Gravity waves and tides in the polar atmosphere above ALOMAR

Scope

Gravity waves are periodic oscillations of temperature, density, pressure and wind which can be observed in the entire middle atmosphere. Their periods range between a few minutes and many hours. Gravity waves have horizontal wavelengths between 10 km and a few thousand kilometer. The vertical wavelengths of these waves lie between a few kilometer and 30 km. Their are different excitation mechanisms for gravity waves like orographic obstacles as mountains or mountain ranges, strong convection cells like strong thunderstorms, the jet-stream in the tropopause region or wave-wave interaction.

Gravity waves are an important driver for the atmospheric circulation because they transport energy and momentum from the excitation altitude upwards in the atmosphere. The energy and momentum is transferred to the background atmosphere at larger heights when the waves break. Through this mechanism they drive amongst others the residual meridional circulation from the summer pole to the winter pole which is the cause of the very cold summer mesopause. Only through the effect of the gravity waves the mesopause region deviates from radiative equilibrium conditions by up to 90 K and only then ice particles can form in the summer mesopause region that are observed with the ALOMAR RMR lidar as noctilucent clouds and with the ALWIN VHF radar as polar mesosphere sommer echoes (PMSE). Breaking gravity waves also generate turbulence which is observed with the CONE-Instrument in the mesosphere and with balloon instruments in the stratosphere.

Methods

We use the ALOMAR RMR lidar to study gravity waves in the temperature measurements above ALOMAR. Some results of these analyses are presented on this page. Wind measurements in the lower and middle atmosphere are at present only done with radar instruments and are analysed for gravity waves in the troposphere/lower stratosphere.

Determining gravity wave amplitudes and energies in the RMR lidar measurements is done by integrating the lidar raw data from one hour, calculating the corresponding temperature profile and then shifting the integration time by 10 min. After the subtraction of the mean of all the single profiles during one measurement, the remaining temperature fluctuations are due to gravity wvaes and tides. More details about this procedure can be found in the description of the derivation of gravity wave potential energy densities from RMR lidar measurements. An example for the gravity waves observed with the RMR lidar is given in the figure. It shows the temperature deviations from the the night-mean during the RMR lidar measurement on 2/3 February 2001 17 UT–05 UT. One can see downward propagation of the wave phases. This corresponds to upward propagating gravity waves. The figure also shows that we observe above ALOMAR always various waves with different parameters simultaneously.

Temperature deviations

Example of the temperature deviations from the night-mean during the RMR lidar measurement on 2/3 February 2001 17 UT–05 UT. The crosses in the upper part of the diagram mark the single profiles. Regions where the temperature is higher than the night-mean temperature are shown in yellow and red, temperatures below the night-mean are marked in blue. One can see downward phase propagation. This corresponds to upward propagating gravity waves.