Does air pollution fuel stronger thunderstorms?The project aims to find out
A rotating radar monitors cloud activity, hoping to map a gathering storm from birth to dissipation.Image: Scott Collis
The skies outside were overcast and ominous, with the possibility of rain at any moment – the perfect backdrop to discuss thunderstorms with Marcus van Lier-Walqui via Zoom.
Van Lier-Valkey Studying Clouds and Precipitation at Columbia Climate Institute Climate System Research Center and NASA’s Goddard Institute for Space Studies. He focuses on the complex interactions of tiny particles and microscopic physics in clouds that can lead to rain, hail, thunderstorms and tornadoes.
On a stormy day like this, he still feels a visceral excitement — “I still think, ‘Oh, cool, lightning!'” he says — but he also keeps a close eye on the weather app. A tablet showing radar data briefly appears in the Zoom screen.
Although he typically handles climate and weather models on computers, van Lier-Walqui recently returned from field work near Houston, Texas, where he was deploying radar, lidar, radiometers, aerosol measurements, drones A co-investigator on the project, and the Lightning Mapping Array for studying thunderstorms.The project is called Tracking Aerosol Convective Interaction Experiments (TRACER), which aims to understand how thunderstorms form and how particles in air pollution affect their intensity and life cycle to improve weather and climate forecasting.
Some studies suggest that air pollution makes storms stronger, but it’s a bone of contention among scientists.
In the Q&A below, for length and clarity, van Lier-Walqui shares more about the project and how he got involved.
Marcus van Lier-Walqui and his son Oliver visited the field study site in June. In the background, anvil clouds are gathering. Anvils composed of ice grains form in the upper layers of thunderstorms.Photo: Scott Collis
What is the main question that TRACER is trying to answer?
We are studying how aerosols – dust and pollution particles – affect the behavior and evolution of thunderstorms? The Houston area is a great place to study this because you have relatively clean air from the bay and you have a huge source of pollution in downtown Houston and all the companies that refine oil there, so we can compare storm formation in polluted air vs. in less polluted air.
Clouds gather over petrochemical processing facilities. The TRACER project is studying how particles from such facilities affect storms. Photo: Marcus van Lier-Walqui
Our project aims to understand in more detail what happens in a real thunderstorm and try to come up with the best model possible to compare with. Then look at where and how the differences tell us what we don’t understand and what we think we understand, and where we can improve.
Why do we still not understand the role of aerosols?
Much of the advanced radar work has focused on tornado storms, because they cause so much damage, rather than simple and mild thunderstorms, and how their vertical motion develops and interacts with clouds and precipitation particles. Little attention has been paid to gaining insight into how ice particles interact as they grow and collide with other particles in the atmosphere, and how this affects lightning and electrification and the subsequent evolution of storm systems, based solely on observations.
Could studying regular thunderstorms also help us understand tornadoes?
Of course, there should be this chain reaction. The better we can understand microphysics — basically how cloud droplets grow, how they collide to form rain, how liquid particles start to freeze — the better we can make predictive models. Ideally, we would be better able to issue severe weather warnings.
One thing about microphysics is that when you condense water vapor into a liquid, you release energy. When you freeze liquid into ice, you release energy. It is this energy that drives these storms and drives the powerful cycles associated with them. So you absolutely have to handle these things correctly in your model.
Drone footage of the TRACER instrument set in a field near Houston, Texas.Credit: Scott Collis
What is your role in the TRACER project?
Together with Eric Bruning and Kelcy Brunner of Texas Tech University, we propose the deployment of additional lightning-mapping array sensors. These antennas basically detect electrical signals produced by lightning discharges. You have many of these devices around an area that pinpoint exactly where in space and time the discharge occurs. Even though it’s obscured by clouds, you can actually see through the clouds how the branch formed.
Lightning has to do with the micro-physics inside storms — how big of ice particles are produced, how many — which is related to the charging rate of the atmosphere and the amount of lightning that is actually produced.
Some people have the hypothesis that aerosols make thunderstorms stronger. There is a lot of contentious debate on this topic. My colleagues found that while aerosols don’t actually affect the strength of the updraft, they do affect the microphysical details of the updraft — the size of the cloud and rain particles that are produced in it. What this means for lightning is that the size of the ice crystals that might be created there—the ice crystals that then interact with supercooled water to create these charging effects—can change dramatically. So even if we don’t see the effect of aerosols on thunderstorm intensity, we can well study the effects of the detailed microphysics that might affect the lightning and other properties we observe with radar.
We received funding to add antennas to existing lightning mapping arrays in the Houston area, which will improve the accuracy of the network. There is also a lightning modeling component, led by Toshi Matsui of Goddard Space Flight Center in Maryland. Most cloud simulations don’t include the microphysical charging process, but this simulation actually looks at where the charging happens, simulates the cloud carrying that charge, and then discharges it to simulate lightning.
Along with colleagues, van Lier-Walqui helped expand a lightning-mapping array in the region to better understand how air pollution affects lightning patterns.Photo: Scott Collis
What did you do when you were there in June?
I’m involved in some forecasting work. On any given day, the team decides whether to release additional sounding balloons to learn about atmospheric conditions, and whether to switch the radar to this tracking mode, which basically tracks thunderstorms as they move. Such decisions need to be informed by predictions. So we run our model every day to help the prediction work.
Other than that, I’m basically a tourist and get to know different aspects of the project. This is my first time in a field event, so I’m really excited to walk in and see what other groups are doing. I visited the group that did the drone launch. A team from the University of Colorado launched a plane that flew back and forth along its orbit to observe horizontal differences in atmospheric conditions. There is also a helicopter moving up and down the atmosphere to get details of the lower atmosphere.
Oliver unleashes a weather balloon with an instrument hanging below.Photo: Scott Collis
What will happen when the storm comes?
There weren’t many storms when I was there, but I did see some images of one of the groups hiding under one of their tents when it was hit by rain. When this happens, they must shut down helicopters and drone flights to protect their equipment and avoid flying in low visibility. Something like radar, they just keep scanning.
What’s next for the TRACER project?
Field activities will continue until the end of September, and then we will start sifting through the vast amount of data we have collected, running simulations and trying to understand the observations. Specifically, we’re interested in what happens when we capture a thunderstorm from the start until it dissipates. It would be great if we could get a certain number of people in relatively clean and polluted conditions. So we really need to dig through all this data, see what’s out there, and set up the simulation.
what are you doing now?
Most of what I do is developing a new method to simulate microphysics or clouds and precipitation. Most weather models contain certain assumptions about how droplets behave and how their populations evolve. Something is hard to incorporate into a model, it makes progress a bit difficult because in some cases when you build an approximation or an assumption in a weather or climate model, you forget about it. You kind of forget that you did that, but the uncertainty is still there, and the effects of those uncertainties are still there. So it kind of came back and said, well, where did this come from? Can we quantify these uncertainties and reduce them in a systematic way?
Instead of assumptions, we’ll let observations directly constrain physics.We put in the minimum first information and trying to build it from scratch to try and get rid of some of the structural errors that we think are holding back science. This has been funded by the Department of Energy for their climate models. We also received funding from NASA to use it for two of their climate models and the National Center for Atmospheric Research Community Atmospheric Model.
Getting these things right in climate models is important. For example, stratocumulus clouds are very important to climate. They are pure liquid clouds, strongly influenced by what is happening on the microscopic level. I saw a recent paper suggesting that these clouds may disappear with a certain amount of warming, which would be one of the climate feedbacks that would speed up the rate of warming. If you remove them, you remove one of the most powerful cooling mechanisms on Earth. If that’s true, that’s really bad news. We need to make sure our cloud microphysical models are as good as possible and try to better quantify the remaining uncertainties to refine our expectations of the future effects of global warming.
