Observations of Middle Atmopshere Dynamics by Na Lidar

Atmospheric gravity waves have a significant impact on the dynamics of the middle atmosphere. For instance breaking gravity waves lead to a momentum flux convergence and an acceleration of the background flow. Due to the filtering of gravity waves in the stratosphere, the acceleration of the background flow in the summer polar mesosphere leads to a southward and upward drift, cooling the summer polar mesopause to 140 K and sometimes colder. The summer polar mesopause is the coldest place on or near Earth. Although there is little water vapour in the mesosphere, these temperatures and the presence of condensation nuclei lead to the formation of sufficient numbers of ice particles: The Polar Mesosphere Summer Echo (PMSE) particles and noctilucent cloud (NLC ) particles. Although the principal mechanisms sketched here have been known for several decades, several of the detailed processes are still little known. - For each idealized gravity wave packet, the sum of the background wind, tides, planetary waves, a nd all other gravity waves and turbulence in the same volume constitute the background medium through which the gravity wave propagates. Any complete global circulation model must include these processes in addition to radiation, greenhouse gases, and met eorology. Our observational results will help to better parametrize gravity waves in global circulation models. - Other lidar groups as well as we have observed Sudden Sodium Layers (SSL), sometimes also called sporadic Na layers. Many of the observed cas es occurred together with sporadic E layers. Nevertheless, several formation mechanisms are still necessary to explain all existing observations. Our lidar observations of Na density, temperature and wind collocated with many complementary measurements su ch as MF radar, ALOMAR Wind radar (VHF), EISCAT and scientific rocket launches from Andøya Rocket Range will allow us to investigate the occurrence and formation mechanisms of SSL.