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 Universität Hamburg. Zentrum für Marine und Atmosphärische Wissenschaften. Bundesstrasse 53. D Hamburg. Germany AGU Chapman Conference on The Role of the Stratosphere in Climate and Climate Change, Santorini, Greece, 24 – 28 Sept 2007

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

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)

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)

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

(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)

Relevance of scale dependence for polar stratospheric anomalies Radiative decay experiment with numerical model (PUMA) Primitive equations on rotating sphere (T42L30, z max =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 (1) Radiative decay experiments

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

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

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

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

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

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

(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 G u g 2 (t): AR(1) with prescribed Initial conditions: State of rest, U.S. Stand. Atm. Zonal wind forcing

(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

(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 30° half width °lat 2-3 times slower than rad.

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

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

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

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

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

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

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

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

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

(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 forcingmuch longer memory slow forcinglittle additional memory Slower decay at low levels? Longer decorr. time at dist.? Interaction with surf.?