DOE/Pacific Northwest National
December 15, 2009
Wind shear strength
determines whether pollution swells or saps storms
conditions, pollution can either strengthen or weaken thunderstorm
PNNL researchers have
figured out how to factor the effect into climate models.
New climate research reveals how wind
shear - the same atmospheric conditions that cause bumpy airplane
rides - affects how pollution contributes to isolated thunderstorm
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 pollution - 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
"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 climate models to improve
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 climate 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
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
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
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 wind shear conditions, the
cooling effect is always larger than the heating effect, leading to
a reduction in updraft speed.
Jiwen 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.