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Dynamical Time Scales in the Extratropical Lowermost Stratosphere

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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

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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