Dynamics of the mesosphere/lower thermosphere transition region at mid and low latitudes (Invited)

The interface region between the mesosphere and lower thermosphere at mid and low latitudes remains one of the most poorly understood regions of the atmosphere. The altitude range between 90 and 110 km is characterized by rapid changes with height in the turbulence characteristics. Observational data have shown evidence for a transition from more isotropic to stratified turbulence at scales of a few hundred kilometers and, furthermore, the dominance of stratified turbulence in the region immediately above the nominal turbopause height before molecular diffusion becomes dominant. The turbulence characteristics not only influence the transport of mass, energy, and momentum, but may also affect the generation of bulk mean flows. The same region is characterized by large winds and shears. Winds similar to those that are observed are beginning to appear in general circulation models as the spatial resolution in the models improves, but the drivers for the winds are still generally unknown or poorly understood. The influence of plasma and electrodynamical processes become increasingly important with increasing height and can change the effective Coriolis force, which in turn affects planetary wave propagation. In addition, the observational evidence suggests that there is a rapid and generally unexplained increase in the magnitude of the vertical velocities across this transition region with vertical winds of 10 or more meters per second over periods of several hours. The drivers and effects of such large vertical winds are not understood at all. The unique dynamical properties of the mesosphere/lower thermosphere region will be described and arguments will be presented that critical parameters for understanding the various aspects of the dynamics of the region are the vorticity and divergence.