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Aerosol Effects on Rainfall Patterns

Aerosol Effects on Rainfall Patterns. Yi Ming. Geophysical Fluid Dynamics Laboratory Princeton, New Jersey. Most anthropogenic aerosols are in the NH mid-latitudes and tropics. MODIS annual-mean aerosol optical depth (AOD). Will the response be confined locally?. Forcing. Forcing. EQ.

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Aerosol Effects on Rainfall Patterns

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  1. Aerosol Effects on Rainfall Patterns Yi Ming Geophysical Fluid Dynamics Laboratory Princeton, New Jersey

  2. Most anthropogenic aerosols are in the NH mid-latitudes and tropics MODIS annual-mean aerosol optical depth (AOD)

  3. Will the response be confined locally? Forcing Forcing EQ EQ 30N 30N 60N 60N 90N 90N Response Response EQ EQ 30N 30N 60N 60N 90N 90N • Mid-latitudes • Large f • Large MTG balanced by thermal wind or • Tropics • Small f • Upper-level wind constrained by angular momentum conservation • Cannot sustain large meridional temp. gradient (MTG)

  4. A rough sketch of a paradigm Forcing Response EQ 30N 60N 90N Stationary Rossby wave EQ 30N Surface albedo feedback 60N 90N Eddies • Local vs. non-local effects (validity? Ways to distinguish and quantify them?) • Zonal-mean vs. zonal asymmetry

  5. Design of mixed-layer experiments +2.76 K GAS +0.55 K BOTH Aerosol direct + indirect effects -2.1 W m-2 Control -0.62 K (direct effect only) AERO -1.90 K Sign of nonlinearity: BOTH (0.55 K) < AERO + GAS (0.86 K) Ming and Ramaswamy (2009)

  6. Zonal-mean changes

  7. Zonal-mean responses to aerosols and greenhouse gases • Dipole pattern of tropical rainfall change; • Role of the thermodynamic control (C-C). Surface temperature (K) Precipitation (mm day-1) BOTH GAS GAS SUM SUM AERO BOTH AERO

  8. Zonal-mean (Hadley) circulation change Meridional stream function (109 kg s-1) (clockwise circulation is positive) Stronger ascent Weaker ascent

  9. How does the Tropical heat engine response to aerosol forcing, and why? From the viewpoint of atmospheric energy transport, the response gives rise to a cross-equatorial heat flux from SH to NH. Radiative cooling AERO BOTH Dumping energy Picking up energy Even the North Pole kicks in. GAS

  10. Zonal-mean change in atmospheric energy transport (PW) GAS AERO Dry Static Dry Static Total Total Latent Est. latent Latent • Assumptions: • No change in flow • Small

  11. Zonally asymmetric changes (tropics)

  12. Aerosol-induced changes in tropical circulation The thermodynamic argument (TA) (Held and Soden, 2006) For the entire globe (or the Tropics in isolation), Convective mass flux Precip. Mixing ratio of water vapor in BL Clausius-Clapeyron (CC) scaling

  13. If one applies TA to greenhouse gases (GHG)-induced warming, Δ Δ Allen & Ingram (2002) Held and Soden (2006) Stephens & Ellis (2008) -5 ~ -6 % K-1 1 ~ 2 % K-1 Does this apply to aerosol cooling? The mixed-layer GCM simulations suggest Δ Δ StrongerTropical mean circulation -3.0 % K-1 3.8 % K-1

  14. Number-crunching time! Percentage differences in variance of Mc (%) • TA works well for GAS, but only partially for AERO. • The zonal-mean response is dominated by AERO.

  15. A drying trend over central-northern India during the second half of the 20th century JJAS rainfall (mm day-1 50 years-1) • PREC/L • CRU • IMR • UDEL

  16. South Asia – A region under global and regional changes AR5 historical emissions (Tg/yr) of SO2 Larmarque et al. (2010)

  17. How aerosols and greenhouse gases may affect the South Asian summer monsoon? • Aerosols • Atmospheric heating enhances pre-monsoon rainfall (Lau and Kim, 2006); • Surface cooling and reduced Indian Ocean SST gradient weaken monsoon (Ramanathan et al., 2005; Chung and Ramanathan, 2006). • Greenhouse gases • Slower tropical (especially Walker) circulation (Vecchi et al., 2006); • Nonetheless, increased rainfall due to higher moisture content (Ueda et al., 2006).

  18. Model physics and chemistry in the GFDL CM3 Model (used for AR5) • Aerosol-Liquid Cloud Interactions • A prognostic scheme of cloud droplet number concentration (Ming et al., 2007) with an explicit treatment of aerosol activation at cloud base (Ming et al., 2006). • Convection Parameterization • Move from the relaxed Arakawa-Schubert (RAS) in CM2 to the Donner deep convection scheme (Donner, 1993) and the University of Washington (UW) shallow convection scheme (Bretherton et al., 2003). By providing in-plume updraft velocity,the latter two are ideal for implementing aerosol/cloud microphysics. • Online aerosol transport • Troposphericand stratospheric chemistry

  19. Attribution of the recent trend of the South Asian summer monsoon using CM3 historical simulations Linear trends of average JJAS rainfall over central-northern Indian (mm day-1) • GG • AERO • All forcing • CRU

  20. Are the simulated trends statistically significant? JJAS rainfall (mm day-1 50 years-1) • Ensemble-mean • Student’s t-test • Natural variation • Ensemble member

  21. Spatial pattern of linear trends of JJAS rainfall (mm day-1 50 years-1) • All forcing • CRU • AERO • GG

  22. Spatial pattern of linear trends of vertical velocity (hPa day-1 50 years-1) Ascent defined as negative • All forcing • GG • AERO

  23. How Hadley and Walker circulations respond to green-house gases and aerosols? • Climatology • GG • All forcing • AERO

  24. Zonally asymmetric changes (boreal winter mid-latitudes)

  25. Change in surface temp. (K) Different spatial structures over N. Pac. & N. Atl. RFP (W m-2) Not obvious from “forcing”

  26. Change in 300-hPa u (m s-1) Δ(Ts) (K) Consistent with Ts

  27. Change in circulation 500-hPa Z (10 m) SLP (hPa) 300-hPa eddy stream function (ESF) (106 m2 s-1)

  28. El Nino-like? 7 Warm Trop. Pac. minus 7 cold Trop. Pac. 500-hPa Z (10 m) ΔTs(K) Lau (1996)

  29. Change in precip. (mm day-1) EX TR-W TR-E

  30. Setup of idealized model experiments • Dry hydrostatic spectral dynamical core; • T42 and 20 sigma-layers; • No topography; • Zonal-mean winds and temp. are nudged towards GCM simulations; • Forced with GCM-simulated diabatic heating.

  31. Change in 300-hPa ESF (106 m2 s-1) Idealized model (TR-W) Idealized model (TR-E) Idealized model (TR) Trop. heating NW-SE tilt Extratropical heating NE-SW tilt

  32. Conclusions • Zonal-mean changes • In the deep tropics, drier NH & wetter SH  • In the subtropics & extratropics, the rich-get-poorer  • With the purpose of re-establishing interhemispheric energy balance; • Zonally asymmetric changes • In the tropics, Hadley circulation sensitive to aerosols, Walker circulation controlled by the tropical-mean ΔTs; • In the wintertime extratropics, the importance of the stationary Rossby wave excited by the tropical rainfall change.

  33. Zonal-mean change in θ(K) Lower troposphere static stability Δθz(K) (10-35°N)

  34. Zonal-mean change in u (m s-1) Lower troposphere vertical wind shear Δuz(K) (10-35°N)

  35. Zonal-mean change in tropopause height (hPa) 2 K km-1 BOTH Height AERO+GAS Instability – source of nonlinearity? Phillips criterion Latitude

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