Han-Li Liu (liuh@ucar.edu), HAO/NCAR
Mike Taylor (mtaylor@cc.usu.edu), Utah State University
Front Range Theatre, 1:15-3:15
Monday, June 18, 2001
In recent years, continuous sampling of the mesosphere by both UARS satellite and ground based observations are used to study rapid changes in the MLT region, especially around equinox period in association with the transition from winter-to-summer (and vice versa) conditions. These observations indicates the existence of large variations in airglow emission rates in the MLT region in late March around spring equinox (termed as "springtime transition" by G. Shepherd) (Shepherd et al., 1999; Stegman et al., 1992; Stegman and Espy, 1998). For example, measurements taken at Meteorological Institute of Stockholm University in 1992 showed that the greenline emission rates tripled in about 3 days followed by about 10 fold decrease in the next 3 days. A large change on a global scale during the same period of time is captured by the UARS/WINDII instrument. The ground based and, more recently, satellite temperature retrieval (Shepherd et al., 2001) also reveals rapid temperature changes around spring equinox that follow a temporal pattern similar to that found in the green line emission rates. On the other hand, exceptionally large perturbations in mesospheric temperature around fall equinox is recently reported by Taylor et al. (2001). Within the observation period of 1996-1999, this is a quite regular feature and it is shown that a temperature departure of up to 25-30 K from the nominal seasonal trend during a disturbed period of ~3-4 weeks.
Certain planetary wave activities may be closely associated with the equinox transition. Analysis of UARS/HRDI daytime horizontal winds, temperatures, and nighttime atomic oxygen in the mesosphere and lower thermosphere shows that 6.5 day waves peak around equinoxes (Talaat et al., 2000). Their study also shows that the horizontal structure of these variables are indictive of the gravest symmetric wave number one Rossby wave, and the analysis displays possible modification by the zonal mean wind.
Numerical simulations using the NCAR thermosphere-ionosphere- mesosphere-electrodynamics general circulation model (TIME-GCM) were also conducted to investigate possible mechanisms leading to the variations in airglow emission rates and mesospheric temperature around spring and fall equinoxes. It is found that mean circulation change and planetary wave transience can affect the constituents distribution and temperature in the mesosphere (Liu and Roble, 2000; Liu et al., 2001). Atomic oxygen distribution, for example, is sensitive to the mean residual circulation in MLT, which is critically dependent on the gravity wave forcing. Around spring equinox, increasing eastward gravity wave forcing in MLT induces an equator-ward/upward circulation, which decreases the peak atomic oxygen number density in the region. However, the model simulation for the spring equinox did not show the observed increase of atomic oxygen prior to its depletion, probably because no planetary wave forcing is considered in this simulation (Liu and Roble, 2000). In the fall equinox simulation (Liu et al., 2000) on the other hand, realistic planetary waves are forced at the lower boundary of the model. It is found that planetary wave transience during the transition period can lead to fast changes in both phase and amplitude of the planetary waves, and these changes may cause large variabilities in local temperature measurements similar to those reported by Taylor et al. (2001).
These observational and numerical studies suggest that the equinox period may be a time involving rapid dynamical changes on vast spatial scales (mean circulation, planetary waves, and internal gravity waves) in the stratosphere, mesosphere, and lower thermosphere. Therefore, more detailed studies of the equinox transition may provide important insight into the couplings of the dynamical processes from lower to upper atmosphere.
This workshop intends to bring people together to discuss these issues. The format of the discussion will be informal and number of viewgraphs will be limited to 6 or less per person. The list of speakers is: