Salt in Sea Spray Aerosols Inhibits Lightning during Marine Thunderstorms: Study


In a study published this week in the journal Nature Communications, scientists tested the hypothesis that fine and coarse marine aerosols have opposite effects on lightning frequency. Their results explain why levels of lightning over tropical oceans are reduced compared to the number seen over land.

This map shows that lightning occurs mostly over land compared to ocean; the gray rectangle indicates the region of interest from 50°W to 50°E and 20°S to 20°N. Image credit: Pan et al., doi: 10.1038/s41467-022-31714-5.

This map shows that lightning occurs mostly over land compared to ocean; the gray rectangle indicates the region of interest from 50°W to 50°E and 20°S to 20°N. Image credit: Pan et al., doi: 10.1038/s41467-022-31714-5.

Thunderstorm activity over land takes place because of the effects of thermodynamics and aerosol-cloud-precipitation interactions.

However, it is unclear why lightning activity over tropical oceans is significantly reduced in comparison to over land.

“The known effects of thermodynamics and aerosols can well explain the thunderstorm activity over land, but fail over oceans,” said Dr. Daniel Rosenfeld, a researcher in the Institute of Earth Sciences at the Hebrew University of Jerusalem and the State Key Laboratory of Information Engineering in Surveying, Mapping, and Remote Sensing at Wuhan University, and his colleagues.

To investigate why lightning may be less frequent over oceans, the researchers analyzed weather, aerosol and lightning activity data from Africa and the adjacent oceans (between 50°W to 50°E and 20°S to 20°N) from 2013 to 2017.

They found that coarse marine aerosols, such as salt (dry radius > 1 μm), reduced lightning frequency.

Fine aerosols were found to promote the electrification of clouds, as they do over land, whilst coarse salt particles from ocean spray reduced lightning by weakening convection within clouds.

The large particles were found to promote the precipitation of warm rain before cloud water can rise up and reach the levels necessary for super-cooling — a necessary step towards cloud electrification.

This has the effect of reducing the upwards transfer of heat over the sea, affecting the amount of rainfall necessary to drive atmospheric circulation.

“Tracking the full lifecycle of tropical deep convective cloud clusters shows that adding fine aerosols significantly increases the lightning density for a given rainfall amount over both ocean and land,” the authors said.

“In contrast, adding coarse sea salt, known as sea spray, weakens the cloud vigor and lightning by producing fewer but larger cloud drops, which accelerate warm rain at the expense of mixed-phase precipitation.”

“Adding coarse sea spray can reduce the lightning by 90% regardless of fine aerosol loading,” they said.

“These findings reconcile long outstanding questions about the differences between continental and marine thunderstorms, and help to understand lightning and underlying aerosol-cloud-precipitation interaction mechanisms and their climatic effects.”

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Z. Pan et al. 2022. Coarse sea spray inhibits lightning. Nat Commun 13, 4289; doi: 10.1038/s41467-022-31714-5




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