For decades, researchers have puzzled over why lightning seems sparser over the sea. New research suggests the difference may be a dash of salt.
Most lightning occurs over land, but most precipitation occurs over the ocean, said Daniel Rosenfeld, a cloud physicist at the Hebrew University of Jerusalem and a coauthor of a new study in Nature Communications. For the past 20-some years, researchers have been batting about ideas on why. “I had many friendly arguments with my colleagues,” Rosenfeld said.
Over land and sea, lightning forms from the collisions of ice particles and pellets in a cloud. The process starts with small water droplets being wafted high into a cloud, where it’s cold. Even when the temperature is below freezing, the droplets can stay liquid, becoming supercooled. But eventually, some droplets form ice crystals, and when other supercooled droplets ram into them, these droplets freeze fast. These pellets, called graupel, fall under their own weight. When graupel collides with small ice crystals, the collisions create electrical charges. The falling graupel tends to grab the negative charges, whereas rising ice crystals tend to stash positive ones. Eventually, the air between those separated charges breaks down, sending a bolt of lightning crashing through the cloud.
But sea spray forms large salt particles—greater than 1 micrometer. Water gloms onto these aerosols, creating large raindrops. The falling rain starves oceanic clouds of the water needed to create zaps of lightning, researchers reported.
A Salty Hypothesis
One familiar theory suggested to explain the discrepancy in lightning strikes over land and over sea invokes thermodynamic factors, such as differences in heating that drive the motion of air. Over land, for instance, there’s greater solar heating in the afternoon. At sea, the process is more evenly spread across the day, Rosenfeld explained. Differences in circulation and temperature in clouds may contribute to different frequencies of lightning.
Rosenfeld and his colleagues surveyed the global distribution of lightning from 2013 to 2017. They also mapped a quantity called convective available potential energy, or CAPE. CAPE is the potential for air to rise—in essence, it’s the energy that feeds a deep updraft. Even though the research documented much more lightning over land, CAPE was similar over land and ocean, the authors reported. “If anything, it’s larger over the ocean,” Rosenfeld said, which suggested that thermodynamic factors (heating that drives air circulation) weren’t the key difference in the lightning disparity.
To further investigate what was going on in storm systems, the scientists zoomed in on the Atlantic Ocean and surrounding continental areas, including parts of South America and Africa. They used satellite imagery to track the evolution of clouds, paying particular attention to how clouds developed vertically and keeping tabs on key factors such as temperature at the cloud top.
The researchers also collected the time and location of lightning data from the World Wide Lightning Location Network. Drawing on meteorological data and observations, they estimated the concentrations of fine aerosols, particles smaller than 1 micrometer, and larger sea spray aerosols.
An increase in fine aerosols correlated with increased lightning density. But a boost in the concentration of coarse sea salt coincided with a steep drop in lightning density. Sea-salt aerosols could cut lightning by 90%, the researchers reported.
The effects of sea spray salt have never been quantified like this, said Jiwen Fan, an Earth scientist at Pacific Northwest National Laboratory in Richland, Wash., who wasn’t involved with the study. “The suppression effect is so strong—that is a surprise.”
Warm Cloud Depth
But discussion about the roles of thermodynamics and aerosols isn’t over yet. Theoretically, the picture painted by Rosenfeld and his team is correct, said Steven Rutledge, an atmospheric scientist at Colorado State University in Fort Collins, but “they’re glossing over the role of thermodynamics.”
The team is missing the effects of a measure called warm cloud depth, said Rutledge. Warm cloud depth describes the vertical difference between a cloud’s base and where its temperature reaches the freezing point. “The magnitude of CAPE is similar between oceans and land,” Rutledge explained, “but the ‘shape of the CAPE’ is very different and is what really matters.”
Clouds over land experience strong updrafts in their middles because these clouds are much warmer than the environment. But over the oceans, this temperature difference is far less, he said, such that precipitation-sized particles are not carried aloft in clouds to help make lightning.
An ocean cloud’s warm cloud depth can stretch some 4 or 5 kilometers, Rutledge said. When this distance is smaller, a higher proportion of the cloud’s water can reach the colder areas where it can freeze and electrify, contributing to lightning, he explained. In addition, he said, cloud updrafts in the tropics are often too weak to push raindrops up across the freezing level.
In other words, sea-salt aerosols weaken the lightning potential of ocean clouds because their substantial warm cloud depth limits the chances of the aerosols reaching the upper part of the cloud, where it would electrify, Rutledge said. Ultimately, he concluded, “you can’t decouple the role of aerosols versus the role of thermodynamics.”
Missing Aerosol Effects in Models
Fan, however, said the study’s conclusions make sense. She agreed that the study didn’t look at all the thermodynamic factors that affect the intensity of rising air; CAPE can’t represent all severe storm conditions. But for tropical storms, using CAPE and interrogating the magnitude of aerosols’ effects on lightning were reasonable, she said.
Modeling could help researchers better understand how fine aerosols and coarse sea spray change clouds, both Fan and Rosenfeld said. The relationship could have implications far beyond lightning.
“The lightning per se is very interesting and striking, but it is only the start of the story as far as significance for climate,” said Rosenfeld. Lightning provides clues about how clouds are precipitating, he said.
“If you want to better predict your weather and the climate, this needs to be considered,” Fan agreed.
Weather and climate modelers have been reticent to include aerosols because of the hefty computational resources that would require. But “we’ve got to bite the bullet and do that,” Rosenfeld said. “This is a major, major factor that we don’t take into account.”
—Carolyn Wilke (@CarolynMWilke), Science Writer
4 November 2022: This article has been updated to better reflect the input of Steven Rutledge.