Atmospheric dynamics

The atmosphere has been for a long time considered as a stratified medium with negligible interactions between the upper layers and the troposphere in which we live. However, recent studies have begun to reveal the key role that dynamics in the middle and upper atmosphere can play in both tropospheric weather and climate. A key part of this interaction is the understanding of the role of atmospheric oscillations, particularly gravity and planetary waves, in driving this interaction. Interactions of atmospheric oscillations with then mean-flow are at the origin of atmospheric mixing and much of the key large-scale atmospheric global circulation in the middle and upper atmosphere including the Brewer-Dobson Circulation and Quasi-Biennial and Semi-Annual Oscillations in the stratosphere and mesosphere.

Dynamics of the troposphere-stratosphere-mesosphere exchanges including contribution of gravity waves and planetary waves.

Atmospheric waves carry energy and momentum from one region to another. The origin and dynamics of planetary and gravity waves are very different. Planetary waves owe their existence to the equator to pole gradient of potential vorticity and are produced by flow over orography and by contrasts in temperature between the land and ocean. They are most important in the middle and high latitudes and can lead to dramatic deviations of the flow from its climatological mean, as in for example a sudden stratospheric warming when the normal, winter-time polar vortex breaks down due to the propagation and breaking of a planetary wave packet from the troposphere. Critical to understanding and prediction of sudden stratospheric warming events is to understand the location and structure of shear-zones in the mean-flow where planetary waves break. High-resolution observations of the stratosphere could greatly improve our understanding of these so-called ‘critical’ layers.

Gravity waves are more ubiquitous and exist on a range of spatial scales from planetary to a few kilometres. They are produced both by flow over orography and by strong convection for example in thunderstorms. Gravity waves play an important role both in setting the mean climate of the stratosphere and mesosphere and in generating the predictable tropical oscillations of mean wind speed which can lead, in general, to enhanced predictability of the climate system. Most small-scale gravity waves are not resolved by typical climate models and only partially resolved by weather forecasting models. Climate models therefore must parameterize gravity waves to ensure an accurate simulation of middle and upper atmosphere mean climate and variability. Many parameters of the gravity wave parameterizations and particularly gravity wave source parameters are uncertain due to a lack of long-term high-resolution observations.

The amplitudes of the atmospheric tides are large in the middle atmosphere. Tides generated by stratospheric ozone and by the water vapour in the upper troposphere, can induce some systematic difference between non-simultaneous measurements and represent a key issue for satellite validation and long-term variability when times of measurements are changing. If the tidal theory is well know, the exact amplitude and phase characteristics are not yet well characterized as few measurements can observed them. ARISE measurements will allow to validate tidal simulations using numerical models that will be used for systematic corrections of satellite comparisons and trend estimates.

In a higher frequency range, measurements of infrasonic atmospheric wave present a large interest for monitoring of volcanoes and extreme events. Monitoring infrasound from volcanoes provides relevant information about ash injection in the atmosphere when satellite information is not available, because of cloud coverage. The International Airways Volcano Watch Operation Group (IAVWOPSG) supported by the International Civil Aviation Organization encourages the Improvement of tools for detecting and forecasting of volcanic ash. They recognize the interest of the infrasound technology for such purpose. Monitoring of extreme events as thunderstorms and cyclones presents also an interest to determine the evolution of such events with climate change.

Much progress can be expected from the ARISE infrastructure in atmospheric modelling, weather forecasting and monitoring of extreme events in relation with civil security applications, from its new 3D images of atmospheric state and its spatial and temporal variability.

More on ARISE results.