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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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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


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


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.:

    frictional damping

    radiative damping

    See, e.g., Garcia (1987, JAS), Scott & Haynes (1998, QJRMS)


    (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)


    (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


    (1) Radiative decay experiments

    Initial conditions:

    T-anom, U

    Stratopause

    Tropopause

    PBL


    (1) Radiative decay experiments

    T+

    T–

    Decay of anomaly:

    Mechanism:

    Secondary circulation

    compensates rad. damping

    radiative heating/cooling

    ageostrophic velocity


    (1) Radiative decay experiments

    T+

    T–

    Decay of anomaly:

    Mechanism:

    Secondary circulation

    compensates rad. damping

    radiative heating/cooling

    ageostrophic velocity

    30°

    half width °lat


    (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


    (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


    (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


    (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


    (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

    2-3 times

    slower

    than rad.

    30°

    half width °lat


    (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°


    (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


    (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


    (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


    (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


    (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


    (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


    (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


    (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


    (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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