Observations of Aerosol Impacts on Clouds

The effects of aerosols on cloud microphysics has been

demonstrated through field measurements and modeling.

However, the effects on precipitation has so far been done

primarily through the use of hypothesis or model simulations.

The effects of aerosols on cloud microphysical structure

Warner and Twomey, J. Atmos. Sci. 24, 704, 1967

Warm Cumulus

Western Washington4 times more CCN and GCCN

Eastern Washington Eastern

CCN size spectra

Western

Cloud drops

Warm stratocumulus and fair-weather cumulus

Hindman et al, J. Atmos. Sci. 1977

300 m deep400 m deep

689 cm-3 146 cm-3

Aerosols below warm thin Stratocumulus

Garrett and Hobbs, JAS, 2974, 1995

Continental air

Clean air

Cloud drops in warm thin Stratocumulus

Ramanathan et al, Science, 2001

Question: why is the drop concentration so low when the CN is so large?

Ship Tracks

• The prevailing hypothesis is that the ship's trails appear brighter on satellite imagery because the effluent from the ships is rich in CCN particles. The more numerous CCN particles create larger concentrations of cloud droplets which then reflect more solar energy than the surrounding clouds. Aircraft observations in the ship track clouds as well as surrounding clouds (Radke et al., 1989) reveal that the ship track clouds exhibit higher droplet concentrations, smaller droplet sizes, and higher liquid water content than surrounding clouds. In some cases, deeper clouds in the ship tracks are also observed(Ackerman et al.(2000).

• The higher droplet concentrations and smaller droplet sizes are consistent with the hypothesis that the higher cloud brightness is due to a higher concentration of CCN in the ship effluent. The greater liquid water content and deeper clouds in the ship trails, however, is probably a result of dynamic feedbacks.

• It is hypothesized that high CCN reduces the rate of drizzle formation that results in higher liquid water contents and higher droplet concentration in ship track clouds compared to surrounding clouds. Radke et al.~(1989) found that the concentration of drizzle drops (droplets of diameter greater than or equal to 200 microns) in the ship track was only 10\% of that in surrounding clouds.

• Also if cloud LWCs increase then cloud top radiative cooling is enhanced which will invigorate down-up circulations, thus enhancing the dynamics of the stratocu

Ship Tracks?

Observed average cloud drop size distribution for three clouds in ambient air and seven clouds in the effluent of a paper mill.Number concentration ~2200 and 1020 cm-3 LWC = 1.0 and 0.9

Dry CCN size distribution for ambient air and Air affected by the paper mill.Number concentrations= 2200 and 1020

Eagan et al, JAM, 13, 535-552, 1974

ambient air

ambient air

Warm Stratocumulus

Cloud drops

CCN

Effects of large and giant CCN from paper mills

Polluted plume

Polluted

Cooper, Bruintjes and Mather, JAM, 1997.

The effects of GCCN (sea salt and coated dust particles) on the droplet size spectra in convective clouds in the Eastern

Mediterranean; Droplet concentrations 250-400 # / cm3

Levin et al JAM, 1996

With GCCN

Summary of the observations of the effects of CCN on droplet size and concentrations

• Most observations in warm clouds confirm the hypothesis that increasing CCN concentrations from pollution or from other sources leads to an increase in droplet concentrations and a decrease in the effective radius of the drops.

• There are observations that show that GCCN, although found in relatively small concentrations produce larger drops that could initiate collision and coalesscence

• It is not clear, from an observational perspective, what is the difference between the response of continental and maritime clouds to increases in GCCN concentrations.

(Modeling simulations of maritime clouds suggest that increasing the concentrations of GCCN has very little effect on rainfall amounts.)

• Relatively little has been written on the effects of pollution on the ice phase in clouds, those That have, suggest pollution rich in sulfates or other soluble species “poison” IN. Exceptions areIndustries like mining and ore processing that can produce very affective IN

The effects of pollution on precipitation

Warner and Twomey (1967) analyzed the effects of burning sugar cane on droplet concentrations and size in clouds downwind of sugar cane fires and hypothesized that it could reduce rainfall.

Warner (1968) evaluated 60 years of rainfall downwind of sugar cane fieldsand found “......a reduction of rainfall at inland stations coinciding with increasing cane production; ...... The reduction is consistent with the hypothesis that through their activity as condensation nuclei the smoke particles result in great increases in concentration and consequent reduction in the size of cloud droplets, thereby hindering the coalescence process of rain formation. However, the possibility that other factors caused the particular climatic changes observed cannot be eliminated”.

Woodcock and Jones (1970) looked into rainfall trends inHawaii due to the increased burning of sugar cane. They found changes that could NOT be supported by the direct effect of the particles from the sugar cane fires on rainfall.

Early evidence for the effects of pollution on rainfall

Suppression of Rain and Snow by Urban and Industrial Air Pollution(Rosenfeld, 2000, Science)

VIRS retrieved effective radius does not exceed the 14 m precipitation threshold in polluted clouds within area 2 in the Australia image.

0 5 10 15 20 25 30

-10

-5

0

5

reff

T [

oC

]

m]

12 3

15 20 25 30 35 40 450

1

2

3

4

Reflectivity [dBZ]

H [

km]

13

65

VIRS painting yellow pollution tracks in the clouds over South Australia, due to reduced droplets size. PR shows precipitation as white patches only outside the pollution tracks, although clouds have same depth.

TMI shows ample water in the polluted clouds

PR shows bright band in clean clouds. Therefore, pollution suppressed rain and snow in polluted clouds.

Ayers 2003 argued that the conclusions reached by Rosenfeld are

incorrect since on that day there was no rain recorded in any of the

regions shown in Rosenfeld’s paper.

It is possible that light rain not reaching the ground could be detected

by

Effect of pollution on snow fall from orographic clouds

Note the difference in concentrations

Borys et al, GRL, 2003

Polluted case, smaller drops, less riming

Clean case,larger drops,more riming

Table 1. Chemical and Physical Properties of Cloud Droplets and Snow During Two Precipitation Events

February 15 (Polluted) 19 (clean)

Major Habit Planar Dendrite Planar Dendrite Rime Category Unrimed (0.5) Moderate (2.0) Rime Mass Frac. 5% 51% SPL Precip. Rate 0.02 mm hr1 0.38 mm hr1 ISS Precip. Rate 0 to 0.1 mm hr1 1.1 mm hr1 SPL Temperature 13C 4C Snow 18O 22.1 16.5 Cloud d18O 21.1 16.2 18O Snow Mass 14C 4.8C Temp. Of Origin

Cloud Top Temp 19C 22C Snow CAE SO4 = 0.011 mgm 3 0.072 mgm 3 Cloud CAE SO4 = 1.1 mgm 3 0.12 mgm 3 Droplet Mean Dia. 8.3 mm 13.6 mm Droplet Conc. 310 cm 3 74 cm 3 Cloud SCLW 0.13 g m 3 0.14 g m 3

Borys et al, GRL, 2003

Borys et al, GRL, 2003

Feb 19Feb 15

Orographic enhancement factor (Ro)

• Orographic enhancement factor (Ro): ratio between precipitation amounts over hills to precipitation amounts in upwind lowland (Givati and Rosenfeld 2004, 2005)

• Suppression rate downwind of coastal urban areas in California and Israel 15-25% of annual precipitation in hills– Occurred mainly in relatively shallow orographic clouds

within cold air mass of cyclones• Role of pollution aerosols in decreasing Ro over

mountains to E of Salt Lake City (Griffith et al. 2005) but upwind increased!

• Similar decreasing trends in Ro (up to 30%) noted on eastern slopes of Rockies during easterly flows, downwind of Denver and Colorado Springs (Jirak and Cotton 2006)

Ro Trends• Numbers show

end/start of winter Ro (Oct-May) Ro for high rain gauge with respect to low– Red numbers are

smaller than 1.00 that indicate a statistically significant trend

• Ro has decreased significantly over all area except pristine regions of northern CA, OR, southern Idaho, and central Utah

Climatic fluctuations• Dettinger et al. (2004) found

that during negative PDO and positive SOI, westerly wind component strong so mountain-plains orographic factor higher than in the positive PDO (El Nino like) phase

– Less overall precipitation in negative than in positive PDO phase

• Ro weakly negatively correlated with PDO and even more weakly so with the SOI

– These weak relations could still explain trends if PDO and SOI would have large trends with time

Jirak and Cotton(2006)

• Effect of Air Pollution on Precipitation along the Front Range of the Rocky Mountains

Introduction

- Increased concentrations of small CCN are thought to suppress precipitation (all else being equal) and are associated with pollution in urban areas- Givati and Rosenfeld (2004) found a 15-25% reduction in orographic precipiation downwind of urban areas in California and Israel- This study investigates the same phenomenon over the Front Range by comparing trends in precipitation ratios at various pairs of sites

Analysis Methods

• Identified 3 site pairs based on length of precipitation records, correlation of precipitation, and geographic orientation:– Denver (Cherry Creek Dam/Morrison)– Colorado Springs (Colorado Springs Municipal

Airport/Ruxton)– A 'pristine' area for comparison

(Greeley/Waterdale/Estes Park)• Only considered days where wind was upslope at Denver

Stapleton (NNE to SSE)• Looked for trend on Orographic Enhancement Factor

(OEF), ratio of precipitation at higher-elevation site to lower-elevation site

Conclusions No significant trends in total precipitation, or OEF in

total precipitation Significant decreasing trends in upslope OEF for urban

areas, but not pristine Points to pollution as source of suppression

Precipitation losses on order of 1mm/year over 50 years from upslope component (but totals actually trended upward, albeit insignificantly)

Could exacerbate water shortages Greater population -> greater water demand and more

pollution More pollution -> less precipitation -> more severe water

shortages

Alpert et al,(2008)

• Does Air Pollution Really Suppress Precipitation in Israel?

• They reanalyzed Ro precip data and recalculated Ro for Israel

Central Israel

• 5 of the 11 stations included in previous analyses as coastal stations are in fact downwind of the Tel Aviv urban area.

• When including only the 6 along the coast in the analysis, the orographic ratio actually increases with time.

Northern Israel

• 2 of the stations previously included as coastal stations are downwind of the Haifa urban area.

• 4 of the coastal stations are not located along the storm track and so may not be useful in an orographic ratio.

• No change can be seen in the orographic ratio over time.

Northern Israel

• The rainfall ratio of the upslope of the mountains to the lee side has increased with time, contrary to previous studies.

• This suggests that cloud-seeding efforts in Israel are either not working properly or are dwarfed by other factors in precipitation formation.

Findings

• No mountain stations have shown a reduction in rainfall over the past 50 years.

• There is actually an increase in orographic rainfall when compared to the seashore stations.

• The decreased orographic ratio reported previously was due to larger increases in rainfall over the southern coastal plain caused by land-use and synoptic changes.

• Larger increases in rainfall also occur downwind of the urban areas, possibly due to urban heat island effects.

• Any ratio will decrease if a constant is added to both the numerator and denominator.

Summary• Use of Ro method can be misleading

• Decreasing trends can be due to other factors

The effects of forest fires on clouds and precipitation

Five different effects were reported from measurements in the Amazon:

1) Blue ocean: Maritime clouds over the ocean – low CCN concentration; few and large drops; rainfall from clouds that are not very deep

2) Green Ocean: unpolluted Amazon mainly in the rainy season. Low CCN concentrations due to washout by rain.

3) Smoky clouds: High concentrations of CCN from long lasting smoke thatremains in the atmosphere due to the lack of rain. Clouds grow deeper due to slow growth of drops, leading to ice production, hail and lightning and heavy rain.

4) Pyro-clouds: Feed directly on the smoke and the heat from the fires. High concentrations of CCN and small drops. Limited warm rain production. On the other hand, some of the large ash particles can produce large drops and enhance the warm rain development.

5) Smoke particles in cloud drops absorb solar radiation and dissipate the clouds

(1-4) Andreae et al Science, 2004(5) Koren et al, Science, 2004

Note the slow development of the spectrum with height for the smoky andPyro clouds

Andreae et al, Science, 2004

Blue Ocean

Smoky Pyro

Green Ocean

The effects of forest fires on clouds and precipitation

Five different effects were reported from measurements in the Amazon:

5) Smoke particles in cloud drops absorb solar radiation and dissipate the clouds (5) Koren et al, Science,

2004

Smoke dissipating clouds

Unintentional fire in the central region of Brazil, east of Salvador-Bahia, Jan. 28, 2003

Image taken with MEIDEX camera on the Columbia, Space Shuttle during its tragic flight STS-107

Huiwen Xue and Graham Feingold (In Press) show that even without heating due to absorption, high concentration of CCN would lead to rapid evaporation of small warm clouds.

There is plenty of evidence that increasing CCN concentrations increases

cloud droplet concentrations and decreases cloud drop sizes. However, the effect of modifying particles’ size and concentrations on

precipitation is much less certain.

Models show that a few GCCN per liter may strongly affect precipitation

development. Therefore, even for continental clouds, which have large

concentrations of small CCN, it may be enough to inject a few giant

CCN to increase rainfall. We need measurements to prove it.

In cold convective clouds precipitation can grow in a number of ways:

1. collision-coalescence process, 3. riming of ice particles

2. ice growth by deposition, 4. all of the above

Summary

There is some evidence suggesting that increasing droplet

concentrations in cold orographic clouds affected by pollution

reduces the riming process, leading to lower snowfall on the

ground. However, many more measurements are needed to

support these findings.

Large mineral dust particles coated with soluble material, lead to

an increase in droplet sizes. The same dust particles could

increase ice particle concentrations. How do these particles affect

rainfall?

There are a few remote sensing observations and model

simulations that suggest that the rainfall decreases with increased

aerosol loading. Much more work is needed to substantiate it.

Concluding remarks

• There is a good agreement between CCN, updraft and droplet concentrations, but there is not enough information about the role of giant nuclei.

• There has not yet been enough work on the effects of aerosols (pollution or natural) at altitudes above cloud base on the development of the clouds.

• There is a need to expand the evaluation of the effects of pollution on rainfall to scales larger than a single storm or a single cloud.

• Use remote sensing techniques from satellites to superimpose the fields of aerosols - clouds - precipitation on a world wide scale.

• Separate the aerosols into fine and coarse modes, try to determine their chemical properties and correlate with clouds and rainfall.

• The effects of increased CCN and GCCN on the development of the ice phase in clouds needs to be further examined.

The simultaneous use of in situ and, remote sensing measurements,

supplemented by efficient and accurate numerical models, will be

needed to more clearly delineate the roles of these various processes to

the formation of precipitation and to the effects of aerosols on these

processes.