Shipping Lane Lightning Declines Following Fuel Sulfur Regulation

Aerosol-cloud interactions influence the Earth’s energy budget, hydrological cycle, and climate. The mechanisms and magnitude of these interactions are still poorly understood and are consistently identified as one of the largest gaps in our collective understanding about the earth’s climate.

Aerosols may also interact with deep convective cloud (DCC) systems, which are essential to the earth's water and energy cycles. Yet there is no strong consensus on how or to what effect this interaction takes place, as DCC response to aerosol perturbations is obfuscated by rapid phase changes and sensitivity to localized perturbations in stability.

Previous work has documented enhancements in lightning over two heavily trafficked shipping corridors in the Indian Ocean and South China Sea. Lightning frequency is a function of updraft velocity, vertical ice fluxes, and super-cooled water in the mixed-phase region of a DCC cloud. So although the mechanism for the lightning enhancement is not well understood, it is assumed to be related to 1) an increase in the frequency or intensity of deep convection needed to support lightning generation or 2) a perturbation to the ice hydrometeor number-size distribution in the mixed-phase region of deep convective cloud. In either case, the first step has been assumed to be similar to the "Twomey effect" in shallow-clouds, where the proportion of small droplets is expected to increase as additional aerosol serve as cloud condensation nuclei (CCN).

For decades, shipping lanes have proved useful as testing grounds to close this gap. Over the open ocean, ships inject a narrow strip of aerosol and aerosol precursors, such as SO2, into a relatively clean marine boundary layer. The shipping lanes thus provide an excellent natural experiment for probing aerosol perturbations to cloud properties. In January 2020, the International Maritime Organization (IMO) reduced the amount of allowable sulfur in fuel by a factor of seven, from 3.5% to 0.5%, in order to curb negative health impacts of SO2 emitted by ships. Since the regulation, a number of studies have noted the change in cloud brightness, droplet number, and droplet size in shallow marine clouds over shipping lanes. To date, all studies on cloud responses to the IMO sulfur regulation have focused on shallow marine clouds, i.e. "ship tracks".

If the IMO regulation has altered cloud droplet size distributions in shallow clouds, a similar shift could impact lightning in deep convective clouds. We test the hypothesis that the enhancement in lightning has changed since the IMO regulation of fuel sulfur content by examining the variability and trend in lightning stroke density over 1) the two most polluting shipping lanes in the world and 2) globally. We then present a series of complementary cloud resolving model simulations that aim to elucidate the impacts of the regulation on deep convective clouds.

We use 12 years of data from the World Wide Lightning Location Network (WWLLN), Integrated Multi-satellitE Retrievals for GPM (IMERG), ECMWF Re-Analysis interim data, and Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). The data are 3-hourly and analyzed on a 0.5 by 0.625o (lat, lon) grid.

The Port of Singapore accounts for roughly 25% of the world's bunkering fuel demand, and is a common port for the Indian Ocean and South China Sea shipping lanes. Over these shipping lanes, lightning is enhanced by approximately 80%. Beginning in late 2019, bunkering fuel statistics show widespread adoption of low sulfur fuels following the IMO regulation. When comparing the intensity of the peak in lightning over the shipping lane to nearby regions, we see a regime shift in the lightning enhancement as the switch to low-sulfur fuels takes place. The new regime seems to produce lightning enhancements approximately 30% lower than before the IMO regulation. We also characterize the change in the lightning enhancement across thermodynamic regimes by dividing lightning observations into CAPE– precipitation bins to isolate the impacts of the shipping lane on stroke density. In nearly every thermodynamic setting, we observe a strong decrease in the lightning enhancement — between 40–80% on average. Globally, the perturbation to background lightning is not as strong as over the shipping lanes leaving the Port of Singapore; however, we observe a significant change in lightning over the world’s tropical shipping lanes since the adoption of low-sulfur fuels.

Next, we model the sensitivity of DCCs and lightning diagnostics to changes to aerosol composition, size, and number distributions after the regulation. Leveraging the declining lightning enhancements over shipping lanes, we conduct a mechanism denial experiment with two SAM simulations to test agreement between proposed mechanisms for lightning enhancement and the observed response lightning to the regulation.

Our results shed light on the role of ship emissions as cloud condensation nuclei and the relationship between sulfur chemistry and DCC mixed-phase cloud microphysics. We discuss these results in the context of various hypotheses regarding aerosol invigoration of deep convective systems for which lightning can be used as an indicator.