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Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol – Wilcox, 2010

Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol – Wilcox, 2010. The semi-direct aerosol effect: Impact of absorbing aerosols on marine stratocumulus – Johnson et al, 2004 Direct and semi-direct radiative forcing of smoke aerosols over clouds – Wilcox, 2012.

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Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol – Wilcox, 2010

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  1. Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol– Wilcox, 2010 The semi-direct aerosol effect: Impact of absorbing aerosols on marine stratocumulus – Johnson et al, 2004 Direct and semi-direct radiative forcing of smoke aerosols over clouds – Wilcox, 2012 YemiAdebiyi

  2. Overview Aerosols are advected from the adjacent Africa continent over the Atlantic Ocean’s stratocumulus cloud; typically between 2-5km

  3. AIRS air temperature is collocated with AMSR-E SST and OMI AI • MODIS is used to identify grid cells that are completely spanned by clouds. • These grid cells are collocated with AMSR-E LWP = 47,000 grid cells

  4. Shortwave heating rate • The SW heating rate increases with increase in AOD • Due to the increase absorption of SW within the layer • SW heating rate increases with cloud fraction. • Due to absorption of both down-welling and upwelling SW within the layer

  5. How much will this corresponds to on a global scale?

  6. How does the upper level respond to changes in SST for diff. AI? How much connection is there between SST and midlevel air temp. variation? • At 700hPa, high smoke samples are warmer than low smoke samples by nearly 1K. • He reported no systematic difference at 600hPa, except 30% of samples below 293K SST which shows cooler T for higher AI

  7. Cloud thickening effect • Increase SW absorption in smoke layer • Leading to increase in buoyancy of layer above the cloud • Reduced cloud-top entrainment • This preserves the humidity and cloud cover in BL • Result = Increased LWP and shallower BL Negative semi-direct forcing

  8. Putting Aerosol above cloud in LES Simulation

  9. Aerosol within the BL in LES • Aerosol heats the cloud layer • Leading to daytime thinning of cloud • Increase evaporation enhances daytime decoupling of BL • …and hence reduced turbulent fluxes between surface and cloud • Result = decreased LWP Positive semi-direct forcing

  10. Sharp longwave cooling near cloud-top • Leads to sinking parcels due to buoyancy • Increases turbulent mixing at cloud-top that entrains free tropospheric air into the BL. • Entrainment dries the BL and reduced LWP • Lower LWP then lowers entrainment = negative feedback

  11. Evidence of possible non-linearity

  12. How does the radiative forcing change if elevated smoke layer has moisture?

  13. Using Observation Direct radiative warming of smoke above the cloud

  14. Semi-direct radiative cooling by cloud thickening

  15. Summary • Absorbing aerosol layer above cloud leads to warming due to absorption of SW radiation • This warming is increased due to additional component reflected by the underlying cloud • Aerosol above the cloud enhances buoyancy which reduces entrainment and thus increases LWP and hence albedo! • The semi-direct cooling due to cloud thickening is capable of compensating for more than 60% of the warming by direct effect. • What remain to be accessed is the impact of moistened smoke layer on the direct and semi-direct radiative forcing.

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