William Boos & Zhiming Kuang Dept. of Earth & Planetary Sciences Harvard University
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Transcript of William Boos & Zhiming Kuang Dept. of Earth & Planetary Sciences Harvard University
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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
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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
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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
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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
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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
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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
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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
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Precipitation in wide-domain model
0.5 m/s
mm/day
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Zonal meanvertical structure for wide domain
m/s
m/s
m/s
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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
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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
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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
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Test mechanism in dry model
• β-drift biases low-level convergence poleward of free-tropospheric heating
applied (constant) thermal forcing surface meridional wind
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Surface wind in dry model
latit
ude
constant imposed heating
x grid point
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Looks like unstable moisture mode
J/kg
MSE tendencies
composite moist static energy anomaly
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Model tests of instability mechanismmm/dayfixed radiative cooling
fixed surface heat fluxes
control run
Precipitation Hovmollers:
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Instability mechanism is non-unique
Dashed black lines denote latitude of peak moist static energy anomaly
Control run Run with fixed radiative cooling
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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
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Additional slides
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Wide domain permits high amplitude eddiesla
titud
e
x (105 m)
g/kgday 0 day 20 day 30 day 41 day 53
composite 930 hPa wind and humidity
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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
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Propagation speed scaling
• Plots of precip and v wind for beta 0.75, 1, 2
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Observed vertical structuredata: ERA-40 Reanalysis, composite of strong poleward events 1979-2002
latitude
pres
sure
(hPa
)
Note some similarties to eastward moving MJO
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latitude
pres
sure
(hPa
)
Observed vertical structuredata: ERA-40 Reanalysis, composite of strong poleward events 1979-2002
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Behavior depends on zonal width,not zonal d.o.f.
time (days)
latit
ude
latit
ude
5 km resolution with 32 zonal grid points
30 km resolution with 32 zonal grid points
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OLR in wide domain model
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Vertical structure for wide domain
(green line denotes position of peak precip signal used for compositing)
m/s
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Turn off both WISHE & radiative feedbacksno WISHE or radiative feedbacks
control
time (days)
mm/dayPrecipitation:
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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