A quasi-geostrophic analysis on medium-scale waves near the midlatitude tropopause and their relation to the background state

Abstract

We examined medium-scale waves near the midlatitude tropopause and their background state in the framework of the quasi-geostrophic system. We made a 19-day continuous observation of the MU radar and radiosondes in April 1995. Using these observational data, we found that the active periods of the medium-scale waves are connected with extremely high values of poleward gradient of the stretching vorticity that are observed in the ridge phase of background synoptic-scale waves. Operational upper-air network data by JMA (Japan Meteorological Agency) showed that the characteristic vertical structure of static stability in the synoptic-scale ridge causes the large poleward gradient of the stretching vorticity within the latitude band of a few degrees. We also analyzed objective analysis data to examine horizontal distribution of quasi-geostrophic potential vorticity. Two kinds of meridional structure of the medium-scale waves are found: one has no node and the other has one node in the meridional direction. Both are located where horizontal gradient of stretching vorticity is large. These observational results are consistent with the theoretical consideration by Sato et al. (1998) that the medium-scale waves can be interpreted as internal modes trapped into the localized gradient of quasi-geostrophic potential vorticity near the midlatitude tropopause.