1.1 General
Over 50 leading aerosol/climate scientists met in January 2002 to define the research
thrusts of the USA National Aerosol Climate Interaction Program. This meeting was
initiated by NASA’s administrator as a result of his visit to Israel in August 2001 and
his exposure to recent findings of several of the PIs of this proposal. At about the
same time, a group of about 15 scientists from a number of countries in Europe and
Israel met to define a similar program for the Mediterranean region. Here we propose
a more focused Israeli National Aerosol Cloud Interaction Program, which will build
on the knowledge basis connecting the emissions of aerosols and impacts on clouds,
precipitation and changes in the availability of water. This newly acquired knowledge
will deepen our understanding regarding the impact of anthropogenic emissions on
precipitation patterns in Israel and will lead to the development of emission control
technology and strategy. Understanding aerosol-cloud physics-precipitation
relationship will provide a basis for new rain enhancement technologies leading to
enrichment of water resources and provide a technological advantage to these future
Israeli technologies and industry. In addition, the knowledge gained will facilitate
better control of environmental pollution.
1.1 Qualitative evidence for aerosols suppressing precipitation
Aerosols containing large concentrations of small CCN nucleate many small cloud
droplets, which coalesce very inefficiently into raindrops. One consequence of this is
suppression of rain over polluted regions, which has been suspected for many years
(Gunn and Phillips, 1957; Warner, 1969). This was recently confirmed by satellite
observations of the Tropical Rainfall Measuring Mission (TRMM), showing tracks of
reduced cloud particles emanating from forest fires (Rosenfeld, 1999) and from
pollution sources such as coal power plants, refineries, smelters and urban areas
(Rosenfeld, 2000). The clouds within and outside of the pollution tracks had similar
dimensions, and according to the TRMM passive Microwave Imager (TMI) contained
similar amounts of water. The only difference was in the reduction of the cloud
particle effective radius (re) within the pollution tracks to less than 14 µm (Fig. 1), the
precipitation threshold radius below which precipitation particles do not normally
form (Rosenfeld and Gutman, 1994). Respectively, the TRMM Precipitation Radar
(PR) observed precipitation outside the pollution tracks, but not in them. Furthermore,
Satellite observations (Rosenfeld and Lensky, 1998; Rosenfeld, 2000) showed
consistently that the suppression of coalescence by smoke and air pollution induced
lower freezing temperature of the cloud supercooled water and suppression of the ice
precipitation processes as well. High concentrations of desert dust were also observed
to suppress precipitation from warm clouds, but in addition to have a strong ice
nucleating activity (Fig 1), (Rosenfeld et al., 2001). While increased pollution or the
presence of dust storms could cause dramatic reduction in rainfall, the picture is not
so obvious. Measurements (Levin et al, 1996) have shown that some, but not all, of
the dust particles reaching the eastern Mediterranean are coated with a layer of
sulfate, making these particles efficient giant CCN. Subsequent theoretical models
showed that these giant CCN could increase the efficiency and the amounts of rain
(Levin et al., 1998; Yin et al., 2000), for example by 15% as shown in Fig. 2.
However, this enhancement strongly depends on the type of clouds affected. The
maximum increase in rainfall occurs in clouds containing large concentrations of
drops (~1000 cm-3). Although the actual rainfall amounts from these types of clouds is
relatively small, the percent increase is large (see Fig. 2). Maritime clouds, with CCN
concentrations as low as 100-200 drops cm-3 are hardly affected by such giant CCN
and in fact, may lead to a decrease in the rain amounts (Yin et al, 2002).
According to results obtained using a spectral-microphysics 2000-bin cloud
parcel model (Pinsky and Khain, 2002; Khain et al, 2002) effect of large CCN on
precipitation depends on the distance between the level of beginning of intensive rain
drop formation (the level of collision triggering) and the cloud top level. In case the
distance is comparatively small (less than about 1 km) additional large CCN can
increase accumulated rain significantly (up to 5 times), while in case the distance is
large (those clouds realize all their precipitation potential) the additional amount of
large CCN leads to a decrease in precipitation in agreement with results by Yin et al
(2002).
Although the effect on precipitation has been documented so far only on the basis of case studies, it
is likely to be a major factor in reducing or increasing precipitation efficiency of clouds mainly over
land, because most of the CCN there are of anthropogenic source (Ramanathan et al., 2001).
o
T [ C]
Fig. 1: Satellite observed impacts of aerosols on cloud properties: Satellite retrieved median
effective radius of particles near the top of deep convective -30
clouds at various stages of their vertical development, as a
IN D O EX p o llu te d
function of the cloud top temperature, which serves as a
A u stra lia u rb a n
surrogate for cloud top height. This is shown for clouds -20
T h a i p re -m o n s
forming in polluted (solid lines) and pristine air (broken lines).
A m a zo n s m o ke
The red lines denoted by "INDOEX polluted” are for
Israe l dust
-10
data along a track that runs from South West India into
the Indian Ocean. The blue lines are for tracks over
urban southeastern Australia (Rosenfeld, 2000); Violet
0
lines are for Thailand pre-monsoon clouds with suppressed
coalescence; Green lines are for biomass smoke over the
Amazon; and black lines are for desert dust over Israel 10
(Rosenfeld et al., 2001). The vertical green line denotes the 14micron precipitation threshold radius (Rosenfeld and Gutman 20
(1994), Rosenfeld and Lensky (1998) and Rosenfeld (1999,
0
5 10 15 20 25 30
2000).
r
 m ]
e ff
Fig 2: Model results showing the effects on
precipitation amounts due to the introduction
of small amounts of giant CCN into
continental and maritime clouds. Note the
sharp increase in rainfall in the continental
case and the slight decrease in rain in the
maritime case.
35
Quantitative evidence for aerosols suppressing precipitation
It was already shown that rainfall from clouds with moderate depth (up to about 5 km)
could be completely shutoff. However, it was believed that clouds that grow well
above the height of the freezing level have a “second chance” to turn their water into
precipitation by freezing onto sleet and snow
particles. It turns out that it is not always the case:
Fig. 3: Observing storm clouds that keep their water aloft:
This ordinary looking cloud is composed mostly of small liquid
water droplets at temperatures as cold as -37.5oC, the coldest that
was ever documented in cloud by in-situ scientific
measurements. The highly super cooled liquid water, which
amounted to 1.8 gram per cubic meter, remained as small
droplets in the cloud instead of turning back as precipitation.
The cloud was measured by a cloud physics aircraft in west
Texas on 13 August 2000, at flight level of 32,000 feet
(Woodley and Rosenfeld, 2000).
3
Rainfall Volume per cloud [m ]
The water in the cloud shown in the picture is distributed in many small
droplets, with concentrations of up to 1000 per cubic cm (Rosenfeld and Woodley,
2000). Such small droplets freeze slowly at temperatures warmer than -38oC. When
these droplets eventually freeze they become small ice particles that are blown off the
top of the cloud and lose the ability to turn into precipitation.
Each cloud droplet is formed on one aerosol particle. A computer simulation
replicated all the observed properties of the cloud shown in the picture (Khain et al.,
2001), but when this simulation was repeated with cleaner air, the result was a cloud
with only 250 droplets per cubic cm that froze readily onto sleet and snow
precipitation between -10oC and -25oC, and the calculated total rainfall output of the
“clean” cloud was doubled compared to the “polluted” case. The clouds that were
simulated by Khain et al. (2001) reached height of 12 km and top temperatures colder
than –50oC. Simulations of clouds reaching a smaller height of 5 km and a warmer top
temperature of –19oC (Reisin et al., 1996) have shown a factor of 5 less rainfall for
clouds simulated in polluted air containing 1100 Cloud Condensation Nuclei (CCN)
cm-3 and very clean air containing 100 CCN cm-3.
The results of the cloud simulations were supported by radar and aircraft
measurements
(Rosenfeld
and
Woodley, 2002), showing that even the
Strong Drop Coalescence
deepest tropical clouds, which produce
W eak Drop Coalescence
the bulk of the tropical rainfall, can
7
10
precipitate less than half the rainfall
under conditions of suppressed cloud
6
droplets coalescence compared to
10
clouds with strong coalescence.
Fig. 4: Measuring the loss of rainfall: The
measured averaged rainfall production by
individual tropical clouds in northwestern
Thailand, in normal (blue) and inhibited (red)
conditions of rain formation. The clouds
reaching the greatest heights produce most of
10
10
10
10
5
4
3
2
3
5
7
9 11 13 15 17
Maxim um cloud top height [km]
the rainfall. However, even the deepest clouds produce about half of the rainfall when cloud droplet
coalescence was inhibited, as can be caused by impact of smoke and air pollution.
The polluted clouds keep most of the water instead of precipitating it.
Raindrops that manage to form enjoy ample cloud water and can grow to large sizes
and form short intense bursts of rainfall. These larger drops are seen by the radar as
more rainfall than it really is, when not taking this effect into account (Rosenfeld and
Ulbrich, 2002). The measurements presented in Fig. 4 were made with a fixed Z-R
relationship (Z is the intensity of the precipitation radar echoes, R is the rain
intensity). Therefore, the indicated difference is likely an underestimate of the real
magnitude of the difference of rainfall production between these two cloud types.
This effect of the aerosols on the raindrop size distribution has contributed to hide
until now the detection and quantification of the impact of air pollution on
precipitation, in spite of the recent advancements in rainfall measurements by radars
and satellites.
As was pointed out before, aerosols, under certain circumstances, can also
enhance precipitation. Giant CCN, naturally produced by sea salt and by some types
of desert dust particles, can do so (Johnson, 1982; Feingold, 1999; Yin et al., 2000;
Wurzler et al., 2000). Lahav and Rosenfeld (2000) have shown some indications that
sea salt might be important in initiating precipitation from clouds in Israel.
The ice nucleating activity of aerosols can also affect the precipitation, usually
positively; especially in supercooled clouds that drop coalescence is suppressed due to
large concentrations of small CCN. Desert dust was shown to have ice-nucleating
activity in Israel (Levi and Rosenfeld, 1996). Urban and industrial emissions might
also have ice-nucleating activity.
In other words, the effects of aerosols on clouds depend on the chemical
composition and size of the particles as well as on the type of clouds.
In addition to the aerosols directly interacting with clouds and changing their
properties, they can suppress the formation of potential rain clouds altogether through
their direct radiative forcing. By blocking part of the solar radiation from reaching the
surface they reduce evaporation from the oceans and reduce the necessary heating that
energizes the convective clouds (Hansen et al., 1997; Ackerman et al., 2000).
On the global scale, precipitation must balance evaporation, which is dictated by
the solar energy reaching the surface. Therefore, the suppression of precipitation at
one place must be compensated by enhancing precipitation elsewhere. The net effect
would be redistribution of precipitation from the most polluted areas to the oceans and
cleaner land areas. Because precipitation releases vast amounts of latent heat energy,
these shifts in precipitation are likely to cause shifts in the global circulation of the
atmosphere, including storm tracks.
Hints to the relations between aerosol and trends of precipitation can be seen
when comparing the two. Quantifying the impacts on the regional and global scales
remains a major challenge. In this proposal we will focus on the local effects, which
are tied mainly to the scales of the individual cloud and cloud cluster.
1.2 From emissions of pollutants to cloud compositions
Clouds are affected by aerosols containing soluble material that can absorb water
vapor and activate at slight supersaturation. The soluble material can be either
inorganic (such as sulfate and nitrate) or organic (such as organic acids). Recent field
measurements showed that organic aerosol could contribute substantially to the cloud
condensation nuclei (CCN) population (Novakov). The aerosols ability to nucleate
clouds depends on their chemical composition and their size distribution. Both of
these parameters are crucial for understanding and quantifying aerosol-cloudprecipitation relationships. For example, Yin et al. (2000a, 2000b) have shown that
even small number of large CCN may increase precipitation in clouds that otherwise
precipitate little amounts. On the other hand, Rosenfeld et al (2000) have shown that
mineral dust coated by minute amounts of soluble material can suppress precipitation.
Detail measurements of the aerosol population are key for quantifying this complex
phenomenon.
According to the Köhler equation, supersaturation has to exceed a critical value, Sc, not much above
100% saturation, before a droplet starts to grow spontaneously to cloud-droplet size. Sc is a function of,
among other things, the concentration of solute in the wetted aerosol particle and its surface tension. By
adding soluble material to the atmosphere (for example sulfate particles derived from industrial
emissions of SO2, or water soluble organics) we are now increasing the number and the efficiency of
CCN, and thereby increasing the concentration of droplets in the clouds of polluted regions. Wurzler et
al (2000) have shown that SO2 oxidation to sulfate in clouds can convert insoluble aerosols and
therefore inefficient CCN, such as mineral dust particles, into efficient ones. The increase in
concentrations would make the clouds brighter because of an increased surface area per volume of
water produce less precipitation and may also be longer lived. Another factor in the Köhler equation
is the surface tension. The presence of surfactant organic material could lower the surface tension of
the wetted aerosol particles so that the critical supersaturation Sc decreases and allows more particles to
become activated and grow to cloud droplets. This, again, would make the clouds brighter and lead to a
negative climate forcing and less precipitation. The surfactants could be supplied from direct emissions
or from aerosols serving as CCN. In addition, surfactants on small cloud drops resulting from emission
of pollutants could reduce the condensation process resulting in small cloud drops and brighter clouds
(Feingold and Chang, 2002).
This human influence on clouds and climate has been discussed at length during
the past few years as a possible masking effect on greenhouse warming. But because
of the complexity of the processes involved and the difficulty of validating theoretical
estimates by measurements, the magnitude of this indirect aerosol effect on climate
has remained uncertain. Recent comprehensive assessment of the very large aerosol
impacts on climate by affecting both the radiative budget and the hydrological cycle is
given in a review paper in Science (Ramanathan et al., 2001). The main conclusions
are:
a) On a regional scale, the aerosol effects can be much greater than
those of the greenhouse gases, especially in densely populated areas;
b) There are large, yet little known quantitatively, impacts on the
hydrological cycle and through that on the general circulation.
2. Research objectives and significance
The main objective of the proposed research is the documentation of the links leading
from emission of aerosols and pollutants to eventually changing the precipitation and
water availability in Israel. These links include:
1. Emissions of anthropogenic and natural gaseous and particulate pollutants.
2. Transport and chemical interactions that determine the aerosol physical,
chemical and optical properties.
3. The CCN activity of the aerosols depending on their TOTAL size distribution
and chemical composition.
4. The manifestation of the CCN distribution in cloud drop size distribution, in
different cloud dynamic situations.
5. Initiation and development of precipitation in the clouds.
6. Quantification of the differences in rainfall between clouds developing at
different dynamic situations and aerosol contents.
7. Assessment of the integral impact on rainfall, using both climatic records and
direct model calculations.
8. Feedback of the clouds and precipitation on the aerosols modification and
scavenging,
It will be presumptuous for us to promise complete answers to all these questions
within the scope of this proposal. Instead, we view this as a vehicle for continuation
of our pioneering work in the field towards the next phase, which will provide
knowledge basis for the following applications:
1. Evaluation of the levels of air pollution emission on rainfall amounts.
2. Development of technologies for monitoring environmental impacts on
precipitation.
3. Development of technology for reducing rainfall-damaging emissions.
4. Helping the technology of precipitation enhancement.
Major contribution to the understanding of man-made induced climate change