Dust — and the microbes hitching rides on it — influences rain, climate

A press release from the American Chemical Society: Dusty air blowing across the Pacific from Asia and Africa plays a critical role in precipitation patterns throughout the drought-stricken western U.S. Today, a scientist will present new research suggesting that the exact chemical make-up of that dust, including microbes found in it, is the key to how much rain and snow falls from clouds throughout the region. This information could help better predict rain events, as well as explain how air pollution from a variety of sources influences regional climate in general.

...“We’ve learned that not all of the particles in the air at high altitudes have the same influence on clouds. We’re starting to think that these differences contribute to how rain gets distributed,” says Kim Prather, Ph.D.

Most of the dust that Prather’s team at the University of California, San Diego, detects in clouds and precipitation originates in Asian and Chinese deserts. It gets swept westward by the jet stream where it mixes with a variety of other airborne particles such as sea spray and smoke. Prather says that each of these types of particles — collectively known as aerosols — has its own, distinctive impact on clouds.

A major key to what turns ordinary clouds into rainmakers in the first place is their ability to form ice crystals around the microscopic particles that invade and “seed” them, Prather said. Without ice crystals as a catalyst, rain development inside clouds can be impaired.   “The standard belief is the more ice you have in a cloud, the more likely you will get precipitation out of it,” she says. “Our goal is to catch the first stages of ice forming and find out what exactly the chemical constituents are that the ice is forming on.”

...Prather speculates that the microbes hitch a ride on bits of sand, iron and other debris swept aloft from desert regions. Microbes and biological components could also become mixed with the dust as it is transported across the Pacific. She and her team are eager to learn just how important these bioparticles are in the rainmaking process.

...“Long term, our goal is to be able to predict how much precipitation we can expect to form when certain aerosols such as dust are coming toward us,” says Prather. “That’s a lofty goal but we’re making headway.”...

Via NASA, a satellite view of a dust cloud over Asia

No limits to human effects on clouds

The Press Room of the Weizmann Institute of Science (Israel): Understanding how clouds affect the climate has been a difficult proposition. What controls the makeup of the low clouds that cool the atmosphere or the high ones that trap heat underneath? How does human activity change patterns of cloud formation? The research of the Weizmann Institute’s Prof. Ilan Koren suggests we may be nudging cloud formation in the direction of added area and height. He and his team have analyzed a unique type of cloud formation; their findings, which appeared recently in Science indicate that in pre-industrial times, there was less cloud cover over areas of pristine ocean than is found there today.

Clouds need tiny particles called aerosols that rise in the atmosphere, in order to form. These aerosols – natural ones like sea salt or dust, or such human-made ones as soot – form nuclei around which the cloud droplets condense. In relatively clean environments, clouds can only grow as large as the amount of aerosols in the atmosphere allows: They will be the limiting factor in cloud formation.

The question is: Does the current load of aerosols in the atmosphere already exceed that limit, in which case adding extra particles should not greatly affect cloud form
ation; or do they continue to be a limiting factor as pollution rises, so that added aerosols would continue to influence the clouds? A model developed by Koren and his team showed that an increase in aerosols, even in relatively polluted conditions, should result in taller, larger clouds that rain more aggressively. ...

Koren, research student Guy Dagan and Dr. Orit Altaratz in the Earth and Planetary Sciences Department looked to an unlikely place to test their model: near the horse latitudes. These are subtropical regions far out in the oceans that were reviled in the past by sailors because the winds that carried their sails would die out there for weeks on end. Here was a lab for them to test the basic physics of their model: an atmospheric region controlled by well-defined meteorological conditions, which was sometimes pristine, sometimes containing low levels of aerosols. If the model was correct, transitions from one to the other should be dramatic. And they wanted to test their theory on the clouds that do form in this region – warm convective clouds that are fuelled by the ocean’s moisture.

...Koren: “We showed that convective clouds do not necessarily stop being aerosol-limited; under relatively polluted conditions the increase in aerosol loading will make the clouds taller, larger and their rain-rate stronger. As the area of this cloud cover grows, it reflects more of the shortwave radiation; but as the clouds get taller, their greenhouse effect becomes more significant, counteracting about half of their total cooling effect.”  

Convective clouds forming over the Amazon in a blanket of smoke. Image: Prof. Ilan Koren

Solution to cloud riddle reveals hotter future

The Newsroom at the University of New South Wales: Global average temperatures will rise at least 4°C by 2100 and potentially more than 8°C by 2200 if carbon dioxide emissions are not reduced, according to new research published in Nature that shows our climate is more sensitive to carbon dioxide than most previous estimates.  The research could solve one of the great unknowns of climate sensitivity, the role of cloud formation and whether this will have a positive or negative effect on global warming.

“Our research has shown climate models indicating a low temperature response to a doubling of carbon dioxide from preindustrial times are not reproducing the correct processes that lead to cloud formation," said lead author from UNSW's Centre of Excellence for Climate System Science, Professor Steven Sherwood.

“When the processes are correct in the climate models the level of climate sensitivity is far higher. Previously estimates of the sensitivity of global temperature to a doubling of carbon dioxide ranged from 1.5°C to 5°C. This new research takes away the lower end of climate sensitivity estimates, meaning that global average temperatures will increase by 3°C to 5°C with a doubling of carbon dioxide."

...When the processes are correct in the climate model, this produces cycles that take water vapour to a wider range of heights in the atmosphere, causing fewer clouds to form in a warmer climate. This increases the amount of sunlight and heat entering the atmosphere and increases the sensitivity of our climate to carbon dioxide or any other perturbation.

When water vapour processes are correctly represented, the sensitivity of the climate to a doubling of carbon dioxide - which will occur in the next 50 years – means we can expect a temperature increase of at least 3°C and more likely 4°C by 2100.

“Climate sceptics like to criticise climate models for getting things wrong, and we are the first to admit they are not perfect, but what we are finding is that the mistakes are being made by those models which predict less warming, not those that predict more,” said Professor Sherwood. “Rises in global average temperatures of this magnitude will have profound impacts on the world and the economies of many countries if we don’t urgently start to curb our emissions."...

A lenticular cloud above Hawaii, shot by Rootmeansquare, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license

Ozone hole might slightly warm planet

A press release from the America Geophysical Union: A lot of people mix up the ozone hole and global warming, believing the hole is a major cause of the world’s increasing average temperature. Scientists, on the other hand, have long attributed a small cooling effect to the ozone shortage in the hole.

Now a new computer-modeling study suggests that the ozone hole might actually have a slight warming influence, but because of its effect on winds, not temperatures. The new research suggests that shifting wind patterns caused by the ozone hole push clouds farther toward the South Pole, reducing the amount of radiation the clouds reflect and possibly causing a bit of warming rather than cooling.

 “We were surprised this effect happened just by shifting the jet stream and the clouds,” said lead author Kevin Grise, a climate scientist at Lamont-Doherty Earth Observatory of Columbia University in New York City. Grise notes this small warming effect may be important for climatologists trying to predict the future of Southern Hemisphere climate.

The work is detailed in Geophysical Research Letters, a journal of the American Geophysical Union. Grise collaborated on the study with Lorenzo Polvani of Columbia University, George Tselioudis of NASA Goddard Institute for Space Studies, Yutian Wu of New York University, and Mark Zelinka of Lawrence Livermore National Laboratory.

...Dennis Hartmann, an atmospheric scientist at the University of Washington in Seattle unrelated with the project, points out that since predicting cloud behavior is so challenging, the model used in Grise’s study could be underestimating clouds north of the jet stream being pulled toward the equator and in turn reflecting more light, potentially reducing or even negating the warming effect. Hartmann added that he also has some concerns about the modeling of the low-level cloud response. Still, “this is certainly a very interesting topic and potentially important from a practical perspective of predicting Southern Hemisphere climate and even global warming rates,” he commented...

The ozone hole via NASA, on September 22, 2012

Saharan and Asian dust, biological particles end global journey in California

Scripps News at the Scripps Institution of Oceanography: A field study of aerosol impacts on clouds and precipitation in the Sierra Nevada shows that dust and microorganisms transported from as far away as the Sahara desert help to spur the precipitation that California counts on for its water supply.

The CalWater field campaign, funded by the California Energy Commission and led by UC San Diego and NOAA, could help western states better understand the future of their water supply and hydropower generation as climate change influences how much and how often dust travels around the world and alters precipitation far from its point of origin.

"UC San Diego is a leader in addressing complex, multi-disciplinary global challenges, such as water shortages and environmental concerns," said UCSD Chancellor Pradeep K. Khosla. "Our researchers work collaboratively to investigate and produce meaningful and impactful research that will further our understanding of our planet and environment, so we can improve human life and our world."

Jessie Creamean, a postdoctoral associate at NOAA's Earth System Research Laboratory in Boulder, Colo., co-authored the paper appearing in the journal Science with Kaitlyn Suski, a graduate student in the laboratory of Distinguished Chair in Atmospheric Chemistry Prof. Kimberly Prather, who holds appointments at Scripps Institution of Oceanography and the Department of Chemistry and Biochemistry at UCSD.

"We were able to show dust and biological aerosols that made it from as far as the Sahara were incorporated into the clouds to form ice, then influenced the formation of the precipitation in California," said Creamean, who conducted the fieldwork as a UCSD graduate student under Prather, the study leader. "To our knowledge, no one has been able to directly determine the origin of the critical aerosols seeding mid-level clouds which ultimately produce periods with extensive precipitation typically in the form of snow at the ground."...

From NASA: Saharan sand blowing off the coasts of Mauritania and Senegal

Cloud forest trees drink water through their leaves

Robert Sanders in the University of California-Berkeley News Center: Tropical montane cloud forest trees use more than their roots to take up water. They also drink water from clouds directly through their leaves, University of California, Berkeley, scientists have discovered.

While this is an essential survival strategy in foggy but otherwise dry areas, the scientists say that the clouds the trees depend on are now disappearing due to climate change.

“The study highlights the vulnerability of this rare and already endangered ecosystem to climate change,” said Todd Dawson, senior author of the study and UC Berkeley professor of integrative biology. Changes in cloud cover have already been correlated to declines and disappearances of cloud forest animal populations, such as frogs and salamanders.

The new study will be published next year in the journal Ecology Letters and is available online this month.

In tropical montane cloud forests, leaves are constantly bathed in clouds, making them wet. The leaves of the most common cloud forest trees drink this cloud water when water from the soil just isn’t enough, said Greg Goldsmith, lead author of the study and a graduate student in Dawson’s lab.

“Many cloud forests experience an annual dry season when the primary water source isn’t rain, but rather, the moisture from the clouds,” he said. “This is when the trees are most likely to draw water in through their leaves.”...

Graduate student Greg Goldsmith in the montaine cloud forest of Monteverde Cloud Forest Reserve, Costa Rica. Photo courtesy of Drew Fulton (Canopy in the Clouds)

Pollution teams with thunderclouds to warm atmosphere

Pacific Northwest National Laboratory News Center: Pollution is warming the atmosphere through summer thunderstorm clouds, according to a computational study published May 10 in Geophysical Research Letters. How much the warming effect of these clouds offsets the cooling that other clouds provide is not yet clear. To find out, researchers need to incorporate this new-found warming into global climate models.

Pollution strengthens thunderstorm clouds, causing their anvil-shaped tops to spread out high in the atmosphere and capture heat — especially at night, said lead author and climate researcher Jiwen Fan of the Department of Energy's Pacific Northwest National Laboratory.

"Global climate models don't see this effect because thunderstorm clouds simulated in those models do not include enough detail," said Fan. "The large amount of heat trapped by the pollution-enhanced clouds could potentially impact regional circulation and modify weather systems."

Clouds are one of the most poorly understood components of Earth's climate system. Called deep convective clouds, thunderstorm clouds reflect a lot of the sun's energy back into space, trap heat that rises from the surface, and return evaporated water back to the surface as rain, making them an important part of the climate cycle.

...Previous work showed that when it's not too windy, pollution leads to bigger clouds. This occurs because more pollution particles divide up the available water for droplets, leading to a higher number of smaller droplets that are too small to rain. Instead of raining, the small droplets ride the updrafts higher, where they freeze and absorb more water vapor. Collectively, these events lead to bigger, more vigorous convective clouds that live longer.

Now, researchers from PNNL, Hebrew University in Jerusalem and the University of Maryland took to high-performance computing to study the invigoration effect on a regional scale. Fan and colleagues found that for the warm summer thunderstorms, pollution led to stronger storms with larger anvils....

An anvil-shaped cumulus cloud and alto-cumulus, shot by Fir0002, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license

Cloud changes may lower global temperature

University of Auckland: Research from The University of Auckland on changes in cloud height in the decade to 2010 has provided the first hint of a cooling mechanism that may be in play in the Earth’s climate. Published in the journal Geophysical Research Letters, the analysis of the first ten years of data from the NASA Terra satellite revealed an overall trend of decreasing cloud height. Global average cloud height declined by around 1 per cent over the decade, or around 30 to 40 metres. Most of the reduction was due to fewer clouds occurring at very high altitudes.

“This is the first time we have been able to accurately measure changes in global cloud height and, while the record is too short to be definitive, it provides just a hint that something quite important might be going on,” explains lead researcher Professor Roger Davies. Longer-term monitoring will be required to determine the significance of the observation for global temperatures.

A consistent reduction in cloud height would allow the Earth to cool to space more efficiently, reducing the surface temperature of the planet and potentially slowing the effects of global warming. This may represent a “negative feedback” mechanism – a change caused by global warming that works to counteract it. “We don’t know exactly what causes the cloud heights to lower,” says Professor Davies, “but it must be due to a change in the circulation patterns that give rise to cloud formation at high altitude.”

Until recently however, it was impossible to measure the changes in global cloud heights and understand their contribution to global climate change.

“Clouds are one of the biggest uncertainties in our ability to predict future climate,” says Professor Davies. “Cloud height is extremely difficult to model and therefore hasn’t been considered in models of future climate. For the first time we have been able to accurately measure the height of clouds on a global basis, and the challenge now will be to incorporate that information into climate models. It will provide a check on how well the models are doing, and may ultimately lead to better ones.”...

Image by Ted Garvin, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 2.5 Generic license

CERES continues legacy of cloud study on a crucial climate monitoring satellite program

From NASA: In October when NASA launches its next-generation Earth-observing satellite, NPP (NPOESS Preparatory Project), one of the passengers aboard will be the latest in a series of instruments that has studied the Earth's climate for nearly 30 years. The first Clouds and the Earth's Radiant Energy System (CERES) instrument launched in 1997. Before that, the job was done by the Earth Radiation Budget Experiment (ERBE) beginning in 1984.

For 27 years without a break, the instruments collectively have returned a vast amount of data about the solar energy reflected and absorbed by Earth, the heat the planet emits, and the role of clouds in that process. "Like wine, CERES gets better with time," said Norman Loeb, CERES principal investigator at NASA's Langley Research Center in Hampton, Va.

Amassing a long record of data is important because CERES monitors minute changes in the Earth's energy budget - the balance between incoming and outgoing energy - that can lead to serious longer-term consequences, such as polar ice melting and rising sea levels, said Loeb.

Scientists are studying the planetary energy balance that results from these interactions primarily because the Earth's atmosphere is influenced by the buildup of human-released carbon dioxide and other greenhouse gases. "Clouds both reflect sunlight and block energy from radiating to space," Loeb said. "Which of these two dominates depends upon the properties of clouds, such as their amount, thickness and height."…

This image from NASA's Clouds and the Earth's Radiant Energy System (CERES) sensor on the Terra satellite show reflected solar radiation. Dark blue in the Arctic regions of the right image show the lack of reflected radiation. Greens, yellows, and whites indicate higher levels of reflected radiation in higher latitudes. Credit: NASA/T. Wong, CERES Science Team

CLOUD experiment provides unprecedented insight into cloud formation

University of Leeds News: Research from CERN involving University of Leeds scientists provides news insights into cloud formation in the atmosphere. In a paper published in the journal Nature today, the CLOUD experiment - designed to study the effect of cosmic rays on the formation of atmospheric aerosols under controlled laboratory conditions - reports its first results.

Aerosols are tiny particles suspended in the atmosphere which are thought to be responsible for a large fraction of the seeds that form cloud droplets. Understanding the process of aerosol formation is therefore important for understanding the climate.

The CLOUD results show that trace vapours assumed until now to account for aerosol formation in the lower atmosphere can explain only a tiny fraction of the observed atmospheric aerosol production. The results also show that ionisation from cosmic rays significantly enhances aerosol formation. Precise measurements such as these are important in achieving a quantitative understanding of cloud formation, and will contribute to a better assessment of the effects of clouds in climate models.

"These new results from CLOUD are important because we've made a number of first observations of some very important atmospheric processes," said the experiment's spokesperson, Jasper Kirkby from CERN. "We've found that cosmic rays significantly enhance the formation of aerosol particles in the mid troposphere and above. These aerosols can eventually grow into the seeds for clouds. However, we've found that the vapours previously thought to account for all aerosol formation in the lower atmosphere can only account for a small fraction of the observations - even with the enhancement of cosmic rays."

Co-author Professor Ken Carslaw from the University of Leeds, said: "Twenty years of research has told us that the formation of atmospheric particles is important for Earth's climate. Now the CLOUD experiment has given us a unique insight into how the particles form and will completely change our understanding of what's happening in the atmosphere."...

A roll cloud in Racine, Wisconsin in 2007, shot by Eazydp, Wikimedia Commons

Clouds, clouds, burning bright

NASA/Goddard Space Institute: High up in the sky near the poles some 50 miles above the ground, silvery blue clouds sometimes appear, shining brightly in the night. First noticed in 1885, these clouds are known as noctilucent, or "night shining," clouds. Their discovery spawned over a century of research into what conditions causes them to form and vary – questions that still tantalize scientists to this day. Since 2007, a NASA mission called Aeronomy of Ice in the Mesosphere (AIM) has shown that the cloud formation is changing year to year, a process they believe is intimately tied to the weather and climate of the whole globe.

"The formation of the clouds requires both water and incredibly low temperatures," says Charles Jackman, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., who is NASA's project scientist for AIM. "The temperatures turn out to be one of the prime driving factors for when the clouds appear."

So the appearance of the noctilucent clouds, also known as polar mesospheric clouds or PMCs since they occur in a layer of the atmosphere called the mesosphere, can provide information about the temperature and other characteristics of the atmosphere. This in turn, helps researchers understand more about Earth's low altitude weather systems, and they've discovered that events in one hemisphere can have a sizable effect in another.

…AIM researchers also believe there is a connection between seemingly disparate atmospheric patterns in the north and south. The upwelling of polar air each summer that contributes to noctilucent cloud formation is part of a larger circulation loop that travels between the two poles. So wind activity some 13,000 miles (20,920 km) away in the northern hemisphere appears to be influencing the southern circulation.

…"The real importance of all of that," says Hampton's Russell, "is not only that events down where we live can affect the clouds 50 miles (80 km) above, but that the total atmosphere from one pole to the next is rather tightly connected."…

Looking down from above, AIM captured this composite image of the noctilucent cloud cover above the Southern Pole on December 31, 2009. The 2009 cloud season began a month earlier than the 2010 season did. Credit: NASA/HU/VT/CU LASP

Scientists fly through the clouds to piece together climate puzzle

NASA: As scientists try to better understand and put together the puzzle of Earth's climate, the role of clouds remains one of the most important missing pieces. Researchers from four NASA centers, other U.S. agencies and several colleges and universities are set to participate in the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX), an airborne field campaign based at Ellington Field, Texas, that aims to answer some major questions about clouds. The campaign is scheduled to begin March 14, with science flights onboard the NASA WB-57 from March 28 to April 29.

"Initially it started as a question about the real characteristics of ice clouds in the lowest portion of the atmosphere, the upper troposphere. Past observations had shown ice particles are a lot smaller and lot more numerous than people thought they were," said Ken Jucks, Upper Atmosphere Program manager at NASA Headquarters in Washington. "More recent measurements done at the tropics have shown that may not actually be true, but no measurements have been made at the mid-latitudes."

The campaign will study the composition of cirrus clouds -- high, wispy clouds made of ice -- and their relationship to Earth's "energy budget," the combination of incoming and outgoing energy or heat.

"Recent advances in instrumentation greatly improve our ability to accurately measure the sizes and numbers of ice crystals in cirrus clouds," said MACPEX project scientist Eric Jensen of NASA's Ames Research Center in Mountain View, Calif. "This information is critically needed to evaluate and improve representations of cirrus in climate models. We anticipate that MACPEX will ultimately lead to improved accuracy of climate predictions."…

The WB-57 airplane can fly as high as 63,000 feet. Credit: NASA

Cloud study predicts more global warming

Science Daily: Current state-of-the-art global climate models predict substantial warming in response to increases in greenhouse gases such as carbon dioxide. The models, though, disagree widely in the magnitude of the warming we can expect. The disagreement among models is mainly due to the different representation of clouds. Some models predict that global mean cloud cover will increase in a warmer climate and the increased reflection of solar radiation will limit the predicted global warming. Other models predict reduced cloudiness and magnified warming.

In a paper that has just appeared in the Journal of Climate, researchers from the University of Hawaii Manoa (UHM) have assessed the performance of current global models in simulating clouds and have presented a new approach to determining the expected cloud feedbacks in a warmer climate.

Lead author Axel Lauer at the International Pacific Research Center (IPRC) at UHM notes, "All the global climate models we analyzed have serious deficiencies in simulating the properties of clouds in present-day climate. It is unfortunate that the global models' greatest weakness may be in the one aspect that is most critical for predicting the magnitude of global warming."

To study the clouds, the researchers applied a model representing only a limited region of the atmosphere over the eastern Pacific Ocean and adjacent land areas. The clouds in this region are known to greatly influence present climate, yet current global models do poorly in representing them. The regional model, developed at the IPRC, successfully simulates key features of the region's present-day cloud fields, including the observed response of clouds to El Nino. Having evaluated the model's simulation of present-day conditions, the researchers examined the response of simulated clouds in a warmer climate such as it might be in 100 years from now. The tendency for clouds to thin and cloud cover to reduce was more pronounced in this model than in any of the current global models.

Co-author Kevin Hamilton concludes, "If our model results prove to be representative of the real global climate, then climate is actually more sensitive to perturbations by greenhouse gases than current global models predict, and even the highest warming predictions would underestimate the real change we could see."…

A sharp mountain peak with cloud formed by orographic lifting of the air. A view from Lynn Canal. Shot by Commander John Bortniak, NOAA Corps

Sandy spores help seed clouds

Kate Ravilious in Environmental Research Web: For most of us sandy beaches are associated with sandcastles and sunbathing, but now it turns out that these golden stretches of coastline could also play an important role in the kind of weather that we receive. New research shows that bacterial and fungal spores in windblown sand may be influencing the amount and type of clouds in the sky.

Brian Palenik from the Scripps Institution of Oceanography, US, and colleagues collected air samples from the end of the Scripps Institution of Oceanography pier – located around 300 m from the shore. An analysis of the aerosols in the samples showed that two types of fungi – Basidiomycota and Ascomycota – and two types of bacteria – Firmicutes and Proteobacteria – were common constituents of the mix.

….Palenik and colleagues' data suggest that sandy beaches could be a significant source of biological aerosols with the potential to affect cloud formation. Since sandy beaches are found all over the world they may be an important provider of biological aerosols and hence play a previously unappreciated role in our weather. And as our climate changes the role that some of these biological aerosols play could alter too. …

Crepuscular rays through clouds, shot by PiccoloNamek, Wikimedia Commons, under the Creative Commons Attribution ShareAlike 3.0

NOAA scientists uncover oscillating patterns in clouds

NOAA: A new NOAA study has found that rain clouds form synchronous patterns in which individual clouds in a large cloud field respond to signals from other clouds, much like chirping crickets or flashing fireflies on a summer night. The study, published online today in the journal Nature also has significant implications for our understanding of climate change research.

This research, led by Graham Feingold of NOAA’s Earth System Research Laboratory in Boulder, Colo., shows for the first time that interactions between certain types of neighboring clouds can result in synchronized rain patterns within a large cloud system.

“Clouds organize in distinct patterns that are fingerprints of myriad physical processes,” Feingold explained. “Precipitation can generate fascinating honeycomb-like patterns that are clearly visible from satellites. Cloud fields organize in such a way that their components ‘communicate’ with one another and produce regular, periodic rainfall events.”

While the discovery of synchronized behavior in clouds is one of many recent findings on self-organization in nature, the study also examines how suspended particles, or aerosols, in the atmosphere can influence these patterns and be a factor in climate change.

…“Once precipitation ensues and an open structure has formed, it is difficult to revert the cloud field to a closed-cell, or overcast state,” Feingold said. “Rain keeps the oscillating, open honeycomb pattern in motion, which allows more sun to reach Earth’s surface.”

The scientists say that their findings point to a significant influence of particulate matter, or aerosols, on the large-scale structure of clouds and therefore on climate change. Scientists have long known that aerosols can influence local rain formation and block solar energy from reaching the Earth’s surface—for an overall surface cooling effect.

…“Our work also suggests that we should expand our thinking about interactions between aerosols and clouds,” Feingold said. “Integrating our current focus on fundamental physical processes with broader studies on system dynamics could give us a more complete understanding of climate change.”

Satellite image above Peru shows self-organizing honeycomb cloud pattern. From NASA

Dust beats biology for cloud ice formation

Liz Kalaugher in Environmental Research Web: Biological particles have been touted as potential key players in the nucleation of ice particles in clouds, as they are able to help ice form at warmer temperatures than many other aerosols. But new modelling work in Norway and Germany indicates that biological particles are not that important for cloud ice formation after all.

"We are confident now that on a global scale, biological particle concentrations are too low to play a significant role in cloud ice formation, and that instead mineral dust is the major component," Corinna Hoose from the University of Oslo, Norway and Karlsruhe Institute of Technology, Germany told environmentalresearchweb. "This is in agreement with many measurements of ice crystal residuals."

Together with colleagues from Oslo and the Max Planck Institute for Chemistry, Germany, Hoose calculated that biological particles contributed a maximum of 0.6% to the global average ice nucleation rate. The team used the CAM–Oslo aerosol climate model and newly available estimates and laboratory measurements to come up with their results.

…Ice nucleation in clouds is of particular interest as it is the first step in the creation of snow and most types of rain. Previously, high concentrations of biological ice nuclei have been found in a wave cloud over Wyoming and in the Amazon basin, and the particles have been found to be ubiquitous in precipitation on different continents. "[Our study] means that living or dead micro-organisms are less involved in cloud ice and precipitation formation than has been speculated, and that the observed cases with high biological ice nucleus concentrations are not representative of average atmospheric conditions," said Hoose....

Cumulus clouds just floating along, shot by Michael Jastremski., Wikimedia Commons, under the Creative Commons Attribution-Share Alike 2.0 Generic license.

Amount of dust, pollen matters for cloud precipitation, climate change

Science Daily: A lot of large particles of dust and pollen in the atmosphere may make your nose twitch, but they can lead directly to greater precipitation in clouds, Colorado State University atmospheric scientists have discovered for the first time.

The amount of ice crystals necessary to form precipitation in clouds is linked to the abundance of larger aerosol particles in the atmosphere, according to a study by Paul DeMott and Anthony Prenni, research scientists in the Atmospheric Science department at Colorado State. Their findings appear in this week's issue of the Proceedings of the National Academy of Sciences.

Using these new findings, a global climate model predicted that clouds have a stronger cooling effect on the globe than previously estimated. However, future increases in these ice nuclei for cold clouds would reduce the cooling impact on climate and vice versa, the scientists found.

Special particles called aerosols -- resulting from desert dust, some biological processes and possibly from pollution -- are needed as catalysts to form ice in clouds, which can influence precipitation and cloud dynamics. These particles can serve as the center, or nuclei, for cloud droplets that combine to form raindrops. "The catalysts for most ice nuclei are primary emissions -- from pollution or sea spray or dust," DeMott said. "The bigger the particles, the better it is for ice nuclei."

At the same time, pinpointing a number of particles at a specific temperature is too simple for climate models to accurately represent what's occurring in nature, DeMott said…..

Clouds shot by Ted Garvin, Wikimedia Commons, under the Creative CommonsAttribution-Share Alike 2.5 Generic license

Laser creates clouds over Germany

Colin Barras in New Scientist: A laser has been used to generate small clouds on demand in lab, and real-world experiments suggest this could be a way to call down rain when it's needed. People have experimented with cloud seeding for decades in the hope of boosting rainfall, usually by sprinkling silver iodide crystals into clouds high in the atmosphere.

These crystals encourage large water droplets to form around them, and the droplets then fall as rain – in theory, at least. "The efficiency of this technique is controversial," says Jérôme Kasparian at the University of Geneva, Switzerland, one member of a research team that think lasers may be a better way to trigger rain on demand.

Kasparian and colleagues have just reported the first successful use of this technique to summon clouds from air both in the lab and in the skies over Berlin, Germany. In the lab the team fired extremely short pulses of infrared laser light into a chamber of water-saturated air at -24 °C. Linear clouds could be seen to form in the laser's wake, like a miniature airplane contrail.

Kasparian says that the laser pulses generate clouds by stripping electrons from atoms in air, which encourage the formation of hydroxyl radicals. Those convert sulphur and nitrogen dioxides in air into particles that act as seeds to grow water droplets. Each laser pulse packs a 220-millijoule punch into just 60 femtoseconds – an intensity "equivalent to the power of 1000 power plants," says Kasparian….

Cirrocumulus stratiformis and blue blue sky, shot by Simon Eugster, Wikimedia Commons, under the Creative Commons Attribution ShareAlike 3.0 License

Strong evidence that cloud changes may exacerbate global warming

Rosenstiel School of Marine and Atmospheric Science: The role of clouds in climate change has been a major question for decades. As the earth warms under increasing greenhouse gases, it is not known whether clouds will dissipate, letting in more of the sun’s heat energy and making the earth warm even faster, or whether cloud cover will increase, blocking the Sun’s rays and actually slowing down global warming.

In a study published in the July 24 issue of Science, researchers Amy Clement and Robert Burgman from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and Joel Norris from Scripps Institution of Oceanography at UC San Diego begin to unravel this mystery. Using observational data collected over the last 50 years and complex climate models, the team has established that low-level stratiform clouds appear to dissipate as the ocean warms, indicating that changes in these clouds may enhance the warming of the planet.

Because of inconsistencies in historical observations, trends in cloudiness have been difficult to identify. The team broke through this cloud conundrum by removing errors from cloud records and using multiple data sources for the northeast Pacific Ocean, one of the most well-studied areas of low-level stratiform clouds in the world. The result of their analysis was a surprising degree of agreement between two multi-decade datasets that were not only independent of each other, but that employed fundamentally different measurement methods. One set consisted of collected visual observations from ships over the last 50 years, and the other was based on data collected from weather satellites.

“The agreement we found between the surface-based observations and the satellite data was almost shocking,” said Clement, a professor of meteorology and physical oceanography at the University of Miami, and winner of the American Geophysical Union's 2007 Macelwane Award for her groundbreaking work on climate change. “These are subtle changes that take place over decades. It is extremely encouraging that a satellite passing miles above the earth would document the same thing as sailors looking up at a cloudy sky from the deck of a ship.”

What was not so encouraging, however, was the fact that most of the state-of-the-art climate models from modeling centers around the world do not reproduce this cloud behavior….

Clouds over Grand Junction, Colorado, shot by Tewy, Wikimedia Commons, under the Creative Commons Attribution ShareAlike 3.0 License