Constraining the aerosol influence on cloud liquid water path

Abstract

Abstract. The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic radiative forcing of the climate. Significant progress has been made in constraining this forcing using observations, but uncertainty remains, particularly in the magnitude of cloud rapid adjustments to aerosol perturbations. Cloud liquid water path (LWP) is the leading control on liquid-cloud albedo, making it important to observationally constrain the aerosol impact on LWP. Previous modelling and observational studies have shown that multiple processes play a role in determining the LWP response to aerosol perturbations, but that the aerosol effect can be difficult to isolate. Following previous studies using mediating variables, this work investigates use of the relationship between cloud droplet number concentration (Nd) and LWP for constraining the role of aerosols. Using joint-probability histograms to account for the non-linear relationship, this work finds a relationship that is broadly consistent with previous studies. There is significant geographical variation in the relationship, partly due to role of meteorological factors (particularly relative humidity). The Nd–LWP relationship is negative in the majority of regions, suggesting that aerosol-induced LWP reductions could offset a significant fraction of the instantaneous radiative forcing from aerosol–cloud interactions (RFaci). However, variations in the Nd–LWP relationship in response to volcanic and shipping aerosol perturbations indicate that the Nd–LWP relationship overestimates the causal Nd impact on LWP due to the role of confounding factors. The weaker LWP reduction implied by these “natural experiments” means that this work provides an upper bound to the radiative forcing from aerosol-induced changes in the LWP.

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 (2019), and are in line with the values for higher N d from Gryspeerdt, Goren et al (2019) and lower r e from Toll et al (2019). 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, 2019), there is uncertainty inherent in applying the λ values estimated here more broadly (particularly in regions with greater precipitation).…”

“…Our results join a growing literature of studies using "natural experiments" like volcanic eruptions and the inadvertent modification of clouds from international shipping, which overall have tended to find that climate model estimates of the effective radiative forcing due to aerosol-cloud interactions are likely too negative due to overly strong adjustments that reinforce the Twomey effect (Gryspeerdt, Goren et al, 2019;Malavelle et al, 2017;Toll et al, 2017Toll et al, , 2019. These previous papers, however, have generally taken a "bottom-up" approach of aggregating statistics of individual pollution tracks to reach their conclusions, whereas we take a "top-down" approach to study aerosol-cloud interactions using climatological fields.…”

“…This may be due to undercounting, as clearly visible ship tracks are relatively rare (Gryspeerdt, Smith, et al, 2019). Hundreds or thousands (Campmany et al, 2009;Christensen et al, 2014;Gryspeerdt, Goren et al, 2019;Toll et al, 2017Toll et al, , 2019 have been identified by satellite per year even though there are on order 100,000 ships in the global fleet (Eyring et al, 2010). 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, 2018).…”

“…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, 2015;Gryspeerdt et al, 2016;Stevens & Feingold, 2009). 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, 2019;Malavelle et al, 2017;McCoy & Hartmann, 2015;Toll et al, 2017Toll et al, , 2019 and ship tracks (Chen et al, 2012;Gryspeerdt, Goren et al, 2019;Toll et al, 2017Toll et al, , 2019, may represent our best opportunity for constraining ACI absent controlled experiments .…”

“…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, 2019;Toll et al, 2017Toll et al, , 2019.…”