From the DOE/Pacific Northwest National Laboratory, a new paper in GRL saying something that doesn’t make much sense to me. As shown in the diagram above, thunderstorms transport heat from the lower troposphere upwards. The heat source at the base of the atmosphere (at the surface) is the absorption of sunlight by the surface of the Earth. That transfers heat to the lower atmosphere by conduction (a small amount), and mostly be re-radiated Long Wave IR. Heat is then transported upwards by convection, which is done by clouds (cumulus for example) and especially thunderstorms. So, given the amount of energy transport, I’m puzzled as to how they think this new theory works as a net warming, especially when all they are doing is running a model, and providing no hard data. They say:
Pollution strengthens thunderstorm clouds, causing their anvil-shaped tops to spread out high in the atmosphere and capture heat — especially at night
Basically what they are saying is that thunderstorm anvils are enhanced by pollution, probably due to increased condensation nuclei, and those anvils act as IR reflectors at night…but…they also act as strong sunlight reflectors, something that goes on every day in the ITCZ, as Willis has pointed out with his Thermostat Hypothesis, now a peer reviewed paper. Steve McIntyre also offered a view that clouds offer a strong net negative feedback here.
But when an abstract ends with this:
The positive aerosol radiative forcing on deep clouds could offset the negative aerosol radiative forcing on low clouds to an unknown extent.
I wonder how this speculation got published in the first place.
Pollution teams with thunderclouds to warm atmosphere
New simulation study shows that atmosphere warms when pollution intensifies storms
RICHLAND, Wash. — Pollution is warming the atmosphere through summer thunderstorm clouds, according to a computational study published May 10 in Geophysical Research Letters. How much the warming effect of these clouds offsets the cooling that other clouds provide is not yet clear. To find out, researchers need to incorporate this new-found warming into global climate models.
Pollution strengthens thunderstorm clouds, causing their anvil-shaped tops to spread out high in the atmosphere and capture heat — especially at night, said lead author and climate researcher Jiwen Fan of the Department of Energy’s Pacific Northwest National Laboratory.
“Global climate models don’t see this effect because thunderstorm clouds simulated in those models do not include enough detail,” said Fan. “The large amount of heat trapped by the pollution-enhanced clouds could potentially impact regional circulation and modify weather systems.”
Clouds are one of the most poorly understood components of Earth’s climate system. Called deep convective clouds, thunderstorm clouds reflect a lot of the sun’s energy back into space, trap heat that rises from the surface, and return evaporated water back to the surface as rain, making them an important part of the climate cycle.
To more realistically model clouds on a small scale, such as in this study, researchers use the physics of temperature, water, gases and aerosols — tiny particles in the air such as pollution, salt or dust on which cloud droplets form.
In large-scale models that look at regions or the entire globe, researchers substitute a stand-in called a parameterization to account for deep convective clouds. The size of the grid in global models can be a hundred times bigger than an actual thunderhead, making a substitute necessary.
However, thunderheads are complicated, dynamic clouds. Coming up with an accurate parameterization is important but has been difficult due to their dynamic nature.
Inside a thunderstorm cloud, warm air rises in updrafts, pushing tiny aerosols from pollution or other particles upwards. Higher up, water vapor cools and condenses onto the aerosols to form droplets, building the cloud. At the same time, cold air falls, creating a convective cycle. Generally, the top of the cloud spreads out like an anvil.
Previous work showed that when it’s not too windy, pollution leads to bigger clouds . This occurs because more pollution particles divide up the available water for droplets, leading to a higher number of smaller droplets that are too small to rain. Instead of raining, the small droplets ride the updrafts higher, where they freeze and absorb more water vapor. Collectively, these events lead to bigger, more vigorous convective clouds that live longer.
Now, researchers from PNNL, Hebrew University in Jerusalem and the University of Maryland took to high-performance computing to study the invigoration effect on a regional scale.
To find out which factors contribute the most to the invigoration, Fan and colleagues set up computer simulations for two different types of storm systems: warm summer thunderstorms in southeastern China and cool, windy frontal systems on the Great Plains of Oklahoma. The data used for the study was collected by different DOE Atmospheric Radiation Measurement facilities.
The simulations had a resolution that was high enough to allow the team to see the clouds develop. The researchers then varied conditions such as wind speed and air pollution.
Fan and colleagues found that for the warm summer thunderstorms, pollution led to stronger storms with larger anvils. Compared to the cloud anvils that developed in clean air, the larger anvils both warmed more — by trapping more heat — and cooled more — by reflecting additional sunlight back to space. On average, however, the warming effect dominated.
The springtime frontal clouds did not have a similarly significant warming effect. Also, increasing the wind speed in the summer clouds dampened the invigoration by aerosols and led to less warming.
This is the first time researchers showed that pollution increased warming by enlarging thunderstorm clouds. The warming was surprisingly strong at the top of the atmosphere during the day when the storms occurred. The pollution-enhanced anvils also trapped more heat at night, leading to warmer nights.
“Those numbers for the warming are very big,” said Fan, “but they are calculated only for the exact day when the thunderstorms occur. Over a longer time-scale such as a month or a season, the average amount of warming would be less because those clouds would not appear everyday.”
Next, the researchers will look into these effects on longer time scales. They will also try to incorporate the invigoration effect in global climate models.
The research was supported by the U.S. Department of Energy Office of Science. The data from China were gathered under a bilateral agreement with the China Ministry of Sciences and Technology.
Reference: Jiwen Fan, Daniel Rosenfeld, Yanni Ding, L. Ruby Leung, and Zhanqing Li, 2012. Potential Aerosol Indirect Effects on Atmospheric Circulation and Radiative Forcing through Deep Convection, Geophys. Res. Lett. May 10, DOI 10.1029/2012GL051851 (http://www.agu.org/pubs/crossref/2012/2012GL051851.shtml)
Here’s the abstract:
GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L09806, 7 PP., 2012
Potential aerosol indirect effects on atmospheric circulation and radiative forcing through deep convection
- Aerosol invigoration (AIV) on deep convective clouds incurs positive radiative forcing
- AIV also leads to enhanced regional convergence, and a strong thermodynamic forcing
- Wind shear and cloud base T determine significance of aerosol invigoration effect
Jiwen Fan Pacific Northwest National Laboratory, Richland, Washington, USA
Daniel Rosenfeld Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel
Yanni Ding Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA
L. Ruby Leung Pacific Northwest National Laboratory, Richland, Washington, USA
Zhanqing Li Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA
Aerosol indirect effects, i.e., the interactions of aerosols with clouds by serving as cloud condensation nuclei or ice nuclei constitute the largest uncertainty in climate forcing and projection. Previous IPCC reported negative aerosol indirect forcing, which does not account for aerosol-convective cloud interactions because the complex processes involved are poorly understood and represented in climate models. Here we elucidated how aerosols change convective intensity, diabatic heating, and regional circulation under different environmental conditions. We found that aerosol indirect effect on deep convective cloud systems could lead to enhanced regional convergence and a strong top-of-atmosphere warming. Aerosol invigoration effect occurs mainly in warmed-based convection with weak shear. This could result in a strong radiative warming in the atmosphere (up to +5.6 W m−2), a lofted latent heating, and a reduced diurnal temperature difference, all of which could potentially impact regional circulation and modify weather systems. The positive aerosol radiative forcing on deep clouds could offset the negative aerosol radiative forcing on low clouds to an unknown extent.