William Boos & Zhiming Kuang Dept. of Earth & Planetary Sciences Harvard University

1. Mechanisms of poleward propagating, intraseasonal convective anomalies in a cloud-system resolving model William Boos & Zhiming Kuang Dept. of Earth & Planetary Sciences Harvard University October 16, 2009

2. 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

3. 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

4. Viewed as poleward migration of ITCZ NOAA OLR anomalies, 80-100°E, summer 2001 1.5 m/s  Several events typically occur each boreal summer, modulating intensity of South Asian monsoon

5. 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

6. 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

7. Model with wider zonal dimension Precipitation snapshots when ITCZ is near 10N: 32 zonal grid points 4 zonal grid points mm/day 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 60 40 20 0 -20 -40 -60 60 40 20 0 -20 -40 -60 latitude 0 500 960 x (km) x (km)

8. Precipitation in wide-domain model mm/day 0.5 m/s

9. Zonal meanvertical structure for wide domain m/s m/s m/s

10. composite relative vorticity 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) latitude zonal mean vorticity

11. Animation of two events Shading: 930 hPa relative vorticity Black contours: precipitation latitude  Poleward drift of vorticity patch/strip on β-plane… coupled to moist convection x grid point

12. Schematic: propagation mechanism Convectively-coupled beta-drift of vortex strip 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 vorticity anomaly deep ascent y z x y deep ascent 2. perturbed vortex strip migrates poleward vorticity anomaly

13. Test mechanism in dry model surface meridional wind • β-drift biases low-level convergence poleward of free-tropospheric heating applied (constant) thermal forcing

14. Surface wind in dry model constant imposed heating latitude x grid point

15. Looks like unstable moisture mode composite moist static energy anomaly J/kg MSE tendencies

16. Model tests of instability mechanism mm/day fixed radiative cooling Precipitation Hovmollers: control run fixed surface heat fluxes

17. Instability mechanism is non-unique Run with fixed radiative cooling Control run Dashed black lines denote latitude of peak moist static energy anomaly

18. 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

19. Additional slides

20. Wide domain permits high amplitude eddies composite 930 hPa wind and humidity day 0 day 20 day 30 day 41 day 53 g/kg latitude x (105 m)

21. Why does the wide domain make a difference? It’s the eddies… composite moist static energy anomaly J/kg MSE tendencies total & zonal mean advection advective components

22. Propagation speed scaling • Plots of precip and v wind for beta 0.75, 1, 2

23. Observed vertical structure data: ERA-40 Reanalysis, composite of strong poleward events 1979-2002 pressure (hPa)  Note some similarties to eastward moving MJO latitude

24. Observed vertical structure data: ERA-40 Reanalysis, composite of strong poleward events 1979-2002 pressure (hPa) latitude

25. Behavior depends on zonal width,not zonal d.o.f. 5 km resolution with 32 zonal grid points latitude 30 km resolution with 32 zonal grid points latitude time (days)

26. OLR in wide domain model

28. Vertical structure for wide domain (green line denotes position of peak precip signal used for compositing) m/s

29. Turn off both WISHE & radiative feedbacks Precipitation: no WISHE or radiative feedbacks mm/day control time (days)

30. moist static energy anomaly J/kg MSE budget for run without WISHE or radiative feedbacks pressure (hPa) “Convective downdraft instability” latitude (degrees) moist static energy tendencies W m-2