Mechanisms of poleward propagating, intraseasonal convective anomalies in a

cloud-system resolving model

William Boos & Zhiming KuangDept. of Earth & Planetary Sciences

Harvard UniversityOctober 16, 2009

Outline• Background and observations

• Results from quasi-2D models with explicit convection

• Mechanisms of instability and propagation

Main message:

For intraseasonal convective anomalies during boreal summer:• Poleward propagation occurs due to convectively-coupled beta-drift of a vorticity strip• Instability occurs due to moisture-radiation feedback

Borealsummer MJO lifecycle of TRMM precip

diagnostic from CLIVAR MJO working group, based on EOFs after Wheeler & Hendon (2004)

propagation has prominent poleward component

some events do exhibit poleward propagation without eastward propagation

Viewed as poleward migration of ITCZ

1.5 m/s

NOAA OLR anomalies, 80-100°E, summer 2001

Several events typically occur each boreal summer, modulating intensity of South Asian monsoon

History of axisymmetric model studies

• Land-atmosphere interactions (Webster & Chou 1980)

• Poleward gradient of convective instability (Gadgil & Srinivasan 1990)

• Dynamical coupling of anomalies to baroclinic mean state (Bellon & Sobel 2008, Jiang et al. 2004)

… but all of these studies use idealized parameterizations of moist convection, and mode characteristics depend on convective closure

Test in model with explicit convection

• System for Atmospheric Modeling (SAM, Khairoutdinov & Randall 2003)

• 1 km horizontal resolution• Beta-plane, 70°N – 70°S• 4 zonal grid points• Oceanic lower boundary

with prescribed SST

precipitation

Model with wider zonal dimension4 zonal grid points 32 zonal grid points

Precipitation snapshots when ITCZ is near 10N:

60

40

20

0

-20

-40

-60

latit

ude

60

40

20

0

-20

-40

-60

Old domain: 140° meridional x 4 km zonal

New domain: 140° meridional x 960 km zonal

For computational efficiency, use RAVE methodology of Kuang, Blossey & Bretherton (2005):

30 km horizontal resolution, RAVE factor 15

Similar results obtained for RAVE factors ranging from 1-15 at 30 km resolution, and for one standard run with 5 km resolution

x (km)x (km)

mm/day

0 500 960

Precipitation in wide-domain model

0.5 m/s

mm/day

Zonal meanvertical structure for wide domain

m/s

m/s

m/s

Composite 950 hPa vorticity

• Zonal mean vorticity satisfies necessary condition for barotropic instability

• Anomalies form closed cyclone for part of poleward migration, and zonal strip for remainder

• Suggestive of “ITCZ breakdown” (Ferreira & Schubert 1997)

zonal mean vorticity

compositerelative vorticity

latit

ude

Animation of two events

Poleward drift of vorticity patch/strip on β-plane… coupled to moist convection

latit

ude

x grid point

Shading: 930 hPa relative vorticity

Black contours: precipitation

Schematic: propagation mechanism

1. deep ascent creates (barotropically unstable) low-level vortex strip

3. Ekman pumping in vortex strip humidifies free-troposphere poleward of original deep ascent, shifting convection poleward

Convectively-coupled beta-drift of vortex strip

deep ascent

2. perturbed vortex strip migrates poleward

deep ascent

vorticity anomaly

xy

vorticity anomaly

yz

Test mechanism in dry model

• β-drift biases low-level convergence poleward of free-tropospheric heating

applied (constant) thermal forcing surface meridional wind

Surface wind in dry model

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ude

constant imposed heating

x grid point

Looks like unstable moisture mode

J/kg

MSE tendencies

composite moist static energy anomaly

Model tests of instability mechanismmm/dayfixed radiative cooling

fixed surface heat fluxes

control run

Precipitation Hovmollers:

Instability mechanism is non-unique

Dashed black lines denote latitude of peak moist static energy anomaly

Control run Run with fixed radiative cooling

Summary• Axisymmetric cloud permitting models fail to produce robust poleward

propagating, intraseasonal convective anomalies

• Meridional “bowling alley” domains O(1000 km) wide do produce such anomalies

– Suggested propagation mechanism:convectively-coupled beta-drift of vortex strip

– Anomalies destabilized by moisture-radiation feedback

– Perhaps slowed and made more coherent by WISHE

– Multiple instability mechanisms can operate, with structural changes

• Future work:– Behavior in wider domains

– Validation of mechanism in simpler models

Additional slides

Wide domain permits high amplitude eddies

latit

ude

x (105 m)

g/kgday 0 day 20 day 30 day 41 day 53

composite 930 hPa wind and humidity

Why does the wide domain make a difference? It’s the eddies…

J/kg

MSE tendencies

composite moist static energy anomaly

advective componentstotal & zonal mean advection

Propagation speed scaling

• Plots of precip and v wind for beta 0.75, 1, 2

Observed vertical structuredata: ERA-40 Reanalysis, composite of strong poleward events 1979-2002

latitude

pres

sure

(hPa

)

Note some similarties to eastward moving MJO

latitude

pres

sure

(hPa

)

Observed vertical structuredata: ERA-40 Reanalysis, composite of strong poleward events 1979-2002

Behavior depends on zonal width,not zonal d.o.f.

time (days)

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ude

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5 km resolution with 32 zonal grid points

30 km resolution with 32 zonal grid points

OLR in wide domain model

Vertical structure for wide domain

(green line denotes position of peak precip signal used for compositing)

m/s

Turn off both WISHE & radiative feedbacks

no WISHE or radiative feedbacks

control

time (days)

mm/dayPrecipitation:

MSE budget for run without WISHE or radiative feedbacks

moist static energy anomaly

latitude (degrees)

pres

sure

(hPa

)

moist static energy tendencies

W m

-2

“Convective downdraft instability”

J/kg