Evidence for critical level filtering of atmospheric gravity waves

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Abstract
Radiosonde measurements were used to investigate gravity wave activity within the lower stratosphere above the Canadian High Arctic. The sondes were launched from the weather station at Eureka (80°N, 86°W) on Ellesmere Island. Gravity waves were detected by the fluctuations they induced in temperature and the associated perturbation potential energy density was used to gauge the amount of wave activity. It was found that, over a 15 month period, there were isolated episodes of enhanced wave activity rather than any distinct seasonal cycle. Profiles of wind velocity were also measured and used here to show that periods of high wave activity occurred only when there would be no (or very little) critical level filtering of the stationary waves generated by flow over the rough terrain in the vicinity of Eureka. It was also found that the potential energy spectral density was enhanced at all resolvable scales during the periods of high wave activity.