1. Background Tropopause theta composites Summary Development of TPVs is greatest in the Baffin Island vicinity in Canada, with development possibly having connections to the northern Pacific Ocean. TPVs often incur a period of deepening, for reasons that are unclear, but may have links to water vapor which can lead to the destruction of potential vorticity conservation. WRF simulations This poster may be downloaded from: http://www.atmos.washington.edu/~scavallo/cavallo_hakim_ams05.ppt Life cycles of tropopause polar vortices Steven M. Cavallo and Gregory J. Hakim University of Washington Poster JP2.1 Tropopause vortices are ubiquitous disturbances that play an active role in extratropical weather. Despite their abundance, particularly over polar latitudes, there remain a number of fundamental questions regarding their origin and dynamics. Over the arctic in particular, surface arctic cyclones are relatively better documented than are tropopause polar vortices (TPVs). Observations have shown that TPVs can drift from polar regions into jet stream regions enhancing the possibility of surface cyclogenesis. This study examines the life cycles of observed TPVs to gain a better understanding of the physical processes that govern their growth and decay. We first survey historical data to locate regions with highest vortex genesis, and then use numerical model simulations to test a hypothesis that diabatic effects play a significant role in the structure and lifecycle of TPVs. Distinguishing between waves and vortices Vortices are very localized structures making them difficult to analyze in fields such as geopotential height Tropopause  is a dynamical tracer, thus if there exist closed contours then a fluid parcel located within these closed contours is bound to stay within these contours and must travel with the vortex. Number of tropopause polar cyclones that developed within a 2.5° latitude by 7.5° longitude box normalized by cos(latitude) centered at the location. The 65°N line (bold black) is shown for reference. Observations Composite of tropopause  anomalies from TPV genesis locations inside 2.5° latitude by 7.5° longitude boxes centered at each location. Composites of tropopause theta along the 1.5 PVU surface were created from NCEP/NCAR reanalysis data for the period 1948- 1999. Only those cyclones that spent 60% of their lifetime north of 65°N and had lifetimes over 2 or more days were kept in the analysis. Vortex  amplitude as a function of age for the vortices lasting at least 5 days (120 hours). Circles correspond to the cyclones while squares the anticyclones (Figure from Hakim & Canavan 2005). Skewt plots from radiosonde observations at YUX (Hall Beach, NT) at 00 UTC on (top left) 19 Nov. 2004 (top right) 20 Nov. 2004, (bottom left) 21 Nov. 2004, and (bottom right) 22 Nov. 2004. Note the tropopause height drop to ~700 hPa on 11/20. Radiosonde Observations at Hall Beach, Nunavut, Canada WRF model tropopopause pressure and winds on 2 PVU surface for times corresponding to soundings on left. Magenta crosses mark the radiosonde stations. WRF simulations corresponding to radiosonde observation times in center plots were performed on a 30km grid using GFS initial and boundary conditions. WRF provides a way to examine TPV life cycles as the observations and simulations agreed reasonably well several days after initialization. However, GFS analyses are not always accurate. This suggests that GFS covariances are suboptimal for these phenomena, thus we employ an Ensemble Kalman Filter (EnKF) which has flow dependent covariances. An EnKF also provides a sample for which to explore statistical relationships, as is shown to the right. WRF EnKF simulations Similar setup as above for WRF model, but using an Ensemble Kalman Filter with 90 ensemble members where simulated radiosonde observations at actual stations are assimilated every 12 hours. Tropopause  as a function of time from the EnKF mean analyses is plotted in the middle, with red circles highlighting the time just before rapid deepening and the time of maximum strength. Arrows point to the corresponding ensemble mean analysis tropopause  (on the 2 PVU surface) fields. Ensemble covariance plots The sensitivity of some metric, y (here tropopause  ), at a later time to changes in the state, x, at an earlier time is given by: Two tropopause vortices as seen in the tropopause  field over northern Canada on November 19, 2004. Both an anticyclonic (in red) and a cyclonic (in blue) vortex are located adjacent to each other. Contour lines of  are at 5K intervals from 260-290K. (Left): Correlations of select state variables at the vortex core as a function of height. (Right): Sensitivity of tropopause  at 12 UTC 21 November 2004 for a 1 g/kg change in 500 hPa water vapor mixing ratio everywhere at 6 UTC 19 November 2004. In the example below, x is an ensemble state matrix on 19 November 2004 at 06 UTC, and y is the ensemble tropopause  averaged within the 280 K contour (normalized by the standard deviation) on 21 November 2004 at 12 UTC. The sensitivity of tropopause  at 6 UTC 19 November 2004 when 600 hPa heights are changed by 1 m everywhere on the domain at 6 UTC 19 November 2004. Here, y is tropopause  averaged within the 280 K contour (shown by the gray contour) and normalized by the ensemble standard deviation, and x is the 600 hPa heights.