Forging cloud anvils: Pollution particles enlarge and extend the lifetime of storm clouds

Dec 15, 2010
Cloud anvils form when rising air expands as it bumps up against the tropical tropopause layer, between the troposphere and the stratosphere.

Tiny particles of pollutants in the lower atmosphere have a striking effect on cloud anvils, which are created by thunderstorms. That's the conclusion of a team of atmospheric scientists at Pacific Northwest National Laboratory. They conducted simulations of these storm clouds under two contrasting atmospheric conditions: humid and clean, and dry and polluted. They found that increasing the concentrations of those tiny particles increases anvil cloud size and lifetime in both cases.

Our insight into the impact of human activity on may hinge on understanding the effects of , those tiny unseen that come from pollution. How aerosols contribute to the formation of clouds will further that understanding. Until now, how aerosols affect cloud anvils has not been well understood.

In the atmosphere between the troposphere and the stratosphere is a thin layer called the tropical tropopause layer (TTL). Rising air from expands when it meets this layer to form cloud anvils. suggest that aerosol particles from forest fires might prolong the life of cloud anvils. Pollution might also increase water vapor in the TTL because cloud anvil properties are changed.

Time evolution of cloud anvil sizes from the sensitivity studies for the humid and clean (FEB06) and dry and polluted (NOV16) cases. The studies show that increasing aerosol particles increases anvil size in both cases.

To explore how aerosol particles affect cloud anvils and water vapor in the TTL, scientists at PNNL conducted simulations of thunderstorms forming in the tropics around Darwin, Australia. They performed sensitivity studies for two deep convective clouds, which develop in contrasting conditions: one started with humid and clean conditions, the other dry and polluted. Scientists found that in both cases, aerosols enlarge the cloud anvil size and lifetime. This finding could not be explained by convection, which is enhanced by aerosols in the humid case but suppressed in the dry case.

By looking at the microphysics of both cloud cases, scientists found that the increase of aerosols, especially aerosols at the lower levels, induced more ice particles along with a decrease in their individual sizes, which helped cloud anvils spread in the TTL. Aerosols from the pollution created numerous cloud droplet particles. The more particles there are, the smaller each droplet's size becomes. The smaller droplets don't fall out as rain. Instead, they are pushed upwards, where temperatures freeze them into ice particles. As a result, more but smaller ice particles form. Because smaller ice particles fall slowly, they stay longer in the layer, which contains strong horizontal winds and, therefore, spread farther than larger ice particles. As pollution increases the anvil size, it also increases the water vapor in the TTL clean air because of the moistening effect of clouds.

In addition, aerosols in the lower troposphere modified convection and the upper-level cloud properties. Lastly, the study showed the effects that ice nuclei and cloud condensation nuclei—which are parts of aerosol particles in the lower and middle troposphere—have on upper-level clouds depend on the humidity, resolving some contradictory results from past studies.

Researchers will be using a similar approach to study aerosol effects in more complicated, large-scale weather systems, such as the Monsoon system in the Australia region. In addition, they will investigate how those effects are sensitive to different ice nucleation parameterizations.

Explore further: Dead floppy drive: Kenya recycles global e-waste

More information: Fan J, J Comstock, and M Ovchinnikov. 2010. "The Cloud Condensation Nuclei and Ice Nuclei Effects on Tropical Anvil Characteristics and Water Vapor of the Tropical Tropopause Layer." Environmental Research Letters: 5 (October-December 2010) 044005. doi:10.1088/1748-9326/5/4/044005

add to favorites email to friend print save as pdf

Related Stories

Pollution alters isolated thunderstorms

Dec 15, 2009

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 ...

The insides of clouds may be the key to climate change

Feb 17, 2007

As climate change scientists develop ever more sophisticated climate models to project an expected path of temperature change, it is becoming increasingly important to include the effects of aerosols on clouds, according ...

Tropical cloud 'dust' could hold the key to climate change

Oct 26, 2005

Scientists at the University of Manchester will set off for Australia this week to undertake an in-depth study of tropical clouds and the particles sucked up into them to gain further insight into climate change and the depletion ...

Science paper examines role of aerosols in climate change

Sep 05, 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

Dead floppy drive: Kenya recycles global e-waste

18 hours ago

In an industrial area outside Kenya's capital city, workers in hard hats and white masks take shiny new power drills to computer parts. This assembly line is not assembling, though. It is dismantling some ...

New paper calls for more carbon capture and storage research

23 hours ago

Federal efforts to reduce greenhouse gas emissions must involve increased investment in research and development of carbon capture and storage technologies, according to a new paper published by the University of Wyoming's ...

Coal gas boom in China holds climate change risks

Aug 22, 2014

Deep in the hilly grasslands of remote Inner Mongolia, twin smoke stacks rise more than 200 feet into the sky, their steam and sulfur billowing over herds of sheep and cattle. Both day and night, the rumble ...

User comments : 0