Tuesday, November 14, 2017

Flash! Thunderstorms Intensify Over Busy Shipping Lanes

Lightning can be a destructive force of nature, but it’s not immune to human influence. In fact, new research suggests that the exhaust from ships transporting oil, coffee, and probably this holiday season’s most popular gifts is intensifying thunderstorms and increasing the number of lightning strikes along some of the busiest shipping lanes in the world. Conducted by Joel Thornton, Robert Holzworth, and Todd Mitchell from the University of Washington and Katrina Virts from NASA Marshall Space Flight Center, the research was published earlier this fall in the American Geophysical Union’s journal Geophysical Research Letters.

Lightning flashes as the aircraft carrier USS Abraham Lincoln (CVN 72) transits the Strait of Malacca (10/8/10).
Image Credit: U.S. Navy photo by Mass Communication Specialist 3rd Class Colby K. Neal/Released.
Public domain in the United States.
“Storm intensification has obvious consequences for human life and the global economy through damages from wind and hail, as well as from direct lightning strikes,” say the authors in the paper. If human activity is intensifying storms, it’s certainly important to know how and why and to consider the consequences. This study is the first to demonstrate that ship exhaust likely influences local thunderstorm activity.

By comparing the frequency of lightning strikes directly above two of the busiest shipping lanes to nearby areas of the ocean that have similar meteorological conditions, the team showed that lightning strikes occur almost twice as often above the shipping lanes. Their research shows that this difference can’t be explained by changes in the weather, sea temperature, or other natural factors that usually influence storms.

The study focused on shipping lanes in the Indian Ocean and South China Sea. Although ships aren’t required to follow shipping lanes to the extent that cars obey traffic laws, the busiest lanes reflect favorable wind and current conditions for large ships. The routes in this study include passage from the Indian Ocean through the Strait of Malacca, a narrow passageway that leads to the South China Sea and eventually the Pacific Ocean. The Strait unites many economies and is traveled by nearly 100,000 ships annually; trillions of dollars in goods pass this way each year.

Map showing the Strait of Malacca.
Image Credit: Public domain.
The researchers used data on lighting strikes collected by the World Wide Lightning Location Network, a collection of geographically spaced radio antennae that collectively monitor global lightning activity. The network can detect most storms that produce lightning and has collected data since 2004. For this study, the team analyzed 12 years of data, 2005-2016, which included about 1.5 billion strikes. After noting the higher levels of lightning above shipping lanes and ruling out natural factors as the cause, the team concluded that the ships themselves must play some role—most likely through their emissions.

During a thunderstorm, strong updrafts bring droplets of water up into the atmosphere where they freeze, if they go high enough. At the same time, strong downdrafts carry ice and hail from above the freezing point in the atmosphere down toward the surface. During collisions between the ice, hail, and semi-frozen droplets in a cloud, electrons from rising particles are transferred to descending particles.

As a result of these collisions, an increasingly strong electric field forms in the cloud between the negatively charged lower region and the positively charged upper region. In the meantime, the lower region also induces positive charges to build up in the ground (or ocean) underneath the cloud. If the field within the cloud becomes strong enough, the cloud discharges with an intra-cloud lightning strike. If the field between the cloud and the ground (or ocean) becomes strong enough, the discharge results in a cloud-to-ground lightning strike.

What does all of this have to do with ship exhaust? The seeds for storm clouds are aerosols—tiny particles or droplets around which water vapor condenses. Ships, like anything with a combustion engine, produce exhaust that contains aerosols. This means that the concentration of aerosols is higher over shipping lanes than over nearby empty regions of the ocean.

Because of the high concentration of aerosols, the authors reason, more droplets form over shipping lanes and they are smaller and lighter than their counterparts over empty ocean. During an updraft, the lighter droplets travel higher up into the atmosphere. This means that more droplets cross the freezing line. More ice means more collisions, a greater buildup of charge, and more lightning.

Using results from the Emissions Database for Global Atmospheric Research and published ship emission factors, the researchers calculated the emissions of aerosol particle types and precursors in these shipping lanes. They found aerosol emissions significantly greater than other lanes in the region, among the largest in the world.

We can learn a lot from studying lightning in this open ocean situation, according to the researchers. As they conclude, “Our findings suggest that even small absolute increases in remote marine aerosol particles due to human activities could have a substantial impact on storm intensity and lightning. As such, there have likely been increases in storm vertical development and lightning in remote regions since the preindustrial era, which has consequences not only for human life and property but also for atmospheric composition and climate.”

Kendra Redmond

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