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Dynamical Time Scales in the Extratropical Lowermost Stratosphere

Universität Hamburg . Zentrum für Marine und Atmosphärische Wissenschaften . Bundesstrasse 53 . D-20146 Hamburg . Germany. Dynamical Time Scales in the Extratropical Lowermost Stratosphere. T. Kunz (1), K. Fraedrich (1), R. J. Greatbatch (2)

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Dynamical Time Scales in the Extratropical Lowermost Stratosphere

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  1. Universität Hamburg . Zentrum für Marine und Atmosphärische Wissenschaften . Bundesstrasse 53 . D-20146 Hamburg . Germany Dynamical Time Scales in the Extratropical Lowermost Stratosphere T. Kunz (1), K. Fraedrich (1), R. J. Greatbatch (2) (1) Meteorological Institute, University of Hamburg, Germany (2) Department of Oceanography, Dalhousie University, Halifax, NS, Canada AGU Chapman Conference on The Role of the Stratosphere in Climate and Climate Change, Santorini, Greece, 24 – 28 Sept 2007

  2. Dynamical Time Scales in the Extratropical Lowermost Stratosphere Outline • Radiative decay experiments • Effective decay time scales • Stochastically forced simulations • Dynamical decorrelation time scales • (3) Summary AGU Chapman Conference on The Role of the Stratosphere in Climate and Climate Change, Santorini, Greece, 24 – 28 Sept 2007

  3. Motivation • Stratospheric memory exceeds tropospheric memory • (e.g., decorrelation time of NAM anomalies) • potential for additional tropospheric forecast skill • Winter time stratosphere: • longest memory located in lowermost stratosphere • ? longer radiative damp. time / zonal mean secondary circulation / waves ? • What is the contribution of the zonal mean circulation to • time scale of stratospheric anomalies ? • in particular, longer time scale in lowermost stratosphere ? • See, e.g., Baldwin et al. (2003)

  4. Motivation • Stratospheric memory exceeds tropospheric memory • (e.g., decorrelation time of NAM anomalies) • potential for additional tropospheric forecast skill • Winter time stratosphere: • longest memory located in lowermost stratosphere • ? longer radiative damp. time / zonal mean secondary circulation / waves ? • What is the contribution of the zonal mean circulation to • time scale of stratospheric anomalies ? • in particular, longer time scale in lowermost stratosphere ? • See, e.g., Baldwin et al. (2003)

  5. (1) Radiative decay experiments Decay time scale of damped zonally symmetric anomaly Quasi-Geostrophy, zonally symmetric, beta-plane, Boussinesq QG potential vorticity eq.: frictional damping radiative damping See, e.g., Garcia (1987, JAS), Scott & Haynes (1998, QJRMS)

  6. (1) Radiative decay experiments Decay time scale of damped zonally symmetric anomaly Quasi-Geostrophy, zonally symmetric, beta-plane, Boussinesq With Effective decay time: QG potential vorticity eq.: where See, e.g., Garcia (1987, JAS), Scott & Haynes (1998, QJRMS)

  7. (1) Radiative decay experiments • Relevance of scale dependence for polar stratospheric anomalies • Radiative decay experiment with numerical model (PUMA) • Primitive equations on rotating sphere (T42L30, zmax=105km) • zonally symmetric • Radiative damping – uniform time scale • Rayleigh friction in PBL • Initial conditions: • State of rest + small initially balanced anomaly T’(lat, z) • Vertical T-profile: U.S. standard atmosphere

  8. (1) Radiative decay experiments Initial conditions: T-anom, U Stratopause Tropopause PBL

  9. (1) Radiative decay experiments T+ T– Decay of anomaly: Mechanism: Secondary circulation compensates rad. damping radiative heating/cooling ageostrophic velocity

  10. (1) Radiative decay experiments T+ T– Decay of anomaly: Mechanism: Secondary circulation compensates rad. damping radiative heating/cooling ageostrophic velocity 30° half width °lat

  11. (1) Radiative decay experiments T+ T– Decay of anomaly: Mechanism: Secondary circulation compensates rad. damping radiative heating/cooling ageostrophic velocity 2-3 times slower 30° 30° half width °lat

  12. (1) Radiative decay experiments T+ T– Decay of anomaly: Recirculation at lower levels radiative heating/cooling ageostrophic velocity 2-3 times slower than radiatively lower stratosphere? slower decay

  13. (1) Radiative decay experiments Decay time scale in lower stratosphere relative zonal wind decay: , at 68° (max. u-anom.) Effective decay time 2-3 times slower than radiatively e -1 pressure longer decay time at lower levels lagged maximum >1 time

  14. (2) Stochastically forced simulations • Time dependent zonally symmetric zonal wind forcing • Decay time scale • decorrelation time • Model forcing: • radiative damp. • frictional damp. in PBL • small amplitude u-forcing Gu • g2(t): AR(1) with prescribed • Initial conditions: • State of rest, U.S. Stand. Atm. Zonal wind forcing

  15. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time: T at 7.5 hPa Zonal wind forcing 30° half width °lat

  16. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time: T at 7.5 hPa close to effective decay time Zonal wind forcing 2-3 times slower than rad. 30° half width °lat

  17. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time: u at 7.5 hPa dyn. memory irrelev. Gu quasi white close to effective decay time 30°

  18. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) 7.5 hPa pressure 175 hPa

  19. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) ~2.5 times longer than rad. damp. time 7.5 hPa pressure 175 hPa

  20. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) ~2.5 times longer than rad. damp. time 7.5 hPa pressure longer decorrelation than upper stratosph. x 1.28 175 hPa but small variance

  21. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) Faster frict. damping only short periods retained at surface larger fraction of mass flux in PBL less recirculation at low. stratosph. 7.5 hPa pressure x 1.28 175 hPa

  22. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) 7.5 hPa pressure x 1.28 175 hPa

  23. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) 7.5 hPa x 11 pressure x 1.28 175 hPa

  24. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) 7.5 hPa x 2.5 pressure x 1.10 175 hPa

  25. (2) Stochastically forced simulations Time dependent zonally symmetric zonal wind forcing Decorrelation time , vertical profile (at 68°, max. Gu) Conceptually, related to time scale of tropospheric planetary wave var. Fast forcing mem. above tropop. strongly increased Slow forcing mem. above tropop. weakly increased 7.5 hPa pressure 175 hPa

  26. (3) Summary Very simple model setup: PE, zonally symm., small ampl.; const heating rate Dynamical time scales in Stratosphere / Lowermost Stratosphere ? Contribution of zonally symmetric circulation ? Effective decay time scales (decay experiments) at upper stratospheric levels: 2 – 3 x rad. time scale at lower stratospheric levels: slower decay (recirculation above surf.) …for typical config. (Rossby rad., merid. scale, distance from surf.) Decorrelation time scales (stochastically forced experiments) at upper levels: close to eff. decay time …for… fast forcing close to forc. time scale …for… slow forcing at lower levels: increased decorr. times, up to ~ 30% longer than above Relative increase: Foring time scale Memory just above tropopause fast forcing much longer memory slow forcing little additional memory Slower decay at low levels? Longer decorr. time at dist.? Interaction with surf.?

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