Observations of gravity wave activity in the middle stratosphere on both sides of the Scandinavian mountains

Atmospheric gravity waves are frequently observed in the polar winter stratosphere. The existence of polar stratospheric clouds in the Arctic is closely connected to the occurrence of orographically generated gravity waves. Such waves modulate the atmospheric temperature profile. We measure temperature profiles in the middle atmosphere with the ALOMAR O3-lidar in northern Norway and the Bonn University lidar at Esrange in northern Sweden. Both instruments have been in operation for many years, resulting in a comprehensive data base. We have analysed the data from several winters with respect to atmospheric gravity waves. The gravity wave potential energy density is a measure for the wave activity, which can be derived from the lidar data. Gravity wave activity over ALOMAR is generally as strong as over Esrange. Both vary from year to year and during the year. The difference between gravity wave activities at ALOMAR and Esrange is strongest from December to February. We observe a strong dependence of gravity wave activity on the ground wind direction at ALOMAR while the distribution is isotropic at Esrange. A comparison of the excitation and propagation conditions for orographically excited gravity waves with the observed gravity wave potential energy density reveals that a large fraction of the observed gravity waves has other sources than orography.