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

Gryspeerdt, Edward; Goren, Tom; Smith, Twp

doi:10.5194/acp-21-6093-2021

Cited by 33 publications

(53 citation statements)

References 59 publications

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“…A key result from the heuristic model, for D s ∼ 100 nm, is that the forcing to offset doubled CO 2 should be achiev-able with a total salt mass injection rate of ∼ 50-70 Tg yr −1 . This is much lower than the natural sea salt flux, which studies suggest ranges from 3000 to > 10 000 Tg yr −1 Grythe et al, 2014). The residence time of natural sea spray particles is considerably shorter than the lifetime (τ res = 2 d) of the injected salt particles because sea spray particles have a much larger mean size.…”

Section: Sprayer Number and Injection Ratementioning

confidence: 89%

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Wood

2021

Atmos. Chem. Phys.

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Abstract. A simple heuristic model is described to assess the potential for increasing solar reflection by augmenting the aerosol population below marine low clouds, which nominally leads to increased cloud droplet concentration and albedo. The model estimates the collective impact of many point source particle sprayers, each of which generates a plume of injected particles that affects clouds over a limited area. A look-up table derived from simulations of an explicit aerosol activation scheme is used to derive cloud droplet concentration as a function of the sub-cloud aerosol size distribution and updraft speed, and a modified version of Twomey's formulation is used to estimate radiative forcing. Plume overlap is accounted for using a Poisson distribution, assuming idealized elongated cuboid plumes that have a length driven by aerosol lifetime and wind speed, a width consistent with satellite observations of ship track broadening, and a depth equal to an assumed boundary layer depth. The model is found to perform favorably against estimates of brightening from large eddy simulation studies that explicitly model cloud responses to aerosol injections over a range of conditions. Although the heuristic model does not account for cloud condensate or coverage adjustments to aerosol, in most realistic ambient remote marine conditions these tend to augment the Twomey effect in the large eddy simulations, with the result being a modest underprediction of brightening in the heuristic model. The heuristic model is used to evaluate the potential for global radiative forcing from marine cloud brightening as a function of the quantity, size, and lifetime of salt particles injected per sprayer and the number of sprayers deployed. Radiative forcing is sensitive to both the background aerosol size distribution in the marine boundary layer into which particles are injected and the assumed updraft speed. Given representative values from the literature, radiative forcing sufficient to offset a doubling of carbon dioxide ΔF2×CO2 is possible but would require spraying 50 % or more of the ocean area. This is likely to require at least 104 sprayers to avoid major losses of particles due to near-sprayer coagulation. The optimal dry diameter of injected particles, for a given salt mass injection rate, is 30–60 nm. A major consequence is that the total salt emission rate (50–70 Tg yr−1) required to offset ΔF2×CO2 is a factor of five lower than the emissions rates required to generate significant forcing in previous studies with climate models, which have mostly assumed dry diameters for injected particles in excess of 200 nm. With the lower required emissions, the salt mass loading in the marine boundary layer for ΔF2×CO2 is dominated by natural salt aerosol, with injected particles only contributing ∼ 10 %. When using particle sizes optimized for cloud brightening, the aerosol direct radiative forcing is shown to make a minimal contribution to the overall radiative forcing.

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Section: Sprayer Number and Injection Ratementioning

confidence: 89%

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Wood

2021

Atmos. Chem. Phys.

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“…A strong offsetting warming effect from the negative LWP adjustment is evident in both observational studies (e.g. Possner et al, 2020;Gryspeerdt et al, 2019aGryspeerdt et al, , 2021 as well as large eddy simulation (e.g. Wang et al, 2003;Ackerman et al, 2004;Xue et al, 2008).…”

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confidence: 86%

Albedo susceptibility of Northeastern Pacific stratocumulus: the role of covarying meteorological conditions

Zhang

Zhou

Feingold

2021

Preprint

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Abstract. Quantification of the radiative adjustment of marine low-clouds to aerosol perturbations, regionally and globally, remains the largest source of uncertainty in assessing current and future climate. An important step towards quantifying the role of aerosol in modifying cloud radiative properties is to quantify the susceptibility of cloud albedo and liquid water path (LWP) to perturbations in cloud droplet number concentration (Nd). We use 10 years of space-borne observations from the polar-orbiting Aqua satellite, to quantify the albedo susceptibility of marine low-clouds over the northeast (NE) Pacific stratocumulus region to Nd perturbations. Overall, we find a low-cloud brightening potential of 20.8 ± 0.96 W m−2 ln(Nd)−1, despite an overall negative LWP adjustment for non-precipitating marine stratocumulus, owing to the high occurrence (37% of the time) of thin non-precipitating clouds (LWP < 55 g m−2) that exhibit brightening. In addition, we identify two more susceptibility regimes, the entrainment-darkening regime (36% of the time), corresponding to negative LWP adjustment, and the precipitating-brightening regime (22% of the time), corresponding to precipitation suppression. The influence of large-scale meteorological conditions, obtained from the ERA5 reanalysis, on the albedo susceptibility is also examined. Over the NE Pacific, clear seasonal covariabilities among meteorological factors related to the large-scale circulation are found to play an important role in grouping favorable conditions for each susceptibility regime. Our results indicate that, for the NE Pacific stratocumulus deck, the strongest positively susceptible cloud states occur most frequently for low cloud top height (CTH), the highest lower-tropospheric stability (LTS), low sea-surface temperature (SST), and the lowest free-tropospheric relative humidity (RHft) conditions, whereas cloud states that exhibit negative LWP adjustment occur most frequently under high CTH and intermediate LTS, SST, and RHft conditions. The warm rain suppression driven cloud brightening is found to preferably occur either under unstable atmospheric conditions (low LTS) or high RHft conditions that co-occur with warm SST. Mutual information analyses reveal a dominating control of LWP, Nd and CTH (cloud state indicators) on low-cloud albedo susceptibility, rather than of the meteorological factors that drive these cloud states.

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“…Instantaneous satellite observations from polar-orbiting satellites can be used to infer time-dependent processes. Gryspeerdt et al (2021) used MODIS imagery to study ship track evolution, assuming that the response of clouds to the ship emissions as a function of time is related to the distance from the head of the ship track. This novel methodology allows the use of higher resolution, with respect to most of the older generation geostationary satellites, to investigate aerosol-cloud interaction from satellites.…”

Section: Satellite Observationsmentioning

confidence: 99%

“…This translates to about a 2 µm increase over a distance of 100 km at typical container ship speeds of 24 knots (45 km h −1 ). Durkee et al (2000a) and Gryspeerdt et al (2021) found that the change in the width of the ship track over time depends on the background concentration of N d .…”

Section: Dilutionmentioning

confidence: 99%

“…The depth of the PBL and free tropospheric humidity have been identified as playing significant roles in the strength of the aerosol-cloud metrics shown in Figure S4. As the humidity in the free troposphere (above the cloud tops) becomes drier, polluted clouds with smaller droplets evaporate more efficiently (Ackerman et al, 2004;Dagan et al, 2017) causing liquid water paths and cloud albedo to decrease (Coakley and Walsh, 2002;Christensen and Stephens, 2011;Chen et al, 2012;Toll et al, 2019;Gryspeerdt et al, 2021) (Figure 11). Also, the sign of LWP adjustments is positive when cloud evolution is dominated by precipitation suppression and negative when dominated by evaporationentrainment (Wang et al, 2003;Xue and Feingold, 2006;Small et al, 2009).…”

Section: Meteorologymentioning

confidence: 99%

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Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

Christensen

Gettelman

Čermák

et al. 2021

Preprint

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Abstract. Aerosol-cloud interactions (ACI) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The non-linearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can also be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatio-temporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite data sets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Opportunistic experiments have significantly improved process level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus, demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.

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

Cited by 33 publications

References 59 publications

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Albedo susceptibility of Northeastern Pacific stratocumulus: the role of covarying meteorological conditions

Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

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