When my colleague Professor Tom Gill at University of Texas at El Paso alerted me to a recent study noting that cargo ships may be creating more lightning, my first reaction was that I wasn’t surprised at all. Though this may be a surprising finding to many scientists and the public, I have been conducting research for over two decades on how cities affect rainfall, storms, and lightning. As I read the paper, it was apparent to me that some of the same physical processes were at play with the cargo ship – lightning relationships. So how does a cargo ship create lightning?
A study published in the American Geophysical Union journal, Geophysical Research Letters, examined 12 years of global lightning stroke data (2005 to 2016) from the World Wide Lightning Location Network. Their analysis revealed that the density of lightning doubled over shipping lanes in the South China Sea and northeastern Indian Ocean compared to nearby, similar regions. They also found that this lightning enhancement had some degree of seasonality. November to April was the dominant period for the Indian Ocean while April to December was most significant in the South China Sea. The study, led by Joel Thornton at the University of Washington, concludes that meteorological factors like variations in wind or temperature do not explain the observed differences (see the graphic below). They hypothesize that aerosols (particulate matter) emitted from ship exhausts are the culprit.
To understand how a cargo ship can affect lightning, it is important to understand the physical processes at play. Lightning is caused by glaciated storms. In other words, the storms are composed of a mixture of liquid drops, graupel, and ice crystals (Lightning formation was explained in further detail in one of my previous Forbes articles). A fundamental premise of cloud development is that a “seed” or condensation nucleus is typically required for a cloud droplet (or ice crystal) to form. This is called heterogeneous nucleation. Cloud droplets and ice nuclei rarely form directly from the vapor phase without that “seed.” Thornton and his colleagues believe that the aerosols from ship tracks serve as condensation nuclei for cloud formation, which leads to deeper clouds able to become electrified. The NASA satellite instrument MODIS (first image in the article cited above) reveals ship track clouds over the Pacific Ocean.
It has been a surprise to find this feature in the data and to have it be so clearly pronounced. … Our finding provides one of the clearest examples of a human perturbation to aerosol particles and lightning in an otherwise clean region. … Our results indicate the ship exhaust particles are, in fact, changing what would be a tropical rain storm into a thunderstorm—from no lightning to a storm with lightning, Or, the particles are increasing the vertical development of thunderstorms to have even more lightning than they otherwise would have.
The researchers confirmed the last statements by comparing surface lightning data with radar data from NASA’s Tropical Rainfall Measuring Mission (TRMM), which had a satellite-borne weather radar. The radar can explore the vertical structure of the cloud in order to compare the depth of vertical liquid with ice particles.
By the way, this concept is not entirely new, although the linkage with ship tracks is novel. A recent paper in the Asia-Pacific Journal of Atmospheric Sciences is a good place to review aerosol-cloud-lightning interactions. I also co-authored a book chapter with colleagues in 2010 summarizing findings that cities can enhance rainfall and lightning activity. In 2003, researchers at Texas A&M University published a study showing significant enhancement of lightning around two cities in Louisiana, Lake Charles and Baton Rouge. They concluded that particulate matter, likely related to refinery activity in the area, was a key factor. Professor Tony Stallins at the University of Kentucky has been one of the leading scholars on this topic. His paper with Shea Rose in the journal Geography Compass summarizes the urban-lightning literature and the role of aerosols.
This is just another piece in the puzzle of how human activity affects weather and climate.
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