SPRG Seminars
February 25, 2014:
"The Polar Vortex and Winter Weather Disturbances in the Middle Atmosphere"
Katelynn Greer, University of Colorado, Boulder
The polar winter middle atmosphere is a dynamically active region that is dominated by the polar vortex and primarily driven by wave activity. Planetary waves intermittently disturbed the region at different levels and the most spectacular type of disturbance is a major Sudden Stratospheric Warming (SSW). However, other types of extreme disturbances occur on a more frequent, intraseasonal basis. One such disturbances are synoptic-scale "weather events" observed in lidar and rocket soundings, soundings from the TIMED/SABER instrument and UK Meteorological Office (MetO) assimilated data. These disturbances are most easily identified near 42 km where temperatures are elevated over baseline conditions by a remarkable 50 K and an associated cooling is observed near 75 km. As these disturbances have a coupled vertical structure extending into the lower mesosphere, they are termed Upper Stratospheric/Lower Mesospheric (USLM) disturbances. A discussion of the dynamical mechanisms responsible for USLM disturbances using the above mentioned observations in addition to model outputs from the Whole Atmosphere Community Climate Model (WACCM4) is presented. Results indicate that WACCM reliably reproduces USLM disturbances. Analysis of planetary wave breaking and EP-flux of individual and composite USLM events indicate an increase in breaking near the 0.1 hPa level, approximately 10 km above the extreme thermal anomaly at the stratopause in the days leading up to the peak of the event. Vertical coupling of the atmosphere during this event is illustrated in the progression of these events and their impact on the thermal structure, zonal mean wind, polar vortex and conditions that have the potential to support a secondary baroclinic instability (including the Charney-Stern criteria for instability the role of baroclinic/barotropic instabilities). Broader impacts of these disturbances and the dynamics associated with them influence gravity wave generation/propagation, vertical air motion, chemical tracer transport, precondition of the atmosphere for SSWs and the potential to couple with the thermosphere through tides.