Mechanisms of poleward propagating, - PowerPoint Presentation

Slide1

Mechanisms of poleward propagating, intraseasonal convective anomalies in a cloud-system resolving model

William Boos & Zhiming KuangDept. of Earth & Planetary SciencesHarvard UniversityOctober 16, 2009Slide2

Outline

Background and observationsResults from quasi-2D models with explicit convectionMechanisms of instability and propagationMain 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 feedbackSlide3

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 propagationSlide4

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 Slide5

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 closureSlide6

Test in model with explicit convection

System for Atmospheric Modeling (SAM, Khairoutdinov & Randall 2003)1 km horizontal resolutionBeta-plane, 70°N – 70°S4 zonal grid pointsOceanic lower boundary with prescribed SST

precipitationSlide7

Model with wider zonal dimension

4 zonal grid points32 zonal grid pointsPrecipitation snapshots when ITCZ is near 10N:

60

40

20

0

-20

-40

-60

latitude

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 960Slide8

Precipitation in wide-domain model

0.5 m/smm/daySlide9

Zonal mean

vertical structure for wide domainm/sm/sm/sSlide10

Composite 950 hPa vorticity

Zonal mean vorticity satisfies necessary condition for barotropic instabilityAnomalies form closed cyclone for part of poleward migration, and zonal strip for remainderSuggestive of “ITCZ breakdown” (Ferreira & Schubert 1997)

zonal mean vorticity

composite

relative vorticity

latitudeSlide11

Animation of two events

 Poleward drift of vorticity patch/strip on β-plane… coupled to moist convectionlatitudex grid pointShading: 930 hPa relative vorticityBlack contours: precipitationSlide12

Schematic: propagation mechanism

deep ascent creates (barotropically unstable) low-level vortex strip3. Ekman pumping in vortex strip humidifies free-troposphere poleward of original deep ascent, shifting convection polewardConvectively-coupled beta-drift of vortex strip

deep ascent

2. perturbed vortex strip migrates poleward

deep ascent

vorticity anomaly

x

y

vorticity anomaly

y

zSlide13

Test mechanism in dry model

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

applied (constant) thermal forcing

surface meridional windSlide14

Surface wind in dry model

latitudeconstant imposed heatingx grid pointSlide15

Looks like unstable moisture mode

J/kgMSE tendencies

composite moist static energy anomalySlide16

Model tests of instability mechanism

mm/day

fixed radiative cooling

fixed surface heat fluxes

control run

Precipitation Hovmollers:Slide17

Instability mechanism is non-unique

Dashed black lines denote latitude of peak moist static energy anomaly

Control run

Run with fixed radiative coolingSlide18

Summary

Axisymmetric cloud permitting models fail to produce robust poleward propagating, intraseasonal convective anomaliesMeridional “bowling alley” domains O(1000 km) wide do produce such anomaliesSuggested propagation mechanism: convectively-coupled beta-drift of vortex stripAnomalies destabilized by moisture-radiation feedbackPerhaps slowed and made more coherent by WISHEMultiple instability mechanisms can operate, with structural changesFuture work:Behavior in wider domainsValidation of mechanism in simpler modelsSlide19

Additional slidesSlide20

Wide domain permits high amplitude eddies

latitude

x (10

5

m)

g/kg

day 0

day 20

day 30

day 41

day 53

composite 930 hPa wind and humiditySlide21

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

J/kgMSE tendencies

composite moist static energy anomaly

advective components

total & zonal mean advectionSlide22

Propagation speed scalingPlots of precip and v wind for beta 0.75, 1, 2Slide23

Observed vertical structure

data: ERA-40 Reanalysis, composite of strong poleward events 1979-2002latitude

pressure (hPa)



Note some similarties to eastward moving MJOSlide24

latitude

pressure (hPa)

Observed vertical structure

data: ERA-40 Reanalysis, composite of strong poleward events 1979-2002Slide25

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

time (days)latitudelatitude

5 km resolution with 32 zonal grid points

30 km resolution with 32 zonal grid pointsSlide26

OLR in wide domain modelSlide27
Slide28

Vertical structure for wide domain

(green line denotes position of peak precip signal used for compositing)m/sSlide29

Turn off both WISHE & radiative feedbacks

no WISHE or radiative feedbacks

control

time (days)

mm/day

Precipitation:Slide30

MSE budget for run without WISHE or radiative feedbacks

moist static energy anomalylatitude (degrees)

pressure (hPa)

moist static energy tendencies

W m

-2

“Convective downdraft instability”

J/kg