Aerosol-cloud-precipitation Interaction

Professor Menglin Jin

METR215: Advanced Physical Meteorology

•Basic aerosol-cloud processes•Current key hypotheses•Focus on Lifetime effect in buffered system

What is aerosol optical thickness? What is it at 0.55 micrometer? What are the typical values for this variable?

Indirect Effect: serve as CCNAerosol Direct Effect: Scattering/absorbing SW/LW

Cloud dropRain dropIce crystalIce precipitation

0oC

surface

Total solar radiation decreased by aerosol= 20Wm-2

(Jin, Shepherd, and King, 2005, JGR)

Aerosol decreases surface insolation

Based on M-D. Chou’s radiative transfer model

Aerosol reduce surface insolation for extreme polluted days

45.5

105.5

29.2

52.8

0

20

40

60

80

100

120

January July

So

lar

Rad

iati

on

(W

m-2

)

NYC

BJ

Jin, Shepherd and Zheng 2010

Observed aerosol reducing cloud droplet size

Jin and Shepherd 2008, JGR

Different Aerosol Effects(from various papers)

• Biomass burning (sugar cane fields): This can greatly reduce or shut-off precipitation processes because of slowed down collision and coalescence, mainly due to the substantial increases in CCN concentrations from the smoke. The increased CCN concentrations leads to smaller sized drops, which have smaller collections kernels.

• Paper and pulp mills: These emissions change the size spectrum by introducing large and giant CCN, while not altering the small CCN concentrations. This increases the collision/coalescence efficiency, leading to enhanced precipitation (also maybe due to additional moisture emitted from such mills).

• Ship tracks: The smoke stack and emissions from a ship can change the number of CCN particles that then change the cloud structure. There is more liquid water in these clouds, higher CCN concentrations, and therefore these clouds reflect more solar energy. These clouds can also be somewhat deeper than surrounding clouds. Drizzle processes maybe reduced or shut off (which may explain the unexpected higher liquid water contents).

• Daniel Rosenfeld

http://www.met.sjsu.edu/~jin/AGU03urban.html

GRL 2003

The difference between the cloud clear air equivalent anthropogenic aerosol sulfate concentrations on the two days is nearly an order of magnitude, but in absolute terms it is only 1 g m-3. Astonishingly, this small amount of aerosol can reduce the snowfall rate up to 50%.Evidence is presented to demonstrate the possible magnitude of the secondary indirect aerosol effect on precipitation rates from cold mixed-phase clouds in mountainous regions where a seeder-feeder cloud couplet is present. Changes as small as 1 g m-3 in CCN aerosol concentration can cause significant changes in cloud properties and precipitation efficiencies. (Quoted from Borys et al., GRL 2003).

Polluted

Clean

Rocky Mountains, CO

Droplet diameter [m]

Droplet diameter [m]

Crystal diameter [mm]

Crystal diameter [mm]

Dro

plet

con

cent

rati

on [

cm-3]

Cry

stal

con

cent

rati

on [

cm-3]

The effect of aerosols on precipitation in clouds was calculated from the data of the image above. The warm colors represent efficient precipitation processes, while the cold colors represent suppressed precipitation, due to the pollution. The scale is the maximal cloud top temperature [0C] required for onset of precipitation.

(1) Maritime and Rural aerosols

Clouds from clean maritime air develop precipitation efficiently. After interacting with rural aerosols, the clouds are less efficient in developing precipitation.

(2) Urban air pollution

The blue color indicates detrimental effect of urban air pollution on the precipitation in the clouds.

(3) Smoke from forest fires

Another case of detrimental effect of the interaction of Clouds with biomass burning smoke on the precipitation in the clouds can be seen in the blue color over Sumatra and Kalimantan.

The TOMS aerosol index can be seen below:

Bangkok

Ho Chi Minh

(Saigon)

KalimantanSumatra

So, does air pollution suppress or enhance overall rainfall amount from convective clouds?

Observations and model simulations show that always clouds with more small CCN will rain less for a given maximum vertical development.

Simulations show that in warm base clouds elevating the onset of precipitation can lead to longer time of cloud growth before downdrafts take over, and hence this dynamic feedback causes greater vigor and secondary formation of clouds, leading to more overall precipitation.

This is Rosenfeld’s view. Not necessarily true

Apparent increases in aerosol optical depth in partly cloudy regions.

Confounding Influences on Observations of Cloud Cover Effect

Aerosol-Cloud-Precipitation Interactions in a Buffered System

• Shallow clouds are a crucial part of the earth’s climate system. They radiate in the longwave at approximately the same temperature as the surface but their stark albedo contrast with the dark underlying ocean/land exerts a significant shortwave cooling on the climate system. These clouds also play an important role in the development of deeper convection the details of their treatment in climate models influence climate sensitivity. The effect of aerosol particles on these clouds has been the subject of intense scrutiny for a number of decades. Traditionally aerosol influences have been viewed in terms of their effect on the cloud albedo or on the cloud lifetime. While there is abundant observational evidence for aerosol-induced cloud brightening, the evidence for the proposed increase in cloudiness or “lifetime” is ambiguous. In this presentation we will make the case that this ambiguity stems, at least in part, from the fact that the aerosol-cloud-precipitation system is buffered. This means that the response of the cloud to a forcing is weaker than would have been expected had internal mechanisms not been accounted for. A number of different examples of this buffering capacity will be presented. A strategy that includes field experiment and modeling work aimed at improving our understanding of the system will be proposed.

shallow cumulus convection, Shallow water clouds (Kaufman et al. )

• only a few hundred meters thick

• an increase in shallow cloud cover by only 0.04 is enough to offset 2-3°K of greenhouse warming

• By reflecting sunlight back to

• space, stratiform clouds are “the vast climate refrigerator of the tropics and subtropics

Longitudinal dependence of the shallow cloud fraction (Left) and droplet effective radius (Right) for the northern tropical Atlantic with dust intrusions (Upper) and southern tropical

Atlantic with smoke intrusion (Lower).

Kaufman Y J et al. PNAS 2005;102:11207-11212

©2005 by National Academy of Sciences

Aerosol Effect in a Buffered SystemStevens and Feingold 2009, Nature

• 1. What is Lifetime effect (LE) hypothesis?

• 2. Why LE is important to shallow martine cloud system?

• 3. What is the authors’ theory? (paragrahy 3)

Stevens and Feingold 2009, Nature (cont.)

• satellite observations– What solid observations on cloud droplet size

vs. aerosols? (open cellular, fig. 2)– Global magnitude– Cloud fraction

Stevens and Feingold 2009, Nature (cont.)

• Two line of arguments related satellite observations for LE– The artefact argument

Stevens and Feingold 2009, Nature (cont.)

• A Buffered System

- negative feedback

- result of negative feedback