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

Overview

Aerosols are advected from the adjacent Africa continent over the Atlantic Ocean’s stratocumulus

cloud; typically between 2-5km

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

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

Shortwave heating rate

How much will this corresponds to on a global scale?

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

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?

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

Cloud thickening effect

Negative semi-direct forcing

Putting Aerosol above cloud in LES Simulation

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 LWPPositive semi-direct forcing

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

Evidence of possible non-linearity

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

Using Observation

Direct radiative warming of smoke above the cloud

Semi-direct radiative cooling by cloud thickening

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.