The project "Understanding the Wave-Driven Circulation and Variability of the Polar Atmosphere through Coordinated Observation, Analysis, and Modeling' is designed to understand the weather and climate of the middle atmosphere: the stratosphere and mesosphere (10-90 km).

From a climate perspective the wintertime middle atmosphere is dominated by the polar vortex that extends through the stratosphere and into the mesosphere. The vortex is a large cyclone that is defined by a strong westerly jet.  From a weather perspective, in the Arctic large-and small scale waves with periods of between minutes and days travel upward through the atmosphere, interact with the winds and break and disturb the middle atmosphere vortex.  In some winters the waves cause the vortex to split, and the circulation is defined by a complex set of non-linear interactions, where large-scale planetary waves break and disrupt the mean flow, then this changed mean flow alters the propagation of small-scale gravity waves, which these changed gravity waves break and alter the mean flow. Each Arctic winter the level of disturbance and interaction is different, and so each winter provides a different test-case to understand these weather-and-climate interactions and test our understanding and models of the atmospheric general circulation. In those winters when the vortex is undisturbed the polar air mass is isolated and colder and there can be significant ozone loss. In disturbed winters the polar air mass mixes with warmer air from lower latitudes and the conditions for ozone loss do not occur. These wintertime disturbances are traditionally called Sudden Stratospheric Warming events (SSW), and also give rise to Elevated Stratopause events (ES), and provide a natural laboratory for understanding weather and climate.

Over the course of this project we have established a multi-year set of lidar observations of the weather of the middle atmosphere that documents the wave activity in disturbed and undisturbed winters. We have combined single-station lidar measurements with satellite data to understand the relative role of small-scale gravity waves (measured by lidar)  and large-scale planetary waves (measured by satellite) in the observed weather patterns.

Our observations have shown that while small-scale wave activity is traditionally understood to have large night-to-night variability, we find pronounced seasonal patterns in the Arctic winter where wave activity is elevated or depressed over periods of months or so in response to SSW and ES events.

Prompted by these observations, we have used model and reanalysis data to understand the physical interactions that underlie the observed weather.  Using free-running circulation models we have shown that the SSW and ES events can be generated spontaneously and extend from the stratosphere upward into the lower thermosphere. Furthermore, model scenarios show that the ES events are not just an extreme of SSW events, but can arise from small-scale wave activity alone. Thus small-scale wave activity can contribute to major disturbances in the middle atmosphere.The accurate representation of small-scale gravity waves in large-scale circulation models continues to be a major challenge in numerical modeling of the atmosphere. The studies provide benchmark scenarios for testing and further development of these models.

As part of this work we have extended the scope of remote sensing instruments and analyses. We have extended the lidar systems at the University of Alaska to provide measurements over a larger altitude range, and at smaller time scales. We have developed new robust algorithms for measuring waves in lidar data. We have also developed new algorithms for determining the structure of the vortex in the stratosphere and mesosphere using observations of carbon monoxide. The developm...

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