Gaussian statistics of vertical velocities. This evidence suggests that our event, and perhaps previous bores, might result from the interplay of gravity waves and turbulent motions.