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Clouds in the Greenhouse

As vexing as they are beautiful, clouds play an important role in Earth's planetary greenhouse.

NASA

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see captionApril 22, 2002:  Clouds may be a delight to children lying in a field on a summer day, letting their imaginations bend the wispy shapes into ducks or boats or dinosaurs.

But clouds can be a real pain in the neck for climate researchers.

Right: Cirrus clouds over Finland Pekka Parviainen. [more]

To understand why, consider again that summer day: If a big, fluffy cumulus cloud comes drifting by, it's usually good news for hot cloud-watchers. Low thick clouds cast a refreshing shadow and reflect sunlight back into space. They cool the planet and the people beneath them.

On the other hand, high wispy clouds drifting by are less refreshing. Such clouds cast meagre shadows and, because they are themselves cold, they trap heat radiated from the planet below. The air temperature near the ground might actually increase.

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It is this schizophrenic behavior that makes clouds so vexing to researchers who are trying to predict the course of climate change.

Clouds are an important part of Earth's planetary greenhouse. Greenhouse gases like carbon dioxide and methane are perhaps more widely discussed, but clouds can do the same thing: they warm our planet by trapping heat beneath them. Yet unlike greenhouse gases, sunlight-reflecting clouds also have a cooling influence. Furthermore, the air temperature, which is affected by clouds, in turn affects cloud formation. It's a circular relationship that makes climate research all the more difficult.

"Clouds remain one of the largest uncertainties in the climate system's response to temperature changes," laments Bruce Wielicki, a scientist at NASA's Langley Research Center. "We need more data to understand how real clouds behave."

see captionLeft: The complex role of clouds in Earth's energy balance. Click on the image to view an easier-to-read version. Credit: NASA/Langley.

How much sunlight do different kinds of clouds reflect? How much heat do they absorb? And how do they respond to ambient temperature changes? Wielicki is the principal investigator for an orbiting instrument that will answer some of these questions. "It's called CERES," he says, "short for Cloud and the Earth's Radiant Energy System."

CERES is a package of three telescopes that watch our planet from Earth orbit. "One telescope is sensitive to ordinary sunlight," says Wielicki. "It tells us how much solar radiation is reflected from clouds or ice." The other two telescopes sense longer-wavelength infrared heat. They reveal how much heat is trapped by clouds and how much of it escapes back to space.

CERES is orbiting Earth now on board NASA's Terra satellite. The instrument was monitoring our planet last summer when a heat wave struck California and produced a remarkable surge in infrared radiation from that region. CERES revealed not only the infrared glow on the ground, but also how much of that heat was absorbed by the atmosphere -- key data for global warming studies.

see caption

Above:  California is glowing in this image of infrared heat radiating from the Earth. CERES on Terra captured the data during the 2001 California heat wave.

NASA's Aqua satellite, slated to launch on May 2nd, will soon carry another package of CERES telescopes to orbit. "Having CERES on board two satellites (Aqua and Terra) will help us cover the entire planet -- to study, for example, day-night variations in Earth's energy balance," explains Wielicki.

CERES is a welcome development for scientists who are often forced to test their ideas about climate change using computer models -- models that may or may not faithfully represent our complicated planet. Using CERES, researchers can now examine some of those theories in the real world.

see captionRight: Clouds in the tropical Pacific. [more]

For example, a group of scientists recently proposed an idea called the "iris hypothesis." They suggested that the canopy of clouds over the tropical Pacific Ocean recedes when the water's surface temperature increases. Fewer clouds would open a window through which heat could escape to space and thus cool the planet. Earth, they argued, has a natural response that counteracts rising temperatures -- a bit like an iris in a human eye dilating to adapt to low light.

But does Earth really respond that way?

Wielicki and other NASA scientists used CERES to test the idea. It turned out that such clouds did trap infrared heat. But even more so they reflected visible sunlight back into space. Fewer of the clouds would mean more global warming, not less.

The iris hypothesis was wrong.

see captionAnother problem CERES will tackle concerns aerosols. Aerosols are tiny particles like volcanic dust, pollution and even sea spray suspended in the air. Aerosols reflect sunlight. They also help clouds form by serving as "nucleation sites" around which water droplets grow. No one knows if increasing numbers of aerosols will cool or warm our planet.

Above: Clouds containing many aerosols (left) also contain many water droplets. Such clouds reflect light well. Clouds containing fewer aerosols (right) tend to harbor larger water droplets; they transmit more solar energy to the planet below. [more]

"The aerosols are a mess," says Thomas Charlock, a senior scientist at NASA's Langley Research Center and co-investigator for CERES. "We don't know how much is out there, and every gosh-darned aerosol particle looks different from every other one. So we just can't estimate their influence with calculations alone."

"What we can do is look at the energy balance in a dusty area and a non-dusty area," Charlock continues. "That's where CERES and MODIS (a Terra instrument that can sense aerosol properties) used together will be very powerful."

see captionRight:  Tracks in Earth's atmosphere left by ... ocean vessels! The clouds pictured here were created when aerosols from the ships' exhaust caused moisture in the air to condense into clouds. [more]

When Aqua joins Terra in orbit, it will bring its own special set of tools to bear on climate research. Says Charlock: "Part of our mission we can do much better with [instruments on board] Aqua -- things relating to humidity and water clouds."

Scientists hope the unprecedented "cloud watching" power of these two satellites will reveal much about the inner workings of climate change. Don't expect any pictures of ducks or dinosaurs, though. Neither satellite has that kind of imagination. Yet in their own way, they will reveal the complex beauty of clouds as never before.

 

 

NASA NEWS & THE ENVIRONMENT

Earth’s Bio Response

 

 

Public release date: 13-Mar-2003

Contact: Jeff Key
jeff.key@ssec.wisc.edu
608-263-2605
University of Wisconsin-Madison

 

Clouds mitigate effects of warming on arctic

 

MADISON - Cloudy weather may dampen the human spirit, but it also may dampen the effects of global warming on the Arctic, according to new study published in the March 14 issue of the journal Science.

 

Data from dozens of meteorological stations show that the surface temperature across Arctic land and water keeps getting warmer. However, researchers at the University of Wisconsin-Madison now show that Arctic clouds and the climate conditions with which the clouds interact produce a cooling effect, possibly offsetting to some degree the effects of global warming in this region.

 

Xuanji Wang, UW-Madison graduate student and lead author of the paper, and Jeff Key, a scientist with the National Oceanic and Atmospheric Administration (NOAA) at UW-Madison's Cooperative Institute for Meteorological Satellite Studies (CIMSS), studied a number of climate changes across the Arctic region during the period of 1982 to 1999. Specifically, they noted changes in the surface temperature of the land and ocean, cloud coverage, and surface albedo - the amount of light reflected off surfaces, such as snow or ice.

 

While a number of researchers have monitored Arctic surface temperature and sea ice extent over the years, the Wisconsin scientists say few have studied other conditions, such as cloud cover, and none have examined how changes in these conditions interact.

 

"To understand how and why the climate is changing, you have to think about the feedback systems," says Wang. One of the most important feedback systems, he notes, is "cloud forcing." This system involves the interplay among clouds, surface temperature and surface reflectivity (albedo).

 

Clouds play an important role in climate: not only do they reflect energy from the sun to the ground, but they also can trap heat emitted by the earth and re-emit some of that energy back to the surface. Depending on other climate conditions, such as surface albedo, clouds can either enhance or inhibit surface warming, says Wang.

 

For instance, when the ground is covered by snow, as is the case for much of the Arctic, solar energy reflects off the snow and is absorbed by clouds. The result: the surface stays cool. But once the covering melts, the ground absorbs the solar energy and surface temperatures rise.

 

Because cloud coverage, albedo and surface temperature all contribute to the outcome, small changes in one of the factors can produce big changes overall: as the surface warms, ice begins to melt, the ground absorbs solar energy and the surface temperature rises even more.

 

Recognizing the interplay among climate factors, Wang and Key set out to understand how aspects of the Arctic climate respond to changes in surface temperature.

 

"Surface temperature is the most important variable of the energy budget," says Key. "But to understand why it is changing, we need to measure other characteristics of the climate."

 

To do this, the researchers used satellite data collected across the Arctic region to compute surface temperature, albedo and cloud properties. This information helped the Wisconsin team determine cloud forcing - a measurement of the warming or cooling effect of clouds that depends on the interactions among the various climate conditions. They averaged data for each season, as well as for each year.

The researchers found that Arctic surface temperature during the spring, summer and autumn has warmed at decadal rates of 1.1, 0.7 and 0.7 degrees Celsius, respectively. This data confirms similar trends noted in previous studies.

 

Adding to this information, the researchers also found that the amount of light reflected off the ground or water during these three seasons has decreased. A lower albedo in autumn, they say, indicates a longer melt season and a later onset of freezing or snowfall.

 

The researchers also found that spring and summer cloud coverage has increased by 2 to 4 percent per decade, but that winter cloud coverage has decreased over the years. When data for cloud coverage was averaged over the year, no changes were noticed.

 

"The average annual change doesn't tell the whole story," says Key. "Opposite trends in different seasons can cancel on an annual scale, but their seasonal effects are important."

 

Some of the seasonal changes the researchers found may seem inconsequential, but Key says they are significant: "The Arctic is a place where small changes can have big effects. These effects can signal climate changes elsewhere." He adds, "That's why it's so important to monitor the Arctic."

 

To understand the cumulative effects of these small changes on the Arctic, the researchers calculated cloud forcing. They found no trend during the spring, but they did find trends toward increasing cloud cooling during the winter, summer and fall seasons. Cloud cooling during the summer, the researchers say, was due in large part to the increased cloud coverage.

 

"It appears that if clouds conditions weren't changing," says Key, "the Arctic would be getting even warmer," which means even more ice would be melting. More clouds in spring and summer and fewer in winter, he says, appear to have dampened the consequences of global warming on this region.

 

Because of the height at which the clouds formed, the researchers say the trends they report are the result of not local processes, such as water evaporation, but large-scale circulation patterns. More research on this possible link, they add, needs to be conducted.

 

Wang and Key say the findings they present confirm and, more importantly, augment the existing data on Arctic climate change with information related to changes in albedo, cloud cover and cloud forcing. "We have added new information on how the climate responds to warming by looking at parameters not previously examined," adds Wang.

 

This information, the two atmospheric scientists say, will help researchers understand the effects of global warming on the Arctic and, ultimately, the rest of the globe.

###

-- Emily Carlson, (608) 262-9772, emilycarlson@facstaff.wisc.edu

 

 

 

Features: NASA's New Satellite Takes on Global Change

The Greenhouse Effect - It's Mostly About Water

A real greenhouse is made of glass, which lets visible sunlight through from the outside. This light gets absorbed by all the materials inside, and the warmed surfaces radiate infrared light, sometimes called "heat rays", back. But the glass, although transparent to visible light, acts as a partial barrier to the infrared light. So some of this infrared radiation, or heat, gets trapped inside. The result is that everything inside the greenhouse, including the air, becomes warmer. Similarly, light from the sun passes almost unhindered through Earth's atmosphere. It gets absorbed by the ocean and land surfaces, which warm and radiate infrared energy (heat!) back into the atmosphere. However, some atmospheric gases absorb these heat rays and trap the energy inside the atmosphere. The net result is warming. The most prominent green house gas is water vapor. Carbon dioxide (CO2) is also very important, and there are a number of minor green house gases as well. AIRS is the first satellite to measure the three-dimensional global distribution of atmospheric water vapor and carbon dioxide from space.

Water plays a very complex role in the atmosphere, because it can exist as a gas in the form of water vapor, a liquid in the form of water clouds and rain, and a solid in the form of ice clouds and snow. Although water in its vapor form will usually enhance the greenhouse effect, clouds can sometimes act like a greenhouse gas and sometimes can weaken it. Some types of clouds trap infrared heat rays, and some types of clouds reflect sunlight back to space very efficiently and prevent this energy from contributing to the heating process.

Clouds that are low in the atmosphere are warm and thus radiate heat back to space at a greater rate than high altitude clouds which are colder. High cold clouds are usually reflective, typically because they consist of very small droplets of small to large ice crystals. It's easy to tell by just looking: if a cloud is white, it is reflecting sunlight; if it is dark, it is usually absorbing sunlight (and the darker it is, the more likely it is to rain). Also, how a cloud behaves radiatively depends more on the size of the droplets or ice particles and the water content than on its temperature.

Water cycles through many forms. It falls as rain or snow, soaks into the soil, runs into lakes and oceans, and evaporates again into atmospheric water vapor, where it can condense into cloud droplets and eventually fall as rain and snow. We call this the water cycle.

If Earth's water flows through this cycle at a faster pace, it is likely that more clouds will form. However, because the effects of clouds can vary, its not obvious whether this will lead to global warming or cooling - or neither. This is a primary focus for scientists working with AIRS data.

strange_clouds.jpg

 Science marches on!

Note:

From:  (http://www.greeningearthsociety.org/wca/2003/wca_1aa.html )

( . . . now comes a paper in Geophysical Research Letters written by an international research team headed by Michael Stevens of the Naval Research Laboratory’s E.O. Hulburt Center for Space Studies. Stevens and his fellow researchers suggest that at least some portion of the increase in noctilucent clouds originates in exhaust from the space shuttles’ main engines. As a shuttle rips through the upper atmosphere, it injects a plume of water vapor into the air and – according to these researchers using a combination of observations and models, and tracking the moisture plume as it travels around the globe on upper level winds – is transformed into noctilucent clouds several days after the launch.
     Stevens concludes, “If the phenomenon we describe is repeatable for at least a subset of all rocket launches, [noctilucent cloud] observations could be influenced by space traffic. We propose that this source be considered in the analysis of [noctilucent cloud] trends. We furthermore suggest care be exercised when using the historical record of [noctilucent cloud] observations as an indicator of global change initiated from the lower atmosphere.”

References:
Stevens, M.H., et al., 2003. Polar mesospheric clouds formed from space shuttle exhaust. Geophysical Research Letters, doi:10.1029/2003GL017249.

Thomas, G.E., et al., 1989. Relation between increasing methane and the presence of ice clouds at the mesosphere. Nature, 338:490-492.)

 

Space Station Crew Photographs Mysterious Clouds that Shine at Night

By Tony Phillips
Science.NASA.gov

posted:
19 February 2003

They hover on the edge of space. Thin, wispy clouds, glowing electric blue. Some scientists think they're seeded by space dust. Others suspect they're a telltale sign of global warming.

They're called noctilucent or "night-shining" clouds (NLCs for short). And whatever causes them, they're lovely.

"Over the past few weeks we've been enjoying outstanding views of these clouds above the southern hemisphere," said space station astronaut Don Pettit during a NASA TV broadcast last month. "We routinely see them when we're flying over Australia and the tip of South America."

Skywatchers on Earth have seen them, too, glowing in the night sky after sunset, although the view from Earth-orbit is better. Pettit estimated the height of the noctilucent clouds he saw at 50 to 62 miles (80 to 100 km) ... "literally on the fringes of space."

"Noctilucent clouds are a relatively new phenomenon," says Gary Thomas, a professor at the University of Colorado who studies NLCs. "They were first seen in 1885" about two years after the powerful eruption of Krakatoa in Indonesia, which hurled plumes of ash as high as 80 kilometers into Earth's atmosphere.

Ash from the volcano caused such splendid sunsets that evening sky watching became a popular worldwide pastime. One sky watcher in particular, a German named T. W. Backhouse, noticed something odd. He stayed outside after the sun had set and, on some nights, saw wispy filaments glowing electric blue against the black sky. Noctilucent clouds. Scientists of the day figured the clouds were some curious manifestation of volcanic ash.

Eventually the ash settled and the vivid sunsets of Krakatoa faded. Yet the noctilucent clouds remained. "It's puzzling," says Thomas. "Noctilucent clouds have not only persisted, but also spread." A century ago the clouds were confined to latitudes above 50 degrees; you had to go to places like Scandinavia, Russia and Britain to see them. In recent years they have been sighted as far south as Utah and Colorado.

Astronaut Don Pettit is a long-time noctilucent cloud-watcher. As a staff scientist at the Los Alamos National Laboratory between 1984 and 1996, he studied noctilucent clouds seeded by high-flying sounding rockets. "Seeing these kinds of clouds [from space] ... is certainly a joy for us on the ISS," he said on NASA TV.

"Although NLCs look like they're in space," continues Thomas, "they're really inside Earth's atmosphere, in a layer called the mesosphere ranging from 50 to 85 kilometers high." The mesosphere is not only very cold (-193 Fahrenheit, or -125 Celsius), but also very dry--"one hundred million times dryer than air from the Sahara desert." Nevertheless, NLCs are made of water. The clouds consist of tiny ice crystals about the size of particles in cigarette smoke. Sunlight scattered by these crystals gives the clouds their characteristic blue color.

How ice crystals form in the arid mesosphere is the essential mystery of noctilucent clouds.

Ice crystals in clouds need two things to grow: water molecules and something for those molecules to stick to--dust, for example. Water gathering on dust to form droplets or ice crystals is a process called nucleation. It happens all the time in ordinary clouds.

Ordinary clouds, which are relatively close to Earth, get their dust from sources like desert wind storms. It's hard to waft wind-blown dust all the way up to the mesosphere, however. "Krakatoa may have seeded the mesosphere with dust in 1883, but that doesn't explain the clouds we see now," notes Thomas. "Perhaps," he speculates, "the source is space itself." Every day Earth sweeps up tons of meteoroids--tiny bits of debris from comets and asteroids. Most are just the right size to seed noctilucent clouds.

The source of water vapor is less controversial. "Upwelling winds in the summertime carry water vapor from the moist lower atmosphere toward the mesosphere," says Thomas. This is why NLCs appear during summer, not winter.

One reason for the recent spread of noctilucent clouds might be global warming. "Extreme cold is required to form ice in a dry environment like the mesosphere," says Thomas. Ironically, global warming helps. While greenhouse gases warm Earth's surface, they actually lower temperatures in the high atmosphere. Thomas notes that noctilucent clouds were first spotted during the Industrial Revolution--a time of rising greenhouse gas production.

Are NLCs a thermometer for climate change? A unusual sign of meteoroids? Or both? "So much about these clouds is speculative," says Thomas.

A NASA spacecraft scheduled for launch in 2006 should provide some answers. The Aeronomy of Ice in the Mesosphere satellite, or AIM for short, will orbit Earth at an altitude of 342 miles (550 kilometers). Although it's a small satellite, says Thomas, there are many sensors on board. AIM will take wide angle photos of NLCs, measure their temperatures and chemical abundances, monitor dusty aerosols, and count meteoroids raining down on Earth. "For the first time we'll be able to monitor all the crucial factors at once."

Meanwhile, all we can do is wait ... and watch. There's never been a better time to see noctilucent clouds. "During the summer months, look west perhaps 30 minutes to an hour after sunset when the Sun has dipped 6 to 16 degrees below the horizon," advises Thomas. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted an NLC. Observing sites north of 40 degrees latitude are favored.

One more thing: don't forget your camera. According to astronaut Don Pettit, "you can never have too many pictures of noctilucent clouds."

 

 

 

NASA Selects University for Cloud Mission

Associated Press
Monday, May 10, 2004,
2:32 pm EST

HAMPTON, Va. (AP) -- The National Aeronautics and Space Administration will give Hampton University $101 million to build a satellite to study how clouds form at the edge of space. After two years of proposals and interviews, Hampton University beat out more than 40 schools, including Stanford University, to lead the project. It is the first time a historically black college will manage a NASA mission.

"This is the largest deal for us ever," Hampton University Professor James M. Russell III told the Virginian-Pilot of Norfolk. He worked at NASA for 30 years before moving to Hampton University in 1996 and will be the project`s principal investigator.

The Aeronomy of Ice in the Mesosphere, or AIM, experiment will determine whether clouds forming in the uppermost atmosphere are a sign the global climate is changing. They are becoming brighter and more numerous and are coming closer to Earth, Russell said, which suggests carbon dioxide is building up and cooling the atmosphere. Carbon dioxide is considered a key ingredient in theories of global warming. But if the clouds grow large enough, they could help slow global warming by reducing the amount of sun reaching the Earth`s surface.

"It was a shock to see them, and it points to the fact that we need to find out more about them," Russell said.

Hampton University will act as the prime contractor, managing the development of the satellite and its instruments. It will subcontract work to companies and other universities.

The 3-foot-by-3-foot satellite weighs about 430 pounds and has a 6-foot solar-panel wing span. It will be shipped to Vandenberg Air Force Base in California for launch aboard a Pegasus XL rocket. The launch is scheduled for Sept. 29, 2006.

"We`re working very hard to hold to that date," Russell said. "A delay means money."

One of NASA`s requirements is that Hampton keep the project within budget. Hampton University students will work on the project, making it a good recruiting tool for at least the next four years, Russell said. Two graduate students and six undergraduates will be analyzing the data and contributing to scientific papers. The satellite will orbit for at least two years.

"We think we`ve set a good platform for the future," Bill Thomas, the university`s director of governmental relations, told the Daily Press of Newport News.

Two years ago, the school`s Center for Atmospheric Sciences won a $97 million contract to launch SAGE III, a satellite instrument that studies the chemical changes in the atmosphere that deplete the ozone layer. Next year, the school and its partners, including the French space agency, will launch CALIPSO, or Cloud Aerosol Lidar and Infrared Pathfinder Satellite. It will photograph Earth`s atmosphere.

AIM`s next milestone will be in October, when Russell`s team meets with NASA for a design review.

Ice Clouds

Large-Scale Ice Clouds in the GFDL SKYHI General Circulation Model

L.J. Donner, C.J. Seman, B.J. Soden, R.S. Hemler, J.C. Warren, J. Strom, K.N. Liou
Geophysical Fluid Dynamics Laboratory/NOAA Princeton University P.O. Box 308 Princeton, NJ 08542 USA

Abstract:

Ice clouds associated with large-scale atmospheric processes are studied using the SKYHI general circulation model (GCM) and parameterizations for their microphysical and radiative properties. The ice source is deposition from vapor, and the ice sinks are gravitational settling and sublimation. Effective particle sizes for ice distributions are related empirically to temperature. Radiative properties are evaluated as functions of ice path and effective size using approximations to detailed radiative-transfer solutions (Mie theory and geometric ray-tracing).

The distributions of atmospheric ice and their impact on climate and climate sensitivity are evaluated by integrating the SKYHI GCM (developed at the Geophysical Fluid Dynamics Laboratory) for six model months. Most of the major climatological cirrus regions revealed by satellite observations appear in the GCM. The radiative forcing associated with ice clouds acts to warm the earth-atmosphere system. Relative to a SKYHI integration without these clouds, zonally averaged temperatures are warmer in the upper tropical troposphere with ice clouds. The presence of ice produced small net changes in the sensitivity of SKYHI climate to radiative perturbations, but this represents an intricate balance among changes in clear-, cloud-, solar-, and longwave-sensitivity components. Deficiencies in the representation of ice clouds are identified as results of biases in the large-scale GCM fields which drive the parameterization and neglect of subgrid variations in these fields, as well as parameterization simplifications of complex microphysical and radiative processes.

AGU Index Terms: 3319 General circulation; 3359 Radiative processes; 3309 Climatology
Keywords/Free Terms: Ice clouds, Radiative Forcing, Parameterization, Radiative Properties

JGR-Atmospheres 97JD01488
Vol. 102 , No. D18 , p. 21,745


1997 AGU

 

 

 

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Title:

 

Inference of Cirrus Cloud Properties Using Satellite-observed Visible and Infrared Radiances. Part I: Parameterization of Radiance Fields.

Authors:

 

Minnis, Patrick; Liou, Kuo-Nan; Takano, Yoshihide

Journal:

 

Journal of Atmospheric Sciences, vol. 50, Issue 9, pp.1279-1304

Publication Date:

 

05/1993

Origin:

 

WEB

Bibliographic Code:

 

1993JAtS...50.1279M

Abstract

Current techniques for deriving cirrus optical depth and altitude from visible (0.65 m) and infrared (11.5 m) satellite data use radiative transfer calculations based on scattering phase functions of spherical water droplets. This study examines the impact of using phase functions for spherical droplets and hexagonal ice crystals to analyze radiances from cirrus. Adding-doubling radiative transfer calculations are used to compute radiances for different cloud thicknesses and heights over various backgrounds. These radiances are used to develop parameterizations of top-of-the-atmosphere visible reflectance and infrared emittance utilizing tables of reflectance as a function of cloud optical depth, viewing and illumination angles, and microphysics. This parameterization, which includes Rayleigh scattering, ozone absorption, variable cloud height, and an anisotropic surface reflectance, reproduces the computed top-of-the-atmosphere reflectances with an accuracy of 6% for four microphysical models: 10-m water droplet, small symmetric crystal, cirrostratus, and cirrus uncinus. The accuracy is twice that of previous models.Bidirectional reflectance patterns from theoretical ice-crystal clouds are distinctly different from those of the theoretical water-droplet clouds. In general, the ice-crystal phase functions produce significantly larger reflectances than the water-droplet phase function for a given optical depth. A parameterization relating infrared emittance to visible optical depth is also developed. The effective infrared emittances computed with the adding-doubling method are reproduced with a precision of 2%. Infrared scattering reduces emittance by an average of 5%. Simulated cloud retrievals using the parameterization indicate that optical depths and cloud temperatures can be determined with an accuracy of 25% and 6 K for typical cirrus conditions. Retrievals of colder clouds over brighter surfaces are not as accurate, while those of warmer clouds over dark surfaces will be more reliable. Sensitivity analyses show that the use of the water-droplet phase function to interpret radiances from a theoretical cirrostratus cloud will significantly overestimate the optical depth and underestimate cloud height by 1.5-2.0 km for nominal cirrus clouds (temperature of 240 K and visible optical depth of 1). The parameterization developed here is economical in terms of computer memory and is useful for both simulation and interpretation of cloud radiance fields.


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Title:
Testing the impact of clouds on the radiation budgets of 19 atmospheric general circulation models
Authors:
Potter, Gerald L.; Cess, Robert D.
Journal:
Journal of Geophysical Research, Volume 109, Issue D2, CiteID D02106 (JGRD Homepage)
Publication Date:
01/2004
Origin:
AGU
AGU Keywords:
Global Change: Climate dynamics (3309), Global Change: Atmosphere (0315, 0325), Meteorology and Atmospheric Dynamics: Radiative processes, Meteorology and Atmospheric Dynamics: Climatology (1620)
Abstract Copyright:
(c) 2004: American Geophysical Union
DOI:
10.1029/2003JD004018
Bibliographic Code:
2004JGRD..10902106P

Abstract

We compare cloud-radiative forcing (CRF) at the top-of-the atmosphere from 19 atmospheric general circulation models, employing simulations with prescribed sea-surface temperatures, to observations from the Earth Radiation Budget Experiment (ERBE). With respect to 60N to 60S means, a surprising result is that many of the 19 models produce unusually large biases in Net CRF that are all of the same sign (negative), meaning that many of the models significantly overestimate cloud radiative cooling. The primary focus of this study, however, is to demonstrate a diagnostic procedure, using ERBE data, to test if a model might produce, for a given region, reasonable CRF as a consequence of compensating errors caused either by unrealistic cloud vertical structure, cloud optical depth or cloud fraction. For this purpose we have chosen two regions, one in the western tropical Pacific characterized by high clouds spanning the range from thin cirrus to deep convective clouds, and the other in the southeastern Pacific characterized by trade cumulus. For a subset of eight models, it is found that most typically produce more realistic regionally-averaged CRF (and its longwave and shortwave components) for the southeastern region as opposed to the western region. However, when the diagnostic procedure for investigating cloud vertical structure and cloud optical depth is imposed, this somewhat better agreement in the southeastern region is found to be the result of compensating errors in either cloud vertical structure, cloud optical depth or cloud fraction. The comparison with ERBE data also shows large errors in clear-sky fluxes for many of the models.
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