Measurements of gravity wave activity in the lower stratosphere by Doppler lidar

Abstract

This study presents results obtained with a Rayleigh‐Mie Doppler wind lidar on mesoscale fluctuations of horizontal winds in the lower stratosphere. The instrument, which is located at the Observatory of Haute‐Provence, France (44°N, 6°E), and its capabilities are briefly described. The technique enables the computation of the kinetic energy content in the [13 km; ≈20 km] altitude range, as well as the vertical wavenumber and apparent frequency spectra, respectively in the [≈300 m; 7 km] and [20 min; ≈10 hours] spectral domains. A large variability of gravity wave activity is found. The kinetic energy can be as low as 3 J kg−1 or as high as 50 J kg−1 with an annual mean of 10 J kg−1. This variability seems to be rather independent of the season, and no annual cycle of gravity wave activity has been detected. As an example of mesoscale fluctuations which may be responsible for the variability of the spectra, we analyzed a high‐amplitude (≈10 m s−1), inertio‐gravity wave detected in the lower stratosphere and propagating energy upward. According to this, the vertical wavenumber spectrum does not show any significant annual variation but exhibits important variability at shorter timescales. The mean spectrum has a spectral index of −2.8 in the high wavenumber regime and a characteristic vertical wavelength (1/m*) of 2 km. The apparent frequency spectrum exhibits a −1.7 slope in the whole frequency domain studied.