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

Observation Model Normalized Anomaly

Observation Total Cloud High Cloud

Total Cloud High Cloud Model If indirect effects were included we would not expect the reduced cloud amount.

Observation Cloud- Top Pressure Height LWP

OLR Model OLR

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 CCN  Decrease precip in shallow clouds  Invigorate 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 Autoconversion Pseudo-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 largest When 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 thickness Where CCN 0.4 is the concentration of active CCN at a supersaturation of 0.4% If k=0.825 and N C =2000 cm -3 Then: (CCN 0.4 )=10,000 cm -3 and AOT=1

CCN, AOT, and Cloud Depth N C is also related to the depth above cloud base required for the onset of rain 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 Example To prevent tropical rainout before reaching the -4C (~5 km), CCN 0.4 ~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 CAPE Radiative: Transmission At max microphysical invigoration (CAPE) AOT≈0.25 Added 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 precipitation Polluted 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.