Anthropogenic Air Pollution Delays Marine Stratocumulus Breakup to Open Cells

Abstract

Marine stratocumulus cloud (Sc) decks with high cloud fraction typically breakup when sufficient drizzle forms. Cloud breakup leads to a lower cloud radiative effect due to the lower cloud amount. Here we use realistic Lagrangian large eddy simulations along a 3-day trajectory, evaluated with satellite observations, to investigate the timing of Sc breakup in response to aerosol conditions. We show that the timing of the breakup is strongly modulated by the diurnal cycle and large-scale meteorology but varies systematically with the initial aerosol concentration: the more polluted the clouds, the later the breakup. This indicates that the cloud radiative effect via cloud cover adjustments is not saturated, in contrast to the effect of aerosol on cloud albedo at fixed cloudiness, which weakens with increasing aerosol levels. The results also show that the cloud radiative impact of anthropogenic aerosol is strongest far from its origin over land.Plain Language Summary Marine stratocumulus clouds (Sc) cover large areas of the subtropical oceans. They frequently form as fully overcast cloud decks near the western coasts of continents, and break up as they move westwards, driven by the prevailing winds, towards the remote ocean. Their breakup is accompanied by a reduction in cloud cover, which in turn reduces the amount of solar radiation that is reflected by the cloud deck. Aerosol particles, as well as meteorological factors, determine the time it takes for a fully overcast cloud deck to break up. Here we use high-resolution simulations of Sc combined with satellite observations to investigate the roles of aerosol and meteorology in Sc break-up time and location. We demonstrate that an observed Sc deck off the coast of Western Europe would have broken up sooner had it not been influenced by anthropogenic aerosol from Europe. Aerosol was the main factor determining the break-up time of the cloud deck, with a sometimes significant contribution by largescale meteorology. Our results show that even at present-day pollution levels, aerosol perturbations can extend the lifetime of the Sc deck in its overcast state, and exert a significant cooling effect.Sc covers extensive regions over the eastern parts of the major subtropical oceans (Klein & Hartmann, 1993;Muhlbauer et al., 2014). These regions are characterized by decks of closed cells that form near the coast and breakup into open cells and trade wind cumulus as they move westward towards the remote oceans

Substantial Cloud Brightening From Shipping in Subtropical Low Clouds

“…For MODIS/Terra, λ is −0.09 (95% confidence interval: −0.31 to 0.08) and for MODIS/Aqua, λ is −0.39 (−0.68 to −0.14), giving a daily mean λ value of −0.24 (−0.42 to −0.08). These λ values are at the more negative end of estimates previously reported in the literature, as compiled by Sato and Suzuki (), and are in line with the values for higher N d from Gryspeerdt, Goren et al () and lower r e from Toll et al (). However, as the relationship between liquid water path and cloud droplet number is not necessarily linear in relative changes or even monotonic (Gryspeerdt, Goren et al, ), there is uncertainty inherent in applying the λ values estimated here more broadly (particularly in regions with greater precipitation).…”

“…For the increase in cloud droplet number concentration with aerosol ( β ) and cloud albedo with droplet concentration ( χ ), we would expect errors here to tend to overestimate the changes (i.e., effects may saturate more rapidly in reality than under an assumption of linearity). For liquid water path changes with droplet number ( λ ), the effects could be subtler as factors such as the free tropospheric relative humidity and likelihood of clouds to precipitate in the absence of an aerosol perturbation could cause λ to vary in magnitude and sign with meteorological conditions (Gryspeerdt, Goren et al, ; Toll et al, , ).…”

“…This may be due to undercounting, as clearly visible ship tracks are relatively rare (Gryspeerdt, Smith, et al, ). Hundreds or thousands (Campmany et al, ; Christensen et al, ; Gryspeerdt, Goren et al, ; Toll et al, , ) have been identified by satellite per year even though there are on order 100,000 ships in the global fleet (Eyring et al, ). However, high‐resolution model results suggest there can be radiatively important impacts even when a clear ship track is not easily discernible (Possner et al, ).…”

“…However, the confounding of aerosol effects and other meteorological variations that can significantly influence cloud properties remains a serious challenge for disentangling the magnitude of the aerosol effects alone (Adebiyi et al, ; Gryspeerdt et al, ; Stevens & Feingold, ). For this reason, “natural experiments” in which there is a clear aerosol perturbation independent of meteorological influence, such as volcanic eruptions (Gryspeerdt, Goren et al, ; Malavelle et al, ; McCoy & Hartmann, ; Toll et al, , ) and ship tracks (Chen et al, ; Gryspeerdt, Goren et al, ; Toll et al, , ), may represent our best opportunity for constraining ACI absent controlled experiments (Wood et al, ).…”

Observing the timescales of aerosol-cloud interactions in snapshot satellite images

“…With a higher background CF, there is less scope for a forcing due to a CF increase (Goren et al, 2019). This interpretation is supported by the results at a constant cloud fraction (Fig.…”