EQUATORIAL WAVES EXCITED BY THE TIEDTKE CONVECTIVE SCHEME

STUDIED WITH WACCM

Acknowledgements: Byron Boville, Marco Giorgetta (MPI, Hamburg), Jim Hack, Phil Rasch, Fabrizio Sassi, John Truesdale, Stacy Walters

This work was supported by NSF grant ATM-0221628

Lucrezia RicciardulliRemote Sensing Systems, Santa Rosa, California

Rolando R. GarciaNational Center for Atmospheric Research, Boulder, Colorado

The Model• WACCM3: Whole Atmosphere Community Climate

Model, v.3

• Surface to 140 km (CCSM up to 40 km)• Standard Resolution: ~ 1.9o x 2.5o or 4o x 5o horizontal

1.5 – 3 km vertical (66 levels)

• Finite volume dynamical core • Interactive chemistry disabled in our experiments

Convective variability at short time scales and wave excitation

LOCAL FORCING GLOBAL EFFECT

QBO

Equatorial waves excited by convection transport energy and momentum in the middle atmosphere and they affect tropical dynamics. Phenomena like the Quasi-Biennial Oscilation (QBO) and Semi Annual Oscillations (SAO) in the winds are driven by momentum deposition by these waves on the mean flow.

From Baldwin et al, Rev. Geophys, 2001

SHORT-TERM VARIABILITY OF CONVECTIVE HEATING RATE

TROPOSPHERIC CONVECTIVE

HEATING RATE 20°N-20°S

(K/day)2

GCI

ISCCP

SATELLITE(estimate from

DCC)

WACCM

CONTROL(Zhang-McFarlane

+ Hack)

WACCM(Hack only)

WACCM

RAS(Relaxed Arakawa

Schubert)

Variance

>10 days2.1 2.0 14.7 2.7

Variance

2-10 days4.5 1.1 17.0 1.8

Variance

< 2 days11.7 0.5 1.8 1.5

The problem: Models with these convective schemes underestimate temporal variability of convection at short periods and, consequently, underestimate wave excitation and propagation in the middle-upper atmosphere

We analyzed 3 convective schemes already available for this model, and looked at variability at short time scales (less than 2 days, necessary for wave excitation)

OUR APPROACH: THE TIEDTKE CONVECTIVE SCHEME

• We implemented the Tiedtke (1989) parameterization from ECHAM into WACCM because ECHAM shows good wave excitation (and produces a QBO when run at sufficiently high vertical resolution).

• Tiedtke is a mass flux scheme based on moisture convergence.

• We had serious problems with excessive low clouds (global low cloud fraction ~ 65% versus 42% in control case) and TOA balance (off by 30 W/m2)

• After making sure this was not a BUG, we fixed this problem by calling the Hack scheme after the Tiedtke scheme, as is done in standard WACCM, which uses Zhang McFarlane + Hack.

• Apparently this is necessary because the Tiedtke scheme can support only ONE type of convection for an atmospheric column at each time step (deep, mid-level OR shallow). Tiedtke alone was not efficient at removing instability at lower levels (~900 mb).

SOME 5-YR DIAGNOSTICS: PRECIPITATION RATE

WACCM TIEDTKE

WACCM CONTROL

XIE-ARKIN

SEASONAL CYCLE

No double ITCZ, improved N. Pacific storm track,better seasonal cycle, but too intense, “spotty”precipitation areas in warm pool.

OTHER VARIABLES:MOST STATIC ENERGY PROFILE

• One interesting change we noticed with CAM is that the vertical profile of the moist static energy greatly improves over the ocean with Tiedtke. (But it degrades significantly over certain continental locations, e.g., the Great Plains –not shown) .

• OBS

CONTROL

TIEDTKE

Marshall Islands Midway Island

Changes in other variables: • Some change in TOA radiative balance, same magnitude as

control (~4 W m-2) but different sign. • Small changes in clouds, precipitable water and latent heat flux• Improvement in annual surface winds• CONCERNS: increase in SW cloud forcing, significant increase

in cloud liquid water, Land temperature too warm, land diurnal cycle in precipitation small, ocean heat transport degenerated, large changes in SLP in winter extratropics.

These changes require a closer look and more tests to identify

the reasons behind them. But this was beyond the scope of our project.

VARIABILITY AT SHORT TIME SCALES (from 6 hr to monthly)

TIEDTKE CONTROL

CONVECTIVE HEATING RATE (K/day) ~ precipitation

GCI

SATELLITE

(estimated)

WACCM

4x5

CONTROL

WACCM

4X5 TIEDTKE

WACCM

1.9X2.5

TIEDTKE

Variance

>10 days2.1 0.7 4.7 5.2

Variance

2-10 days4.5 0.4 3.3 8.2

Variance

< 2 days11.7 0.9 6.2 5.5

Total variance

18.2 2.0 14.3 18.9

Tiedtke scheme implemented in low and high horizontal resolution WACCM

Wave activity flux (EP flux)

Westward waves

GCI

SATELLITE

WACCM

4x5

CONTROL

WACCM

4x5

TIEDTKE

WACCM

1.9x2.5

TIEDTKE

Rossby 1.75 0.32 2.22 2.17Rossby-gravity

0.09 0.02 0.20 0.20

Gravity 0.83 0.06 0.21 0.39Eastward waves

Kelvin -0.34 -0.07 -0.40 -0.38Rossby-gravity

-0.13 -0.01 -0.05 -0.07

Gravity -0.32 -0.05 -0.20 -0.26

Using an analytical model (Ricciardulli and Garcia 2000) we calculated the momentum flux that would be associated with vertically propagating waves excited by OBS or MODEL (just above forcing, 12 km). The EP flux is expressed in units of 10-2 m2 s-2.

COMPARISON WITH SPECTRA FROM SABER SATELLITE DATA: Temperature (2003) for zonal wavenumber =1

W E W E

SABER SABERSABER

model model model

30 mb 1 mb 0.01 mb

SIMULATED ZONAL WIND PROFILE (EQUATOR)

CONTROL TIEDTKE @ 4° RES TIEDTKE @ 2° RES

NO QBO

Km

Still NO QBO

Z (

km

)

0

50

100

150

Generation of WESTERLY LAYERdue to Kelvin waves

U(z,t)

U(t), lower stratosphere

time time time

20

-20

10

-10

0

U (

m/s

)

time time time

CONCLUSIONS

• We showed how critical it is for climate models to represent the convective variability at time scales < 2 days for exciting vertically-propagating equatorial waves

• The Tiedtke convective parameterization was implemented in WACCM and resulted in a greatly improved equatorial wave excitation

• Simulated waves in the middle atmosphere can be compared to satellite observations

• The excited waves impact stratospheric dynamics. In order to be able to simulate the QBO, we will likely need to use a high horizontal resolution model at increased vertical resolution to allow propagation in the stratosphere of gravity waves with small vertical wavelengths

FUTURE WORK

• Increase vertical resolution and try to simulate the QBO• Compare model waves with satellite observations• Improve Tiedtke average climatology

• We are not funded anymore for this project• Any student/postdoc who likes to contribute?• Anyone interested in looking at the 5-yr diagnostics and

point out areas of concern or improvement? • Please contact Lucrezia at Ricciardulli@remss.com