Coarse sea spray inhibits lightning

Electrification of deep convective clouds (DCC) occurs when ice crystals collide with graupel in supercooled water, transferring charge: crystals typically gain positive charge and are lofted by updrafts, while graupel tend to gain negative charge and descend, establishing the electric field that leads to lightning. Greater electrification is expected in clouds with stronger updrafts and larger supercooled liquid water content (SLWC). Updraft intensity is dominated by convective available potential energy (CAPE), but can be further invigorated by fine aerosols (dry radius < 1 µm) acting as cloud condensation nuclei (CCN). More CCN nucleate more numerous, smaller droplets that delay coalescence, prolonging ascent to colder levels and increasing SLWC; the enhanced condensation also releases additional latent heat that accelerates updrafts. These mechanisms explain observed increases in lightning with CAPE and CCN and decreases with greater cloud base–freezing level distances over both tropical land and ocean. However, for the same rainfall amount, lightning frequency is generally far smaller over tropical ocean than land despite similar meteorology. Known differences in cloud base heights, updrafts, and CCN cannot explain why CAPE times precipitation amount correlates with lightning over land but not over ocean, leaving much oceanic lightning variability unexplained. Prior work also proposed that ultrafine aerosol particles could invigorate DCC via secondary activation, but this needs further observational verification. Here, the authors hypothesize that coarse sea salt aerosols (dry radius > 1 µm; coarse sea spray, CSS) have the opposite effect of fine CCN: by nucleating fewer but larger cloud drops that coalesce and precipitate faster, CSS should weaken DCC and reduce lightning. They test this hypothesis by tracking tropical DCC clusters and relating their properties and lightning to fine and coarse aerosol concentrations while controlling for meteorology.

Past studies established that DCC electrification arises from ice–graupel collisions in supercooled regions and is strengthened by strong updrafts and abundant SLWC. CAPE is a primary control on updraft strength, and fine aerosols (CCN) can invigorate convection by increasing droplet number, delaying warm rain formation, enhancing condensation and latent heat release, and promoting development of mixed-phase precipitation. Observations showed lightning increases with CAPE and CCN but decreases with larger cloud base–freezing level separation over tropics. Despite these relationships, the pronounced land–ocean lightning contrast for a given rainfall remains unexplained by thermodynamic differences alone; CAPE×precipitation predicts lightning over land but fails over ocean. Additional meteorological controls (precipitable water, wind shear) modulate convection, and ultrafine aerosol particles have been implicated in invigoration via secondary activation, though observational confirmation is pending. In contrast to invigoration by fine aerosols, theory and modeling indicate that giant or coarse CCN, such as sea salt, can enhance warm rain via early coalescence, potentially reducing mixed-phase processes that underpin electrification. This study builds on these findings to quantitatively separate and compare the effects of fine versus coarse marine aerosols on DCC and lightning.

Study domain spans tropical Africa and adjacent Atlantic (50°W–50°E, 20°S–20°N). Period: January 2013–December 2017. Data sources: Meteosat Second Generation (MSG) geostationary satellite for cloud properties (phase and cloud top temperature, CTT); GPM Integrated Multi-satellite Retrievals (IMERG) for rainfall; National Centers for Environmental Prediction (NCEP) Final Operational Global Analysis for environmental fields; MERRA-2 reanalysis for boundary-layer aerosol mass concentrations (fine aerosols and coarse sea salt, CSS), which assimilates multiple satellite and AERONET observations and shows high agreement with AERONET (global mean correlation ~0.85; ~0.93 over ocean). Lightning strokes are from the World Wide Lightning Location Network (WWLLN), a global VLF network with ~5 km location accuracy and high temporal resolution; relative detection efficiency is near 100% over land and ocean (excluding polar ice regions) at the scales considered. GPM Dual-frequency Precipitation Radar (DPR) is used to derive ice water path (IWP) for convective precipitation from radar reflectivity relationships. Data collocation: All variables are matched to MSG’s 9 km × 9 km grid using nearest-neighbor in space; temporal matching uses 30-minute IMERG intervals with linear interpolation. Tracking: DCC systems are tracked across their full lifecycle within a moving 10° × 5° (longitude × latitude) window to capture multicellular convective clusters. A convective core is defined as the area with IMERG rainfall rate > 1 mm h⁻¹ and an ice-phase cloud top; each DCC case includes the convective core and associated ice anvil. Lifecycle runs from core initiation to anvil dissipation. The domain-averaged rainfall amount (mm) is the integrated rainfall rate over the DCC lifetime divided by the fixed window area. Normalized lightning frequency is the total strokes over the DCC lifetime per km³ of integrated rainfall volume. Sample: 76,665 convective cases with lightning from 2013–2017, partitioned into 54,523 over land and 22,142 over ocean; additionally, 198 land and 2,439 ocean cases without lightning are noted. Analyses stratify by aerosol concentrations (percentile bins), and by meteorological controls (CAPE, precipitable water, vertical motion, relative humidity, wind shear) to isolate aerosol effects; CSS relationships are also examined across surface wind speeds.

• CAPE alone cannot explain the land–ocean lightning contrast: CAPE is similar or even larger over some oceanic regions, yet lightning is much more frequent over land for a given rainfall amount.
• Fine aerosol effects (invigoration):
  • Over land, increasing fine aerosol mass from clean to polluted conditions cools DCC core CTT by ~12 °C and nearly triples rainfall (to optimal fine aerosol concentration ~5 µg m⁻³).
  • Over ocean, the same fine aerosol increase cools CTT by ~4 °C and increases rainfall by a factor of ~1.6.
  • Despite weaker invigoration in CTT and rainfall over ocean, lightning density per rainfall volume increases much more strongly with fine aerosols over ocean: ×11 from low to high fine aerosol mass, versus ×2.6 over land (robust across CAPE and PW bins).
  • Partitioning by CSS shows that with low CSS, adding fine aerosols over ocean cools CTT by ~6 °C and enhances lightning by ×22; oceanic lightning depends on fine aerosols similarly to land but is ~25% lower in magnitude for the same fine aerosol level.
  • Lightning sensitivity is largest at low fine aerosol concentrations (<1.5 µg m⁻³), more common over ocean; over land, higher fine aerosol conditions place lightning response on a more saturated part of the curve.
• Coarse sea salt (CSS) effects (suppression):
  • Increasing CSS is associated with moderate CTT warming (especially at low PW), enhanced warm rain at the expense of mixed-phase precipitation, and strong lightning suppression.
  • From the lowest to highest 5% CSS bins, lightning can decrease by up to ×0.1 (90% reduction), largely independent of fine aerosol loading.
  • CSS most strongly suppresses lightning when fine aerosol concentrations are high (lightning reduction ~×0.2 at largest fine aerosol levels), consistent with CSS restoring coalescence and warm rain otherwise suppressed by abundant fine CCN.
  • IWP increases with fine aerosols (at fixed CSS), especially under low CSS, while CSS decreases IWP by a comparable magnitude; the CSS inhibition of IWP is strongest at large fine aerosol concentrations, matching observed lightning reductions.
  • CSS lengthens the lifetime of multicellular convective clusters even as individual convective intensity weakens, likely because weaker convection consumes CAPE more slowly.
  • CSS-driven initiation of raindrops accelerates scavenging of small droplets formed on fine particles, cleansing marine air and further inhibiting fine-aerosol invigoration.
• Robustness to meteorology: The aerosol–lightning relationships hold across stratifications by CAPE, precipitable water, mid-tropospheric vertical motion, surface and 450 hPa relative humidity, and 850–200 hPa wind shear; the effect of a given CSS level on lightning shows no dependence on surface wind speed.
• Overall, fine and coarse soluble aerosols exert opposite influences: fine aerosols suppress warm rain and invigorate mixed-phase processes, increasing lightning; coarse sea spray enhances warm rain, weakens mixed-phase precipitation, and suppresses lightning.

The results reconcile the longstanding puzzle of why oceanic thunderstorms produce far fewer lightning flashes than continental storms for comparable rainfall. While thermodynamic controls (e.g., CAPE) and fine aerosol concentrations modulate convective invigoration and electrification in both environments, they cannot by themselves account for the land–ocean contrast. By quantifying the competing effects of fine aerosols versus coarse sea spray (CSS), the study shows that CSS weakens convection by promoting early coalescence and warm rain, thereby reducing supercooled liquid water and ice–graupel interactions that drive electrification. Consequently, lightning over ocean increases sharply with fine aerosol loading when CSS is scarce (approaching continental behavior), but elevated CSS suppresses lightning—most strongly at high fine aerosol concentrations where CSS most effectively restores coalescence. These mechanisms explain why CAPE×precipitation predicts lightning over land but not over ocean and elucidate the large variability of oceanic lightning. The findings imply that marine lightning occurrence is controlled by the balance between fine aerosol invigoration and CSS-induced warm rain enhancement, with CSS often dominating over the ocean.

Fine and coarse soluble aerosols exert opposite controls on deep convective cloud intensity, phase partitioning, and electrification. Fine aerosols (CCN) suppress coalescence, invigorate convection, increase mixed-phase precipitation, and enhance lightning; coarse sea spray (CSS) enhances coalescence, shifts precipitation toward warm rain, weakens convection, and suppresses lightning—by up to ~90% across observed CSS ranges, with the strongest suppression at high fine aerosol concentrations. Accounting for these contrasting aerosol effects resolves much of the land–ocean lightning discrepancy for a given rainfall and clarifies aerosol–cloud–precipitation–lightning interactions. Climatically, CSS-driven conversion of mixed-phase to warm rain reduces latent heat of freezing and upward heat transport for a given surface rainfall, implying a need for more rainfall to balance tropospheric radiative deficits over oceans. Incorporating both fine and coarse marine aerosol effects into weather–climate models should improve simulations of rainfall amounts, vertical latent heating distributions, and atmospheric circulation. Future work could include independent replication of lightning responses using additional calibrated datasets and further observational tests of ultrafine aerosol invigoration mechanisms.

The analysis relies on WWLLN lightning detections; while performance is characterized and relative detection efficiency is high over the study regions, other independent lightning measurements with published sensitivity characterizations at the same time–space scales were not available for replication. Aerosol properties are from the MERRA-2 reanalysis, which, despite strong validation against AERONET, remains an assimilated product and may carry associated uncertainties.