Experiments with WACCM: A sensitivity study.

OUTLINE• Why is a parameterization of gravity waves important?

• Middle atmosphere (stratosphere + mesosphere) depends critically on momentum deposition by unresolved gravity waves.

• Gravity waves scheme and tropospheric sources• Simulations• A flavor of what happens when different assumptions are made.• Conclusions

THE ROLE OF GRAVITY WAVES IN M.A. CLIMATE

• Zonal wind reversal in mesosphere

• Winter polar mesopause warmer than summer polar mesopause

• Higher stratopause in winter than in summer

• Zonal wind strength in the lower stratosphere

• Tropical wind oscillations

• Implementation

Gravity Waves Schemes

1. McFarlane (1987) parameterization is used to account for the momentum deposition due to orographic gravity waves (c=0).

2. Spectrum Base case: gravity wave source is zonally uniform, operates continuously with an intermittency factor of 1/32.

a) Spectrum range is from -80 m/s to +80 m/s by 2.5 m/s (65 waves in total)

b) The spectral shape is gaussian, centered about the wind at the source level.

c) Source level is the first model interface above 500 hPa.

d) In the tropics (between 30S and 30N), the gravity wave spectrum is launched with 1/5 of the amplitude of the extra-tropics (to avoid generation of tropical wind oscillations).

e) Temporal functions are intended to account for the observed seasonal variation of forcing (mid-latitude fronts), reducing the forcing in summer but increasing it in winter.

OTHER GRAVITY WAVES SCHEMES3. Spectrum Beres (2004; JAS) scheme (note: Beres =

Richter):

a) Above convection, the source spectrum is calculated as a two-dimensional momentum flux, its magnitude dictated by the underlying convection scheme (Zhang and McFarlane in our case).

b) The most robust part of this scheme is the momentum flux spectrum which is derived from the tropospheric heating depth and wind. Both these quantities are well represented in the model.

c) ad hoc assumptions include the frequency distribution (red noise) and the fraction of the model cell that is occupied by convection. The latter is the greatest uncertainty.

4. Spectrum Charon and Manzini (2002; JAS) scheme:

a) The Frontogenesis function (F) identifies locations where the circulation is such that it tightens the magnitude of the temperature gradient (Hoskins, 1982),

b) Subjectively, a critical value of Frontogenesis function is chosen: When and where the critical values is exceeded, gravity waves are launched like in the standard scheme.

SUMMARY OF SIMULATIONS• REF: Reference simulation with the standard gravity wave scheme

• UnS: Source spectrum is Gaussian centered at zero phase speed

• nES: As in REF but spectral bins are not equally spaced

• HLL: As in REF but launch level is located at 100 hPa

• TrS: Tropospheric sources (convective and frontal)

Fv-dynamics at 1.9x2.5 horizontal resolution. Time step is 1800 s, dynamics is solved on a time step of 225 s to ensure stability. 66 vertical levels, covering from the surface to about 150 km. Fully interactive chemistry, including physical parameterizations for the upper atmosphere. Simulations are carried out for 1995 chemical composition, solar minimum conditions, and mean climatological SST. Each simulation is at least 20 years long.

TROPICAL OSCILLATION(S)

ZONAL WIND JANUARY

SEASONAL EVOLUTION OF TEMPERATURE @ SOUTH POLE

140 K 160 K 170 K

130 K 110 K

STRATOSPHERIC WARMINGS(Dec 01 – Feb 28)

Simulation

(length in years)

# Major w. (60N; 10 hPa)

(# events/year)

Statistically distinguishable

from REF?

REF (38y) 10

(0.26±0.037)

UnS (26 y) 12

(0.46±0.097)

Y

nES (21y) 5

(0.24±0.070)

N

HLL (21 y) 1

(0.05)

Y

TrS (25 y) 4

(0.16±0.047)

Y

NCEP/NCAR (45y) 27

(0.6±0.100)

Y

Linear correlation of the NH wintertime PC of the leading annular mode at 10 hPa against all other levels.

Christiansen (2005)

All time scales Time scales > 30 days

Autocorrelation at 10 hPa

(e-folding time scale)

Linear correlation of the NH wintertime PC of the leading annular mode at 500 hPa against all other levels.

All time scales Time scales > 30 days

Christiansen (2005)

Autocorrelation at 500 hPa

(e-folding time scale)

CONCLUSIONS

• A gravity wave parameterization is a powerful tool that defines to first order the climate of the middle atmosphere.

• Several metrics have been used to determine those influences:

A. Changing the spectral center (i.e. shifted vrs not shifted) has profound effects in the lower and mid-stratosphere, reducing significantly some errors. However, the upper mesosphere is severely degraded. This may indicate that

1. The tropospheric filtering of gravity waves is a not a trivial matter. Different launching level?

2. Spectral components of the spectrum near the tails are important for mesospheric simulations.

B. Spectral resolution does not seem to affect the stratosphere but it affects adversely the mesosphere:

1. Role of the spectral components near the tails in controlling the mesospheric simulation.

2. We can benefit (performance) from a spectrum with fewer waves.

CONCLUSIONS

• Launching waves at tropopause level does not affect the stratospheric simulation significantly.

• Experimentation with tropospheric sources produces significantly better results in the tropics where waves are mostly convectively generated. Although the extra-tropical simulation is not improved (nor degraded greatly) removing ad hoc parameters is considered an improvement.

• Stratospheric warmings (a critical metric of the goodness of a stratospheric simulation) are still deficient. This could result from either a poor representation of the planetary wave structure that emerges from the tropospheric model, or a lack of ENSO events in the climatological SST.

• Modest changes in aspects of the gravity wave parameterization do not affect significantly the tropospheric structure. On the other hand, the middle atmospheric behavior can change substantially.