Pollution alters isolated thunderstorms

December 15, 2009
Under certain conditions, pollution can either strengthen or weaken thunderstorm clouds. PNNL researchers have figured out how to factor the effect into climate models. Credit: UCAR/Carlyle Calvin

New climate research reveals how wind shear -- the same atmospheric conditions that cause bumpy airplane rides -- affects how pollution contributes to isolated thunderstorm clouds. Under strong wind shear conditions, pollution hampers thunderhead formation. But with weak wind shear, pollution does the opposite and makes storms stronger.

The work improves climate scientists' understanding of how aerosols -- tiny unseen particles that make up -- contribute to isolated thunderstorms and the climate cycle. How aerosols and clouds interact is one of the least understood aspects of climate, and this work allows researchers to better model clouds and precipitation.

"This finding may provide some guidelines on how man-made aerosols affect the local climate and precipitation, especially for the places where 'afternoon showers' happen frequently and affect the weather system and hydrological cycle," said atmospheric scientist Jiwen Fan of the Department of Energy's Pacific Northwest National Laboratory. "Aerosols in the air change the cloud properties, but the changes vary from case to case. With detailed cloud modeling, we found an important factor regulating how aerosols change storms and precipitation."

Fan will discuss her results Thursday, December 17 at the 2009 American Geophysical Union meeting. Her study uses data from skies over Australia and China.

The results provide insight into how to incorporate these types of clouds and conditions into computational to improve their accuracy.

A Model Sky

Deep convective clouds reflect a lot of the sun's energy back into space and return water that has evaporated back to the surface as rain, making them an important part of the cycle. The clouds form as lower air rises upwards in a process called convection. The updrafts carry aerosols that can seed cloud droplets, building a storm.

Previous studies produced conflicting results in how aerosols from pollution affect storm development. For example, in some cases, more pollution leads to stronger storms, while in others, less pollution does. Fan and her colleagues used computer simulations to tease out what was going on. Of concern was a weather phenomenon known as wind shear, where horizontal wind speed and direction vary at different heights. Wind shear can be found near weather fronts and is known to influence storms.

The team ran a computer model with atmospheric data collected in northern Australia and eastern China. They simulated the development of eight deep convective clouds by varying the concentration of aerosols, wind shear, and humidity. Then they examined updraft speed and precipitation.

Storm Forming

In the first simulations, the team found that in scenarios containing strong wind shear, more pollution curbed convection. When wind shear was weak, more pollution produced a stronger storm. But convection also changed depending on humidity, so the team wanted to see which effect -- wind shear or humidity -- was more important.

The team took a closer look at two cloud-forming scenarios: one that ended up with the strongest enhancement in updraft speed and one with the weakest. For each scenario, they created a humid and a dry condition, as well as a strong and weak wind shear condition. The trend in the different conditions indicated that wind shear had a much greater effect on updraft strength than humidity.

When the team measured the expected rainfall, they found that the pattern of rainfall followed the pattern of updraft speed. That is, with strong wind shear, more pollution led to less rainfall. When wind shear was weak, more pollution created stronger storms and more rain -- up to a certain point. Beyond a peak level in weak wind shear conditions, pollution led to decreased storm development.

Additional analyses described the physics underlying these results. Water condensing onto aerosol particles releases heat, which contributes to convection and increases updraft speed. The evaporation of water from the cloud droplets cools the air, which reduces the updrafts. In strong conditions, the cooling effect is always larger than the heating effect, leading to a reduction in updraft speed.

Explore further: Gravity Waves Make Tornados

More information: iwen Fan, "Dominant Role by Vertical Wind Shear in Regulating Aerosol Effects on Deep Convective Clouds" in session A43F, Cloud Properties and Physical Processes, Including Aerosol-Cloud Interactions II on Thursday, December 17, 2009, at 2:10 PM, in Moscone West.

J. Fan, T. Yuan, J. M. Comstock, S. Ghan, A. Khain, L. R. Leung, Z. Li, V. J. Martins, M. Ovchinnikov, Dominant role by vertical wind shear in regulating aerosol effects on deep convective clouds, J. Geophys. Res., 114, D22206, doi:10.1029/2009JD012352

Related Stories

Gravity Waves Make Tornados

March 19, 2008

Did you know that there's a new breakfast food that helps meteorologists predict severe storms? Down South they call it "GrITs."

Dirty air brings rain -- then again, maybe not

September 8, 2008

An international team of scientists, headed by Prof. Daniel Rosenfeld of the Institute of Earth Sciences at the Hebrew University of Jerusalem, has come up with a surprising finding to the disputed issue of whether air pollution ...

Science paper examines role of aerosols in climate change

September 5, 2008

A group of scientists affiliated with the International Geosphere-Biosphere Programme (IGBP) have proposed a new framework to account more accurately for the effects of aerosols on precipitation in climate models. Their work ...

Recommended for you

Mineral resource exhaustion is just a myth: study

April 28, 2017

Recent articles have declared that deposits of raw mineral materials (copper, zinc, etc.) will be exhausted within a few decades. An international team including the University of Geneva (UNIGE), Switzerland, has shown that ...

El Nino and the end of the global warming hiatus

April 27, 2017

A new climate model developed by Yale scientists puts the "global warming hiatus" into a broader historical context and offers a new method for predicting global mean temperature.


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (4) Dec 15, 2009
Trying to dig up another fudge factor they can throw into their climate sims are they? Disgusting.

1st) The title of this article should be "strong wind shear alters isolated t-storms", pollution has nothing to do with it. They would arrive at the same conclusions in the absence of pollution.

2nd) They're trying to spin it so that aerosols (which the more CO2 emissions you have, the more particulate and aerosols there are) appear to worsen global warming by decreasing cloud cover, when in fact the opposite is true, and it is only strong wind shear that is a (microscale) factor.

Windshear, especially strong wind shear, is a microscale meteorological event, meaning it is short in duration and small in size -not applicable to a climate simulation-, the strong portions (and lack of clouds) are manifested in fronts. It is simply a meteorological boundary. The vast majority of a storm's "body" have no vertical wind shear.

Everyone thank "scientist" Fan for the latest bit of AGW fraud.
2.3 / 5 (3) Dec 15, 2009
If strong wind shear was a "microscale meteorological event ... short in duration and small in size" then there would be no such thing as a jet stream.
not rated yet Dec 15, 2009
...pollution has nothing to do with it.

I believe the research described above, attempted to address the question of why pollution seems to have inconsistent effects in different studies (i.e. amplifying storms in some cases, while weakening them in others.) So the title was appropriate, given the context.

They're trying to spin it so that aerosols...appear to worsen global warming by decreasing cloud cover

They are doing no such thing. You are having hallucinations. I suggest you re-read the article, but first switch off the reality distortion field (are you using a Mac? :-P)

Windshear, especially strong wind shear, is a microscale meteorological event...

AFAIK, windshear is a constant phenomenon near mountain ranges. Hence, one would expect effects of pollution to differ somewhat based on terrain topography and prevailing wind patterns.
1 / 5 (1) Dec 16, 2009
Much of what we call pollution, isn't. Nearly everything outgassed by man's activities can be found outgassing "naturally" somewhere on the planet (and in MUCH larger quantities than Man's puny contribution).

Polyaromatic hydrocarbons occur naturally. CO2 occurs naturally (boy howdy does it occur naturally). Ozone forms naturally. Percolates form naturally. H2SO4 occurs naturally. HCL occurs naturally. Against all expectations, even plutonium forms naturally (google "natural reactor" Africa).

But it is hard to feel bad about natural occurrences. And you dang sure won't want to turn over your hard earned cash to some Gov't agency to fix a problem that occurs whether man caused it or not.
1 / 5 (3) Dec 16, 2009
wind shear...jet stream

These are two well defined indisputably separate meteorological terms. Certainly the jet stream can cause wind shear but it is not wind shear. And jet stream originated windshear is still not a sustained and/or meso-scale feature. Please at least have basic knowledge of meteorology before commenting like you do.
pollution seems to have inconsistent effects in different studies

True, but the same findings would be applicable to naturally occuring aerosols and/or any storm forming in the presence of wind shear of the nature they discuss. This is a necessary part to do the following...
They are doing no such thing.

Actually, yeah, I fully believe they are. They want to have some "science" they can point to when they put in this mysterious "wind shear factor" into their models. Otherwise, the increased aerosols (and particulate) that are present with increased CO2 emissions would probably override their CO2 forcing and they can't "allow" that.
2.3 / 5 (3) Dec 16, 2009
You don't have any idea what wind shear is, do you. Shear, whether in wind or any other gas or fluid, occurs when adjacent volumes of the gas or fluid are moving at different velocities relative to each other. That is why the jet stream is specifically mentioned in the wikipedia article for Wind Shear. If a jet stream was not surrounded by a tube of high wind shear, then it would not be moving at a different speed or direction than the surrounding air and could not be considered a stream - it would not be distinguishable from the surrounding air. You seem to be using some sort of bastard definition of wind shear used be airline pilots and the channel 9 weather man, but real scientists use a much more rigorous definition.
3 / 5 (2) Dec 16, 2009
but real scientists use a much more rigorous definition.
That's actually a less rigorous definition as it's more general, but your point stands. The Jet Stream is an example of wind shear as defined by science.

Defunct, give this paper a read. It's focused on the affects of particulate pollution, not AGW pseudo pollution.
1 / 5 (2) Dec 17, 2009
Shear, whether in wind or any other gas or fluid, occurs when adjacent volumes of the gas or fluid are moving at different velocities relative to each other.

You forgot to mention this gradient must occur over a short distance.

But anyway, this not what I'm arguing against. The jet stream is a large sustained (permanent) meteorological phenomenon created by the Coriolis effect and gravity, as well as atmospheric temperature and pressure phenomenon, it IS NOT created by wind shear, as you said. So sorry. You are wrong.

As I willingly admitted it can exhibit shear, though shear is not present everywhere there is jet stream, as the jet stream gradient does not always occur on a sharp gradient. The jet stream is the jet stream.

Wind shear is a micro-scale whether phenomenon, exhibiting shear as you have defined above, over SHORT dist.

As Vel indicated, you have bastardized the definition of jet stream to try and make your original statement seem valid, which it is not. Sorry.
3 / 5 (4) Dec 17, 2009
I've looked repeatedly for definitions of wind shear that mention a short distance or the compound word "micro-scale", but they don't exist. I'd bet money you made it up, but what the heck, why not post a reference to your source?

I never said wind shear created jet streams, but thanks for putting words in my mouth. I was saying that wherever you find a jet stream you will find wind shear. Stating that a jet stream "does not always occur on a sharp gradient" is oxymoronic. If the velocity gradient between a jet stream and the surrounding air is small, that means the surrounding air is moving almost as fast as the jet stream, and therefore that surrounding air is also part of the stream. The reason it is called a JET is because it is moving fast and a STREAM because it is moving at a different velocity relative to the surrounding air. Wind shear by definition, not wind shear as the source.
3 / 5 (2) Dec 17, 2009
Do you accept Merriam-Webster?


And then there's the first sentences of the first and second paragraphs of the wiki page...


Didn't really look too hard did you? That's okay, I wouldn't expect you to provide your own refutation.
I never said wind shear created jet streams

Allow me to refresh your memory. Your OP...
If strong wind shear was a "microscale meteorological event ... short in duration and small in size" then there would be no such thing as a jet stream.

This explicitly states you believe there would be no such thing as a jet stream with wind shear being defined as it is by me and the most authoritative dictionary there is and a peer reviewed widely cited (in forums such as this) website. This implies you think wind shear has a central role in the existence of the jet stream, instead of vice versa-as is reality.

You may remove your foot from your mouth now. Or swallow.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.