NATO Advanced Study Institute: The Stratosphere and its Role in Climate

Québec, Canada, 5-15 September 1995

Director : Guy Brasseur

In early September, 1995, 85 students and approximately 15 teachers from 25 countries gathered in the middle of the Laurentides, 100 km north of Montreal to attend one of the NATO Advanced Study Institutes sponsored by SPARC, and supported by NATO, the US National Aeronautics and Space Administration (NASA), the US National Science Foundation (NSF), the European Commission (EC), several agencies from Canada and Quebec, as well as the National Center for Atmospheric Research (NCAR).

The purpose of the Advanced Study Institute was to provide a detailed view on the coupled chemical, radiative and dynamical processes in the stratosphere that affect the global climate system. Several tutorial lectures focused on fundamental and theoretical questions, while others discussed more applied topics.

The role of dynamics and transport was first emphasised. It was explained that the general circulation in the stratosphere is driven primarily from momentum deposition associated with gravity wave breaking and planetary wave absorption. The concept of the transformed Eulerian mean circulation was presented. The role of tropical waves and their relations with the observed quasi-biennial oscillation were also discussed. The fundamental equations for the dynamics of the atmosphere were established. The theoretical concepts (including that of potential vorticity) were used to analyse and interpret observed situations.

Comprehensive lectures on radiative transfer in the atmosphere were also presented. The emphasis was on the penetration of solar radiation in the stratosphere and troposphere, and specifically the role of absorption and multiple scattering. Another aspect, which received much attention, was the role of terrestrial radiation. Simple techniques to solve the radiative transfer equations were presented, including the concept of bands models which are often used in practical applications. The theory used in satellite remote sensing applications was also presented and discussed in fair detail.

Several lectures were given on the chemistry and photochemistry of the middle atmosphere. The focus was on the processes that lead to the formation and destruction of ozone in the stratosphere. The specific roles of nitrogen, chlorine, and bromine compounds were discussed. Much attention was given to the importance of heterogeneous chemical reactions on the surface of aerosols and ice particles in polar stratospheric clouds. The mechanisms responsible for the formation of the Antarctic springtime ozone hole were presented. Finally, the role played by sulphate aerosols, especially after large volcanic eruptions such as that of Mt Pinatubo (1991) was discussed. The basic techniques used to probe the chemical composition of the stratosphere (in-situ and remote sensing) were intensively discussed.

In addition to tutorial lectures, several presentations focusing on specific subjects related to the physics and chemistry of the stratosphere were given. These included subjects such as the role of trace gases in radiative forcing, the potential importance of solar variability in atmospheric changes, observations of chemical constituents from space, the specific role of aerosols, the methods to develop numerical models of the atmosphere, the key findings provided by recent polar ozone campaigns, etc.

What was unique to this Advanced Study Institute was the active participation of the students in the preparation of a comprehensive textbook, which will summarise the lectures given during the school. Val Morin, the small village of the Laurentides which hosted the meeting, has offered a unique experience to an outstanding group of students and teachers. An unanimous conclusion was that other schools of that nature should be organised, especially for the 300 applicants who were not selected because of limitations in space and funding.

Guy Brasseur