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3/6/2014 Seminar – Disturbances of the Wintertime Polar Upper Stratosphere and Lower Mesosphere: Observations, Modeling, and Mechanisms

Published on March 6, 2014

Speaker:Katelynn Greer (LASP, CU Boulder)
Date:3/6
Time:4:00pm
Location:SPSC W120

Seminar Abstract:

This investigation studies the inherent variability and dynamical mechanisms responsible for upper stratosphere / lower mesosphere (USLM) disturbances in the thermal structure of the wintertime polar region. USLM disturbances are intimately related to the onset and development of minor and major stratospheric warmings. These disturbances appear to contribute to the preconditioning of the middle atmosphere for SSWs by periodically weakening the polar vortex in the early winter season and preceding SSWs in the later winter season. In addition, USLM disturbances are strongly connected with an increase in planetary wave breaking at altitudes near 60 km and 75 km several days prior to the disturbance. Using the Whole Atmosphere Community Climate Model (WACCM4), USLM disturbances in the model reliably reproduce observations in terms of horizontal thermal structure, seasonal distribution of events, and vertical temperature profiles through the warm temperature anomaly (peaks at 2 hPa with temperatures in excess of 300 K).  Composites of the USLM disturbances illustrate the evolution, vertical extent, vertical motion and potential to support baroclinic instabilities. Conditions that differentiate the evolutionary paths of USLM disturbances, minor SSWs and major SSWs are explored. Potential vorticity analysis (including the Charney-Stern criteria for instability the role of baroclinic/barotropic instabilities) is used to elucidate the dynamics in the development of USLM events.  Broader impacts of these disturbances and the dynamics associated with them influence gravity wave generation/propagation, vertical air motion and chemical tracer transport.