Effect of cloud-scattered sunlight on Earth's energy balance depends on wavelength of light

Apr 22, 2011
These cumulus clouds above Oklahoma both shade the earth and make shadows brighter. The larger ones have a distinct cauliflower shape, providing even more opportunities for light to bounce off of them. Credit: Evgueni Kassianov/PNNL

Atmospheric scientists trying to pin down how clouds curb the amount of sunlight available to warm the earth have found that it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.

Additionally, the researchers found that scattered by clouds — the reason why beachgoers can get sunburned on overcast days — is an important component of cloud contributions to the earth's energy balance. Capturing such contributions will increase the accuracy of climate models, the team from the Department of Energy's Pacific Northwest National Laboratory reported in Geophysical Research Letters earlier this month.

"The amount of the sun's energy that reaches the earth's surface is the main driver of the earth's temperature. Clouds are one of the least understood aspects of climate change. They can block the sun, but can also bounce off one cloud into another cloud's shadow and increase the solar energy hitting ," said PNNL atmospheric scientist Evgueni Kassianov.

White clouds

Clouds both cool down and warm up the earth's surface. They cool the earth by reflecting some sunlight up into outer space, and they warm it by bouncing some sunlight down to the surface. Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.

But it's a hard number to get. Fair-weather clouds are big puffy white objects that bounce a lot of light around. They can make the sky around them look brighter when they're there, but they float about and reform constantly. Cloud droplets and aerosol particles in the sky — tiny bits of dirt and water in the air that cause haziness — scatter light in three dimensions, even into cloud shadows.

To determine the net cloud effect, researchers need two numbers. First they need to measure the total amount of sunlight in a cloudy sky. Then they need to determine how bright that sky would be without the clouds, imagining that same sky to be blue and cloudless, when aerosols are in charge of a sky's brightness. The difference between those numbers is the net cloud effect.

Rainbow energy

Researchers have traditionally estimated the net cloud effect by measuring a broad spectrum of sunlight that makes it to the earth's surface, from ultraviolet to infrared. But clouds are white — that's because the large water droplets within them scatter light of all colors almost equally in the visible spectrum, the part of the electromagnetic spectrum that includes the colors of the rainbow.

On the other hand, aerosols — both within clouds and in the open sky — bounce different-colored light unequally. Broadband measurements that fail to distinguish color differences might be covering up important details, the researchers thought.

Instead of taking one broadband measurement that covers everything from ultraviolet to infrared, Kassianov and crew wanted to determine how individual wavelengths contribute to the net cloud effect. To do so, the team used an instrument that can measure brightness at four different wavelengths of color — violet, green, orange, red — and two of infrared.

In addition, this instrument, a spectral radiometer at DOE's Atmospheric Radiation Measurement Climate Research Facility located on the southern Great Plains in Oklahoma, allowed the team to calculate what the brightness would be if the day sported a cloudless, blue sky. The spectral measurements taken by the radiometer can be converted into the amount and properties of aerosols. Then aerosol properties can be used to calculate clear blue sky brightness.

Clouds Gone Wild

Comparing measured values for cloudy sky to the calculated values for clear sky, the researchers found that, on average, puffy fair-weather clouds cool down the earth's surface by several percent on a summer day. Although clouds cool overall, two components that the researchers looked at — from direct and scattered sunlight — had opposite effects.

The direct component accounts for the shade provided by clouds and cools the earth. The second component accounts for the sunlight scattered between and under clouds, which makes the sky brighter, warming the earth.

"The sunlight scattered by clouds can heat the surface," said Kassianov. "We all know that we can still get sunburned on cloudy days. This explains why."

In the Oklahoma summer, the scattered-light effect measured by the researchers could be quite large. For example, if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being "caught on tape" by the radiometer.

"Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference. The one-dimensional cloud simulations currently used in large-scale don't capture this diffuse light," said Kassianov.

Aerosols' Day in the Sky

The team also found that the effect changed depending on the measured visible-spectrum , and whether the light was direct or scattered.

With direct light, the cooling caused by clouds was weakest on the violet end of the spectrum and strongest at infrared. With scattered light, warming caused by clouds was also weakest at violet and the strongest at infrared. Overall, the least cooling and warming occurred at violet, and the most cooling and warming occurred at infrared.

Because large droplets in clouds scatter sunlight almost uniformly across the spectrum, the clouds themselves can't be the reason why different wavelengths contribute differently to the net cloud effect. Compared to cloud droplets, aerosols are more than 100 times smaller and scatter wavelengths differently. These results suggest that aerosols — which not only cause haziness but contribute to cloud formation as well — are responsible for the wavelength differences, something researchers need to be aware of as they study clouds in the sky.

"If you want to study how aerosols and interact," said Kassianov, "you need to look in the region of the spectrum where aerosol effects are significant. If you want to fish, you go where the fish are biting."

Explore further: Quakes destroy or damage 83 houses in Philippines

More information: Kassianov E., Barnard J., Berg L.K., Long C.N., and C. Flynn, Shortwave Spectral Radiative Forcing of Cumulus Clouds from Surface Observations, Geophys Res Lett, April 2, 2011, DOI:10.1029/2010GL046282

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User comments : 16

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omatumr
2 / 5 (4) Apr 23, 2011
Congratulations!

Two vary important variables must be considered to understand how Earth's heat source - the Sun - controls Earth's climate.

1. Variations in the wavelength of light emitted from the Sun.

2. Variations in the transmission of light of different wavelengths through the Earth's atmosphere.

Al Gore, the UN's IPCC, and the army of consensus scientists promoting CO2-induced global warming ignored these important variables and claimed that the Sun could not be the source of climate change because TSI (total solar irradiance) was almost constant.

Again, thanks for acknowledging that one of the important variables was overlooked.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo

See "Earth's Heat Source - The Sun", Energy & Environment 20 (2009) 131-144, and

"Neutron Repulsion", The APEIRON Journal, in press (2011) 19 pages.
that_guy
5 / 5 (1) Apr 23, 2011
Clouds repel neutrons. I don't know how, but I measured a slight variation in a dipolar nucleon. Don't forget that Hydrogen bombs are powered by neutrons and not by fusion. There's too much neutron repulsion for fusion to work. Dark matter, that's from neutrons too!

Regarding...
Shut up already
deepsand
5 / 5 (5) Apr 24, 2011
Al Gore, the UN's IPCC, and the army of consensus scientists promoting CO2-induced global warming ignored these important variables and claimed that the Sun could not be the source of climate change because TSI (total solar irradiance) was almost constant.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo

You've deliberately conflated atmospheric variables with Solar ones.

Shame on you.
beelize54
1 / 5 (1) Apr 24, 2011
..effect of cloud-scattered sunlight on Earth's energy balance depends on wavelength of light..
The same effect is used in fog lamps for years.. I'm afraid, the climatology modeling is still in protoscience age.
omatumr
1 / 5 (2) Apr 24, 2011
You've deliberately conflated atmospheric variables with Solar ones.


Not at all.

My comment concerned two, distinct and important variables:

1. Variations in the wavelength of light emitted from the Sun.

2. Variations in the transmission of light of different wavelengths through the Earth's atmosphere.

deepsand
5 / 5 (6) Apr 24, 2011
You've deliberately conflated atmospheric variables with Solar ones.


Not at all.

My comment concerned two, distinct and important variables:

1. Variations in the wavelength of light emitted from the Sun.

2. Variations in the transmission of light of different wavelengths through the Earth's atmosphere.

The article is specifically about the varying effects of clouds, not about Solar influx.

That atmospheric transmissivity varies by wavelength has been long and been known. To claim that such has been ignored is a blatant lie.
omatumr
1 / 5 (1) Apr 25, 2011
That atmospheric transmissivity varies by wavelength has been long and been known. To claim that such has been ignored is a blatant lie.


Did you think I said that atmospheric transmissivity variations by wavelength were unknown?

My comment was NOT about atmospheric transmissivity varying by wavelength.

If you have a reference for measurements of "Variations in the wavelength of light emitted from the Sun" over one or more solar cycles, please share it.

Thanks.
deepsand
5 / 5 (5) Apr 25, 2011
That atmospheric transmissivity varies by wavelength has been long and been known. To claim that such has been ignored is a blatant lie.


Did you think I said that atmospheric transmissivity variations by wavelength were unknown?

My comment was NOT about atmospheric transmissivity varying by wavelength.

If you have a reference for measurements of "Variations in the wavelength of light emitted from the Sun" over one or more solar cycles, please share it.

Thanks.

Why do you continue to attempt to evade and obfuscate by way of misdirection?

The article that is the subject of discussion is NOT ABOUT SOLAR IRRADIATION.
omatumr
1 / 5 (1) Apr 25, 2011
You can't study one without knowing the other!

That is why I suggested above:

If you have a reference for measurements of "Variations in the wavelength of light emitted from the Sun" over one or more solar cycles, please share it.
deepsand
5 / 5 (5) Apr 25, 2011
You can't study one without knowing the other!

So obviously false that I'll not waste my time explaining why.

omatumr
1 / 5 (1) Apr 25, 2011
If you have a reference for measurements of "Variations in the wavelength of light emitted from the Sun" over one or more solar cycles, please share it.
deepsand
5 / 5 (5) Apr 26, 2011
Immaterial to the subject at hand.
omatumr
1 / 5 (1) Apr 26, 2011
Or, to speak frankly,

You do not know of any measurements of "Variations in the wavelength of light emitted from the Sun" over one or more solar cycles, because that information in unavailable to anyone, including the group that claimed Earth's climate is immune to changes in the Sun.
deepsand
5 / 5 (5) Apr 26, 2011
To speak most frankly, your repeated attempts at diversion are puerile.
omatumr
1 / 5 (2) Apr 26, 2011
Sorry, deepsand, but that is not a convincing substitute for information on "Variations in the wavelength of light emitted from the Sun."

Todays news story by Dr. David Whitehouse may help you understand why information was requested on "Variations in the wavelength of light emitted from the Sun" over one or more solar cycles.

Is It The Sun Wot Done It?

http://thegwpf.or...-it.html
deepsand
5 / 5 (5) Apr 26, 2011
More important is the fact that your fixation on the Sun is immaterial to what clouds do.

It is also immaterial to the fact that increased levels of atmospheric CO2 perforce increases radiative forcing.