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
Yemi Adebiyi

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

4. 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

6. 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

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

8. 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

9. 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

10. Putting Aerosol above cloud in LES Simulation

11. 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

12. 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

13. Evidence of possible non-linearity

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

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

17. Semi-direct radiative cooling by cloud thickening

18. 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.