Background – Building their Case

• “continental” – polluted, aerosol laden

• “maritime” – clean, pristine

• Polluted concentrations are 1-2 orders of magnitude greater than clean maritime

• Preindustrial continental was only up to 2 times greater than maritime

Continue their Case• ~37% of energy into the

atmosphere is from condensational latent heat release

• Heat is consumed through re-evaporation

• Heat is left behind after precipitation (CAPE)

The Opposing Effects of Aerosols on Clouds and Precipitation

• Direct radiative effect suppresses precipitation

Less radiation reaches the surface

Less heat available for evaporation and convection

• Radiation absorbed by the atmosphere heats the air above the surface Stabilizes the atmosphere Suppresses convection

Obs

erva

tion

Mod

el

Normalized Anomaly

Observation

TotalCloud

HighCloud

TotalCloud

HighCloud

Model

If indirect effects were included we would not expect the reduced cloud amount.

Observation

Cloud-TopPressureHeight

LWP

OLR

ModelOLR

Decreased cloud amount , increased OLR , increased meridional divergence , decreased omega

Decreased Precipitation by ~30%

Decrease in convection with polluted clouds.This is due to higher ice particle concentrations: smaller particles, smaller fall velocities.This finding is different from the aerosol-induced convective invigoration theory we will see.

2-D nonhydrostatic anelastic flow model with superperameterization

2-moment bulk scheme

Radiative transfer model is NCAR’s CCSM

Domain is Darwin Australia

Darwin,Australia

Back to Rosenfeld et al.After observational studies of polluted air suppressing precipitation, the WMO concluded...

The National Research Council reported…

How Can Slowing the Conversion of Cloud Droplets to Raindrops Enhance Rainfall?

• Sub-micron CCNDecrease precip in shallow cloudsInvigorate deep convection

• Adding giant CCN to polluted clouds accelerates the autoconversion (growth by collection)

• Global precip = global evaporation Decrease in precip from shallow clouds must be compensated

with increased precip from deep convection

The rate of autoconversion (coalescence) slows

Precipitation is delayed. More water above 0C

Cloud dynamics are invigorated

Greater intensities and more cloud water later in the life cycle

More cloud water means more evaporative cooling in the downdrafts (cold pools)

Cold pools push ambient air upward (upward heat transport)

Consumption of more CAPE is converted to greater kinetic energy

Convection and convective overturning is invigorated, precip increases, static instability is depleted

When cloud base is near 0C and polluted:

Slowing of the autoconversion rate leaves the droplets airborne

Updrafts push them above homogeneous ice nucleation level (~-38C)

Cannot coagulate and fall as precip.

When clean:

Rainfall increased substantially

Invigoration due to Slowing AutoconversionPseudo-adiabatic parcel theory

Case: Tropical parcel ascends adiabatically from sea level with cloud base P = 960hPa, T=22C

When no precip is allowed, it takes 415 J kg-1 to rise to the -4C isotherm

Freezing releases latent heat, warming the air and adding bouyancy to the parcel

Becomes more positively buoyant when hydrometeors precipitate due to reduced weight

The released convective energy at the top of the cloud is largestWhen there is precip below -4C and ice above, released convective energy is ~1000 J km-1 smaller at the top of the cloud

Further delaying the conversion of cloud water to precipitation until the -36C isotherm, 727 J kg-1 are needed to lift the condensates from -4C to -36C, weakening the convection.

CCN, AOT, and Cloud Depth

From Fig. 1, CCN increases with optical thickness

CCN, AOT, and Cloud Depth

From Fig. 1, CCN increases with optical thickness𝐴𝑂𝑇=0.0027× (𝐶𝐶𝑁 0.4 )

0.64

Where CCN0.4 is the concentration of active CCN at a supersaturation of 0.4%

k < 1

If k=0.825 and NC=2000 cm-3 Then: (CCN0.4)=10,000 cm-3 and AOT=1

CCN, AOT, and Cloud DepthNC is also related to the depth above cloud base required for the onset of rain

𝐷=80+(4×𝐶𝐶𝑁0.4)

D determines on which track the parcel will ascend which reveals:• The strength of convection• Extent of overturning• Thus, rainfall amount

CCN, AOT, and Cloud Depth ExampleTo prevent tropical rainout before reaching the -4C (~5 km), CCN0.4 ~1200 cm-3

Invigoration is maximum (cloud parcel is following d)If CCN were to increase, convection is suppressed and the parcel will move toward a.

Importance of CCN on Direct and Indirect Effect

Microphysical: released CAPERadiative: Transmission

At max microphysical invigoration (CAPE) AOT≈0.25Added aerosols:AOT ↑Transmission ↓Convection is energized (?)Decreased CAPE is reinforced by reduced surface flux.

REDUCED PRECIPITATION

My take home message:

Polluted warm-base clouds increased precipitationPolluted cold-based clouds decreased precipitation

Dependent on the optimal aerosol concentration.• Higher than optimal – direct and indirect effects

compliment one another suppressing precip.• Lower than optimal – precip is reduced, but not to

the same extent.