Publications
74 results found
Murray-Watson R, Gryspeerdt E, Goren T, 2023, Investigating the development of clouds within marine cold air outbreaks, Atmospheric Chemistry and Physics, Vol: 23, Pages: 9365-9383, ISSN: 1680-7316
Marine cold air outbreaks are important parts of the high-latitude climate system, and are characterised by strong surface fluxes generated by the air-sea temperature gradient. These fluxes promote cloud formation, which can be identified in satellite imagery by the distinct transformation of stratiform cloud ‘streets’ into a broken field of cumuliform clouds downwind of the outbreak. This evolution in cloud morphology changes the radiative properties of the cloud, and therefore is of importanceto the surface energy budget. While the drivers of stratocumulus-to-cumulus transitions, such as aerosols or the sea surface temperature gradient, have been extensively studied for subtropical clouds, the factors influencing transitions at higher latitudes are relatively poorly understood. This work uses reanalysis data to create a set of composite trajectories of cold air outbreaks moving off the Arctic ice edge and co-locates these trajectories with satellite data to generate a unique view of liquid-dominated cloud development within cold air outbreaks.The results of this analysis show that clouds embedded in cold-air outbreaks have distinctive properties relative to clouds following other trajectories in the region. The initial strength of the outbreak shows a lasting effect on cloud properties, with differences between clouds in strong and weak events visible over 30 hours after the air has left the ice edge. However, while the strength (measured by the magnitude of the marine cold-air outbreak index) of the outbreak affects the magnitude of cloud properties, it does not affect the timing of the transition to cumuliform clouds nor the top-of-atmosphere albedo. In contrast, the initial aerosol conditions do not strongly affect the magnitude of the cloud properties, but are correlated to cloud break-up,leading to an enhanced cooling effect in clouds moving through high aerosol conditions due to delayed break-up. Both the aerosol environment and the strength and
Horner G, Gryspeerdt E, 2023, Do detrained cirrus clouds have memory of the deep convection they came from?
<jats:p>The large cirrus outflows that arise from deep convection play a vital role in modulating the energy balance of the Earth&#8217;s atmosphere. One important question is how much do the initial conditions of the deep convection influence the subsequent evolution of the detrained cirrus, and if these initial conditions are important, over what timescales do they matter? Characterising how these cirrus outflows evolve over their entire lifetime, and how they might change in response to anthropogenic emissions is important in order to understand their role in the climate system and to constrain past and future climate change.Building on the &#8216;Time Since Convection&#8217; product used in Horner & Gryspeerdt (2023), this work investigates how the initial conditions of the deep convection influence the subsequent evolution of the detrained cirrus- in particular, how does the timing, location, and meteorological environment of the deep convection alter the detrained cirrus, and for how long are these initial conditions important for the cirrus properties- is there a &#8216;memory&#8217; of the initial conditions of the deep convection imprinted on the properties of the cirrus hours or days after the initial deep convection has dissipated? To answer this question, data from the DARDAR, ISCCP, and CERES products are used to build a composite picture of the radiative and microphysical properties of the clouds, which is investigated under varying initial conditions.The initial state of the convection is found to have a considerable impact on cirrus development under a variety of conditions. The diurnal cycle, particularly the timing of the convection, is a strong control on the cloud radiative effect, particularly in regions of strong convective activity. The initial aerosol perturbation is also shown to play a role in cirrus development, both in the large scale properties of the cirrus and the microphysical prop
Murray-Watson R, Gryspeerdt E, 2023, The evolution of clouds in Arctic marine cold air outbreaks
<jats:p>Marine cold air outbreaks (MCAOs) are important parts of the high-latitude climate system and are characterised by strong surface fluxes generated by the air-sea temperature gradient. These fluxes promote cloud formation, which can be identified in satellite imagery by the distinct transformation of stratiform cloud 'streets' into a broken field of cumuliform clouds downwind of the outbreak. This evolution of cloud morphology changes the radiative properties of the cloud and therefore is of importance to the surface energy budget.&#160;&#160;While the drivers of stratocumulus-to-cumulus transitions have been extensively studied for subtropical clouds, such as aerosols or the sea surface temperature gradient, the factors influencing transitions at higher latitudes are relatively poorly understood. This work uses reanalysis data to create a set of composite trajectories of cold air outbreaks moving off the Arctic ice edge and co-locates these trajectories with data from multiple satellites to generate a unique view of cloud development within cold air outbreaks.&#160;Clouds embedded in MCAOs have distinctive properties relative to clouds following other, more stable trajectories in the region. The initial instability and aerosol environments have distinct impacts on cloud development within outbreaks. The strength of the outbreak has a lasting effect on the magnitude of cloud properties along the trajectory. However, it does not strongly affect the timing of the transition to cumuliform clouds. In contrast, the initial aerosol concentration changes the timing of cloud break-up rather than the size of the cloud response.</jats:p>
Gryspeerdt E, Povey AC, Grainger RG, et al., 2023, Uncertainty in aerosol-cloud radiative forcing is driven by clean conditions
<jats:p>Atmospheric aerosols and their interaction with clouds are the largest uncertainty in the human forcing of the climate system. Anthropogenic emissions have increased aerosol concentrations, increasing the concentration of cloud droplets and leading to reductions in droplet size and increases in cloud reflectivity (a negative radiative forcing). Central to this climate impact is the susceptibility of cloud droplet number to aerosol. This susceptibility varies widely with the method and data used to estimate it and within global climate models, explaining much of the variation in estimates of the radiative forcing from aerosol-cloud interactions (RFaci). Better constraints on the susceptibility have been a key target for recent observation-based constraints on the aerosol forcing. &#160; &#160; &#160; &#160; &#160;&#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160;Previous work has shown that the aerosol burden of the clean, pre-industrial atmosphere has been demons
Quilelli Correa Rocha Ribeiro R, Gryspeerdt E, Van Reeuwijk M, 2023, Retrieving cloud sensitivity to aerosol using ship emissions
<jats:p>Aerosol-cloud interactions are one of the key uncertainties in understanding future climate change. A commonly used method for constraining these interactions is using ship tracks. Aerosol-containing plumes from ships can develop into linearly shaped clouds identifiable in satellite images, isolating the aerosol impact on clouds. Previous studies have shown that ship tracks form more commonly in clean conditions, but even accounting for this, many ships that might be expected to form ship tracks do not. This leads to uncertainties in aerosol-cloud interactions and their climate impact. &#160;Ship track formation depends on the aerosol-containing plumes from the ship being sufficiently concentrated upon reaching the cloud. The cloud must also be sensitive to aerosol. In focus are updraft-limited clouds: smaller updrafts promote slower cooling as a cloud parcel rises, higher critical supersaturation values and lower aerosol activation fractions. It is not clear which of these are more important, but it is vital to understand them if we are using ship tracks to retrieve cloud sensitivity to aerosol.&#160;&#160; &#160;We develop a plume-parcel model to address these issues to estimate cloud droplet enhancements in ship tracks. Ship aerosol concentrations at the cloud height were modelled as plumes, simulating the shorter timescales of injection. Droplet number concentration enhancements were estimated using K&#246;hler theory for over one hundred thousand ships off the coast of California. &#160;Using a constant updraft, the model was able to achieve reasonable enhancements (r2 ranging between (0.32, 0.4)). These enhancements were shown to be significantly sensitive to the choice of the updraft. In order to examine the hypothetical updraft values required for activation, an optimisation algorithm was developed to fit updrafts to cloud enhancement observations; a 1-1 correlation was achieved between observed a
Murray-Watson RJ, Gryspeerdt E, Goren T, 2023, Supplementary material to "Investigating the development of clouds within marine cold air outbreaks"
Gryspeerdt E, Povey AC, Grainger RG, et al., 2023, Uncertainty in aerosol-cloud radiative forcing is driven by clean conditions, Atmospheric Chemistry and Physics, Vol: 23, Pages: 4115-4122, ISSN: 1680-7316
Atmospheric aerosols and their impact on cloud properties remain the largest uncertainty in the human forcing of theclimate system. By increasing the concentration of cloud droplets (Nd ), aerosols reduce droplet size and increase the reflectivity of clouds (a negative radiative forcing). Central to this climate impact is the susceptibility of cloud droplet number to aerosol (β ), the diversity of which explains much of the variation in the radiative forcing from aerosol-cloud interactions (RFaci) in global climate models. This has made measuring β a key target for developing observational constraints of the aerosol forcing. While the aerosol burden of the clean, pre-industrial atmosphere has been demonstrated as a key uncertainty for the aerosol forcing, here we show that the behaviour of clouds under these clean conditions is of equal importance for understanding the spread in radiative forcing estimates between models and observations. This means that the uncertainty in the aerosol impact on clouds is, counterintuitively, driven by situations with little aerosol. Discarding clean conditions produces a close agreement between different model and observational estimates of the cloud response to aerosol, but does not provide a strong constraint on the RFaci. This makes constraining aerosol behaviour in clean conditions an important goal for future observational studies.
Arola A, Lipponen A, Kolmonen P, et al., 2022, Aerosol effects on clouds are concealed by natural cloud heterogeneity and satellite retrieval errors, Nature Communications, Vol: 13, Pages: 1-8, ISSN: 2041-1723
One major source of uncertainty in the cloud-mediated aerosol forcing arises from the magnitude of the cloud liquid water path (LWP) adjustment to aerosol-cloud interactions, which is poorly constrained by observations. Many of the recent satellite-based studies have observed a decreasing LWP as a function of cloud droplet number concentration (CDNC) as the dominating behavior. Estimating the LWP response to the CDNC changes is a complex task since various confounding factors need to be isolated. However, an important aspect has not been sufficiently considered: the propagation of natural spatial variability and errors in satellite retrievals of cloud optical depth and cloud effective radius to estimates of CDNC and LWP. Here we use satellite and simulated measurements to demonstrate that, because of this propagation, even a positive LWP adjustment is likely to be misinterpreted as negative. This biasing effect therefore leads to an underestimate of the aerosol-cloud-climate cooling and must be properly considered in future studies.
Goren T, Feingold G, Gryspeerdt E, et al., 2022, Projecting stratocumulus transitions on the albedo-cloud fraction relationship reveals linearity of albedo to droplet concentrations, Geophysical Research Letters, Vol: 49, ISSN: 0094-8276
Satellite images show solid marine stratocumulus cloud decks (Sc) that break up over the remote oceans. The Sc breakup is initiated by precipitation and is accompanied by a strong reduction in the cloud radiative effect. Aerosol has been shown to delay the Sc breakup by postponing the onset of precipitation, however its climatic effect is uncertain. Here we introduce a new approach that allows us to re-cast currently observed cloud cover and albedo to their counterfactual cleaner world, enabling the first estimate of the radiative effect due to delayed cloud breakup. Using simple radiative approximation, the radiative forcing with respect to pre-industrial times due to delayed Sc breakup is −0.39 W m−2. The radiative effect changes nearly linearly with aerosol due to the droplet concentration control on the cloud cover, suggesting a potentially accelerated warming if the current trend of reduction in aerosol emissions continues.
Watson-Parris D, Christensen MW, Laurenson A, et al., 2022, Shipping regulations lead to large reduction in cloud perturbations., Proceedings of the National Academy of Sciences of USA, Vol: 119, Pages: 1-5, ISSN: 0027-8424
Global shipping accounts for 13% of global emissions of SO2, which, once oxidized to sulfate aerosol, acts to cool the planet both directly by scattering sunlight and indirectly by increasing the albedo of clouds. This cooling due to sulfate aerosol offsets some of the warming effect of greenhouse gasses and is the largest uncertainty in determining the change in the Earth's radiative balance by human activity. Ship tracks-the visible manifestation of the indirect of effect of ship emissions on clouds as quasi-linear features-have long provided an opportunity to quantify these effects. However, they have been arduous to catalog and typically studied only in particular regions for short periods of time. Using a machine-learning algorithm to automate their detection we catalog more than 1 million ship tracks to provide a global climatology. We use this to investigate the effect of stringent fuel regulations introduced by the International Maritime Organization in 2020 on their global prevalence since then, while accounting for the disruption in global commerce caused by COVID-19. We find a marked, but clearly nonlinear, decline in ship tracks globally: An 80% reduction in SO[Formula: see text] emissions causes only a 25% reduction in the number of tracks detected.
Gryspeerdt E, Povey AC, Grainger RG, et al., 2022, Uncertainty in aerosol-cloud radiative forcing is driven by clean conditions
<jats:p>Abstract. Atmospheric aerosols and their impact on cloud properties remain the largest uncertainty in the human forcing of the climate system. By increasing the concentration of cloud droplets (Nd), aerosols reduce droplet size and increase the reflectivity of clouds (a negative radiative forcing). Central to this climate impact is the susceptibility of cloud droplet number to aerosol (β), the diversity of which explains much of the variation in radiative forcing in global climate models. This has made measuring β a key target for developing observational constraints of the aerosol forcing. While the aerosol burden of the clean, pre-industrial atmosphere has been demonstrated as a key uncertainty for the aerosol forcing, here we show that the behaviour of clouds under these clean conditions is of equal importance for understanding the spread in radiative forcing estimates between models and observations. This means that the uncertainty in the aerosol impact on clouds is, counterintuitively, driven by situations with little aerosol. Discarding clean conditions produces a close agreement between different model and observational estimates of the cloud response to aerosol, but does not provide a strong constraint on the radiative forcing from aerosol-cloud interactions. This makes constraining aerosol behaviour in clean conditions a key goal for future observational studies. </jats:p>
Dipu S, Schwarz M, Ekman AML, et al., 2022, Exploring satellite-derived relationships between cloud droplet number concentration and liquid water path using a large- domain large-eddy simulation, Tellus Series B: Chemical and Physical Meteorology, Vol: 74, Pages: 176-188, ISSN: 0280-6509
Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (Nd) and liquid water path (LWP). Specifically, this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between Nd and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the Nd, we have also considered adiabatic Nd (NAd) from the LES model for comparison. The joint histogram analysis shows that the NAd-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower NAd and LWP; the corresponding mean LWP (LWP) shows a weak relation with NAd. The CP shows a larger spread at higher NAd (>50 cm–3), and the LWP increases non-monotonically with increasing NAd in both cases. Nevertheless, both lack the negative NAd-LWP relationship at higher NAd, the entrainment effect on cloud droplets. In contrast, the model simulated Nd-LWP clearly illustrates a much more nonlinear (an increase in LWP with increasing Nd and a decrease in LWP at higher Nd) relationship, which clearly depicts the cloud lifetime and the entrainment effect. Additionally, our analysis demonstrates a regime dependency (marine and continental) in the NAd-LWP relation from the satellite retrievals. Comparing local vs large-scale statistics from satellite data shows that continental clouds exhibit only a weak nonlinear NAd-LWP relationship. Hence a regime-based Nd-LWP analysis is even more relevant when it comes to warm continental clouds and their comparison to satellite retrievals.
Gryspeerdt E, Glassmeier F, Feingold G, et al., 2022, Observing short timescale cloud development to constrain aerosol-cloud interactions, Atmospheric Chemistry and Physics, Vol: 22, Pages: 11727-11738, ISSN: 1680-7316
The aerosol impact on liquid water path (LWP) is a key uncertainty in the overall climate impact of aerosol. However, despite a significant effort in this area, the size of the effect remains poorly constrained, and even the sign is unclear. Recent studies have shown that the relationship between droplet number concentration (Nd ) and LWP is an unreliable measure of theimpact of Nd variations on LWP due to the difficulty in establishing causality. In this work, we use satellite observations of the short-term development of clouds to examine the role of Nd perturbations in LWP variations. Similar to previous studies, an increase followed by a general decrease in LWP with increasing Nd is observed, suggesting an overall negative LWP response to Nd and a warming LWP adjustment to aerosol. However, the Nd also responds to the local environment, with aerosol production, entrainment from the free troposphere and wet scavenging all acting to modify the Nd . Many of these effects act to further steepen the Nd -LWP relationship and obscure the causal Nd impact on LWP. Using the temporal development of clouds to account for these feedbacks in the Nd -LWP system, a weaker negative Nd -LWP relationship is observed over most of the globe. This relationship is highly sensitive to the initial cloud state, illuminating the roles of different processes in shaping the Nd -LWP relationship. The nature of the current observing system limits this work toa single time period for observations, highlighting the need for more frequent observations of key cloud properties to constrain cloud behaviour at process timescales.
Teoh R, Schumann U, Gryspeerdt E, et al., 2022, Aviation contrail climate effects in the North Atlantic from 2016 to 2021, Atmospheric Chemistry and Physics, Vol: 22, Pages: 10919-10935, ISSN: 1680-7316
Around 5 % of anthropogenic radiative forcing (RF) is attributed to aviation CO2 and non-CO2 impacts. This paper quantifies aviation emissions and contrail climate forcing in the North Atlantic, one of the world's busiest air traffic corridors, over 5 years. Between 2016 and 2019, growth in CO2 (+3.13 % yr−1) and nitrogen oxide emissions (+4.5 % yr−1) outpaced increases in flight distance (+3.05 % yr−1). Over the same period, the annual mean contrail cirrus net RF (204–280 mW m−2) showed significant inter-annual variability caused by variations in meteorology. Responses to COVID-19 caused significant reductions in flight distance travelled (−66 %), CO2 emissions (−71 %) and the contrail net RF (−66 %) compared with the prior 1-year period. Around 12 % of all flights in this region cause 80 % of the annual contrail energy forcing, and the factors associated with strongly warming/cooling contrails include seasonal changes in meteorology and radiation, time of day, background cloud fields, and engine-specific non-volatile particulate matter (nvPM) emissions. Strongly warming contrails in this region are generally formed in wintertime, close to the tropopause, between 15:00 and 04:00 UTC, and above low-level clouds. The most strongly cooling contrails occur in the spring, in the upper troposphere, between 06:00 and 15:00 UTC, and without lower-level clouds. Uncertainty in the contrail cirrus net RF (216–238 mW m−2) arising from meteorology in 2019 is smaller than the inter-annual variability. The contrail RF estimates are most sensitive to the humidity fields, followed by nvPM emissions and aircraft mass assumptions. This longitudinal evaluation of aviation contrail impacts contributes a quantified understanding of inter-annual variability and informs strategies for contrail mitigation.
Gryspeerdt E, McCoy DT, Crosbie E, et al., 2022, The impact of sampling strategy on the cloud droplet number concentration estimated from satellite data, Atmospheric Measurement Techniques, Vol: 15, Pages: 3875-3892, ISSN: 1867-1381
Cloud droplet number concentration (Nd) is of central importance to observation-based estimates of aerosol indirect effects, being used to quantify both the cloud sensitivity to aerosol and the base state of the cloud. However, the derivation of Nd from satellite data depends on a number of assumptions about the cloud and the accuracy of the retrievals of the cloud properties from which it is derived, making it prone to systematic biases.A number of sampling strategies have been proposed to address these biases by selecting the most accurate Nd retrievals in the satellite data. This work compares the impact of these strategies on the accuracy of the satellite retrieved Nd, using a selection of in situ measurements. In stratocumulus regions, the MODIS Nd retrieval is able to achieve a high precision (r2 of 0.5–0.8). This is lower in other cloud regimes but can be increased by appropriate sampling choices. Although the Nd sampling can have significant effects on the Nd climatology, it produces only a 20 % variation in the implied radiative forcing from aerosol–cloud interactions, with the choice of aerosol proxy driving the overall uncertainty. The results are summarised into recommendations for using MODIS Nd products and appropriate sampling.
Glassmeier F, Hoffmann F, Feingold G, et al., 2022, Gaussian-process emulation for integrating data-driven aerosol-cloud physics from simulation, satellite, and ground-based data
<jats:p>&lt;p&gt;Data-driven quantification and parameterization of cloud physics in general, and of aerosol-cloud interactions in particular, rely on input data from observations or detailed simulations. These data sources have complementary limitations in terms of their spatial and temporal coverage and resolution; simulation data has the advantage of readily providing causality but cannot represent the full process complexity. In order to base data-driven approaches on comprehensive information, we therefore need ways to integrate different data sources.&amp;#160;&lt;/p&gt;&lt;p&gt;We discuss how the classical statistical technique of Gaussian-process emulation can be combined with specifically initialized ensembles of detailed cloud simulations (large-eddy simulations, LES) to provide a framework for evaluating data-driven descriptions of cloud characteristics and processes across different data sources. We specifically illustrate this approach for integrating LES and satellite data of aerosol-cloud interactions in subtropical stratocumulus cloud decks. We furthermore explore the extension of our framework to ground-based observations of Arctic mixed-phase clouds.&lt;/p&gt;&lt;p&gt;- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;ul&gt;&lt;li&gt;Glassmeier, F., F. Hoffmann, J. S. Johnson, T. Yamaguchi, K. S. Carslaw and G. Feingold (2019): &amp;#8220;An emulator approach to stratocumulus susceptibility&amp;#8221;, Atmos. Chem. Phys., 19, 10191- 10203, doi: 10.5194/acp-19-10191-2019&lt;/li&gt;&lt;li&gt;Hoffmann, F., F. Glassmeier, T. Yamaguchi and G. Feingold (2020)
Jia H, Quaas J, Gryspeerdt E, et al., 2022, Addressing the difficulties in quantifying droplet number response to aerosol from satellite observations, Atmospheric Chemistry and Physics, Vol: 22, Pages: 7353-7372, ISSN: 1680-7316
Aerosol–cloud interaction is the most uncertain component of the overall anthropogenic forcing of the climate, in which cloud droplet number concentration (Nd) sensitivity to aerosol (S) is a key term for the overall estimation. However, satellite-based estimates of S are especially challenging, mainly due to the difficulty in disentangling aerosol effects on Nd from possible confounders. By combining multiple satellite observations and reanalysis, this study investigates the impacts of (a) updraft, (b) precipitation, (c) retrieval errors, and (d) vertical co-location between aerosol and cloud on the assessment of S in the context of marine warm (liquid) clouds. Our analysis suggests that S increases remarkably with both cloud-base height and cloud geometric thickness (proxies for vertical velocity at cloud base), consistent with stronger aerosol–cloud interactions at larger updraft velocity for midlatitude and low-latitude clouds. In turn, introducing the confounding effect of aerosol–precipitation interaction can artificially amplify S by an estimated 21 %, highlighting the necessity of removing precipitating clouds from analyses of S. It is noted that the retrieval biases in aerosol and cloud appear to underestimate S, in which cloud fraction acts as a key modulator, making it practically difficult to balance the accuracies of aerosol–cloud retrievals at aggregate scales (e.g., 1 grid). Moreover, we show that using column-integrated sulfate mass concentration (SO4C) to approximate sulfate concentration at cloud base (SO4B) can result in a degradation of correlation with Nd, along with a nearly twofold enhancement of S, mostly attributed to the inability of SO4C to capture the full spatiotemporal variability of SO4B. These findings point to several potential ways forward to practically account for the major influential factors by means of satellite observations and reanalysis, aiming at optimal observational estimates of global radia
Gryspeerdt E, Glassmeier F, Feingold G, et al., 2022, Observing short timescale cloud development to constrain aerosol-cloud interactions
<jats:p>Abstract. The aerosol impact on liquid water path (LWP) is a key uncertainty in the overall climate impact of aerosol. However, despite a significant effort in this area, the size of the effect remains poorly constrained, and even the sign is unclear. Recent studies have shown that the relationship between droplet number concentration (Nd) and LWP is an unreliable measure of the impact of Nd variations on LWP due to the difficulty in establishing causality. In this work, we use satellite observations of the short-term development of clouds to examine the role of Nd perturbations in LWP variations. Similar to previous studies, a increase followed by a general decrease in LWP with increasing Nd is observed, suggesting an overall negative LWP response to Nd and a warming LWP adjustment to aerosol. However, the Nd also responds to the local environment, with aerosol production, entrainment from the free troposphere and wet scavenging all acting to modify the Nd. Many of these effects act to further steepen the Nd-LWP relationship and obscure the causal Nd impact on LWP. Using the temporal development of clouds to account for these feedbacks in the Nd-LWP system, a weaker negative Nd-LWP relationship is observed over most of the globe. This relationship is highly sensitive to the initial cloud state, illuminating the roles of different processes in shaping the Nd-LWP relationship. The nature of the current observing system limits this work to a single timeperiod for observations, highlighting the need for more frequent observations of key cloud properties to constrain cloud behaviour at process timescales. </jats:p>
Murray-Watson RJ, Gryspeerdt E, 2022, Stability-dependent increases in liquid water with droplet number in the Arctic, Atmospheric Chemistry and Physics, Vol: 22, Pages: 5743-5756, ISSN: 1680-7316
The effects of aerosols on cloud microphysical properties are a large source of uncertainty when assessing anthropogenic climate change. The aerosol–cloud relationship is particularly unclear in high-latitude polar regions due to a limited number of observations. Cloud liquid water path (LWP) is an important control on cloud radiative properties, particularly in the Arctic, where clouds play a central role in the surface energy budget. Therefore, understanding how aerosols may alter cloud LWP is important, especially as aerosol sources such as industry and shipping move further north in a warming Arctic.Using satellite data, this work investigates the effects of aerosols on liquid Arctic clouds over open ocean by considering the relationship between cloud droplet number concentration (Nd) and LWP, an important component of the aerosol–LWP relationship. The LWP response to Nd varies significantly across the region, with increases in LWP with Nd observed at very high latitudes in multiple satellite datasets, with this positive signal observed most strongly during the summer months. This result is in contrast to the negative response typically seen in global satellite studies and previous work on Arctic clouds showing little LWP response to aerosols.The lower tropospheric stability (LTS) was found to be an important control on the spatial variations in LWP response, strongly influencing the sign and magnitude of the Nd–LWP relationship, with increases in LWP in high-stability environments. The influence of humidity varied depending on the stability, with little impact at low LTS but a strong influence at high LTS. The mean Nd state does not dominate the LWP response, despite the non-linearities in the relationship. As the Nd–LWP sensitivity changed from positive to negative when moving from high- to low-LTS environments, this work shows evidence of a temperature-dependent aerosol indirect effect. Additionally, the LWP–LTS relationship chan
Arola A, Lipponen A, Kolmonen P, et al., 2022, Aerosol effects on clouds are concealed by natural cloud heterogeneity and satellite retrieval errors
<jats:title>Abstract</jats:title> <jats:p>One major source of uncertainty in the cloud-mediated aerosol forcing arises from the magnitude of the cloud liquid water path (LWP) adjustment to aerosol–cloud interactions, which is poorly constrained by observations. Many of the recent satellite-based studies have observed a decreasing LWP as a function of cloud droplet number concentration (CDNC) as the dominating behavior. Estimating the LWP response to the CDNC changes is a complex task since various confounding factors need to be isolated. However, an important aspect has not been sufficiently considered: the propagation of natural spatial variability and errors in satellite retrievals of cloud optical depth (COD) and cloud effective radius (CER) to estimates of CDNC and LWP. Here we demonstrate using satellite and simulated measurements that, because of this propagation, even a positive LWP adjustment is likely to be misinterpreted as negative. This biasing effect therefore leads to an underestimate of the aerosol-cloud-climate cooling and must be properly considered in future studies.</jats:p>
Teoh R, Schumann U, Gryspeerdt E, et al., 2022, Aviation contrail climate effects in the North Atlantic from 2016–2021
<jats:p>Abstract. Around 5 % of anthropogenic radiative forcing (RF) is attributed to aviation CO2 and non-CO2 impacts. This paper quantifies aviation emissions and contrail climate forcing in the North Atlantic, one of the world’s busiest air traffic corridors, over 5 years. Between 2016 and 2019, growth in CO2 (+3.13 % per annum, p.a.) and nitrogen oxide emissions (+4.5 % p.a.) outpaced increases in flight distance (+3.05 % p.a.). Over the same period, the annual mean contrail cirrus net RF (204–280 mW m-2) showed significant interannual variability caused by variations in meteorology. Responses to COVID-19 caused significant reductions in flight distance travelled (-66 %), CO2 emissions (-71 %), and the contrail net RF (-66 %) compared to the prior one-year period. Around 12 % of all flights in this region cause 80 % of the annual contrail energy forcing, and the factors associated with strongly warming/cooling contrails include seasonal changes in meteorology and radiation, time of day, background cloud fields, and engine-specific non-volatile particulate matter (nvPM) emissions. Strongly warming contrails in this region are generally formed in wintertime, close to the tropopause, between 15:00 and 04:00 UTC, and above low-level clouds. The most strongly cooling contrails occur in the spring, in the upper troposphere, between 06:00 and 15:00 UTC, and without lower-level clouds. Uncertainty in the contrail cirrus net RF (216–238 mW m-2) arising from meteorology in 2019, is smaller than the interannual variability. The contrail RF estimates are most sensitive to the humidity fields, followed by nvPM emissions and aircraft mass assumptions. This longitudinal evaluation of aviation contrail impacts contributes a quantified understanding of inter-annual variability and informs strategies for contrail mitigation. </jats:p>
Teoh R, Schumann U, Gryspeerdt E, et al., 2022, Supplementary material to &quot;Aviation contrail climate effects in the North Atlantic from 2016–2021&quot;
Goren T, Feingold G, Gryspeerdt E, et al., 2022, Exploring the Effect of Aerosol on Marine Cloud Cover Using a Counterfactual Approach
<jats:p>&lt;p&gt;Aerosol&amp;#8211;cloud interactions in marine stratocumulus clouds (Sc) are among the most challenging frontiers in cloud&amp;#8211;climate research.&amp;#160;In particular, the cloud cover susceptibility to droplet concentration remained under-represented in the literature.&amp;#160;We developed methodologies to estimate what&amp;#160;would have been the cloud cover and&amp;#160;the associated&amp;#160;radiative&amp;#160;&lt;span&gt;effect&lt;/span&gt; of currently observed Sc, but in a hypothetical cleaner world. The first methodology uses a realistic Lagrangian large eddy simulation coupled with satellite observations and provides a process-oriented analysis. The other uses a &lt;span&gt;simple&lt;/span&gt; model and provides a global estimate of the radiative impact. We found that overcast Sc decks would have broken up sooner had they not been influenced by anthropogenic aerosol, thereby causing a significant effective radiative forcing.&lt;/p&gt;</jats:p>
Sourdeval O, Gryspeerdt E, Krämer M, et al., 2022, Assessment of ice clouds - aerosol interactions in global satellite observations
<jats:p>&lt;p&gt;Interactions between aerosols and clouds, as well as their radiative consequences, have been a long-standing problem to understand cloud physics as well as anthropogenic impacts on climate. Satellite-based investigations of the direct and indirect impact of aerosols on liquid clouds have led to significant progress in the understanding during the last decade. This is partly due to the emergence of adapted cloud properties provided by satellites, such as the droplet number concentration. Ice clouds have suffered from such adapted quantity for much longer, but solutions have recently been appearing.&lt;/p&gt;&lt;p&gt;This study investigates aerosol - ice clouds interactions using ice crystal number concentration (Ni) profiles from a lidar-radar dataset (DARDAR-Nice), used cojointly with with collocated aerosol information from the Copernicus Atmospheric Monitoring Service (CAMS) reanalyses. A multitude of cloud regimes, subdivided into seasonal and regional bins, are considered in order to disentangle meteorological effects from the aci signature. First results of joint-histograms between Ni and the aerosol mass show an overall positive sensitivity of Ni to the aerosols load. This response is particularly strong towards to cloud-top and flattens towards cloud-base, consistently with expectations for homogeneous nucleation processes. The response of the ice water content, in terms of adjustment to the initial aerosol perturbation as also quantified.&lt;/p&gt;</jats:p>
Gryspeerdt E, Louro Coelho M, Smith T, et al., 2022, Measuring cloud sensitivity to aerosols at a global scale using isolated aerosol sources
<jats:p>&lt;p&gt;The sensitivity of clouds to anthropogenic aerosol perturbations remains one of the largest uncertainties in the human forcing of the climate system. A key difficulty is in isolating the impact of aerosols from large-scale covariability of aerosol and cloud properties. Natural experiments, where aerosol is produced independently of the cloud and meteorological properties, provide a pathway to address this issue. These aerosol sources often modify cloud properties, leaving linear cloud features known as shiptracks (when formed by a ship) or pollution tracks (more generally).&lt;/p&gt;&lt;p&gt;In this work, we use a database of point sources of aerosol over both land and ocean to identify clouds that are sensitive to aerosol and to measure their response. Using a neural network to identify when a point source is modifying the cloud, we are able to measure the sensitivity of individual clouds to aerosol at a global scale, looking at over 400 million cases.&lt;/p&gt;&lt;p&gt;We find the probability of track formation is strongly dependent on the background cloud and meteorological state, similar to previous regional studies. With our global database, we identify regions that are strongly susceptible to aerosol perturbations, even where aerosol sources are rare. We find that there are several regions that are highly susceptible to aerosol, but that have been previously overlooked due to a low frequency of pollution tracks. &amp;#160; &amp;#160;&lt;/p&gt;</jats:p>
Dipu S, Schwarz M, Ekman AML, et al., 2022, Exploring satellite-derived relationships between cloud droplet number concentration and liquid water path using large-domain large-eddy simulation
<jats:p>&lt;p&gt;Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (N&lt;sub&gt;d&lt;/sub&gt;) and liquid water path (LWP). Specifically, this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between N&lt;sub&gt;d&lt;/sub&gt; and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the N&lt;sub&gt;d&lt;/sub&gt; (N&lt;sub&gt;Ad&lt;/sub&gt;), we have also considered N&lt;sub&gt;Ad&lt;/sub&gt; from the LES model for comparison. The joint histogram analysis shows that the N&lt;sub&gt;Ad&lt;/sub&gt;-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower N&lt;sub&gt;Ad&lt;/sub&gt; and LWP, and the corresponding mean LWP shows a weak relation with N&lt;sub&gt;Ad&lt;/sub&gt;. In contrast, at higher N&lt;sub&gt;Ad&lt;/sub&gt; (&gt; 50 cm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; ), the CP shows a larger spread; consequently, the mean LWP increases non-monotonically with increasing N&lt;sub&gt;Ad&lt;/sub&gt; in both cases. However, the N&lt;sub&gt;Ad&lt;/sub&gt;-LWP relation lacks, in particular, the negative sensitivity at higher N&lt;sub&gt;Ad&l
Horner G, Gryspeerdt E, 2022, Investigating the evolution of tropical cirrus clouds from deep convection
<jats:p>&lt;p&gt;Tropical convective clouds, particularly their large cirrus outflows, play an important role in modulating the energy balance of the Earth&amp;#8217;s atmosphere. Understanding the evolution of these clouds, and how they change in response to anthropogenic emissions is therefore important to understand past and future climate change. Previous work has focused on tracking individual convective cores and their evolution into anvil cirrus and subsequent thin cirrus clouds in satellite data.&lt;/p&gt;&lt;p&gt;In this work we have introduced a novel approach to investigating the evolution of tropical convective clouds by creating a &amp;#8216;Time Since Convection&amp;#8217; (TSC) dataset. Using reanalysis windspeeds, the time since the air at each location last experienced a convective event (as defined by the presence of a deep convective core) is calculated. Used in conjunction with data from the DARDAR and CERES products, we can build a composite picture of the radiative and microphysical properties of the clouds as a function of their time since convection.&lt;/p&gt;&lt;p&gt;As with previous studies, we find that cloud properties are a strong function of time since convection, with decreases in the optical thickness, cloud top height, and cloud fraction over time. These changes in in cloud properties also have a significant radiative impacts, with the longwave and shortwave component of the cloud radiative effect also being a strong function of time since convection. In addition, using the DARDAR product, a combination of CloudSat radar and the CALIPSO lidar measurements, we build composite cross sections of convective clouds, characterising their vertical evolution and how it is influenced by external meteorological and initial conditions flagged in the TSC dataset.&lt;/p&gt;</jats:p>
Jia H, Quaas J, Gryspeerdt E, et al., 2022, Addressing the difficulties in quantifying the Twomey effect for marine warm clouds from multi-sensor satellite observations and reanalysis
<jats:p>&lt;p&gt;Aerosol&amp;#8211;cloud interaction is the most uncertain component of the overall anthropogenic forcing of the climate, in which the Twomey effect plays a fundamental role. Satellite-based estimates of the Twomey effect are especially challenging, mainly due to the difficulty in disentangling aerosol effects on cloud droplet number concentration (&lt;em&gt;N&lt;/em&gt;&lt;sub&gt;d&lt;/sub&gt;) from possible confounders. By combining multiple satellite observations and reanalysis, this study investigates the impacts of a) updraft, b) precipitation, c) retrieval errors, as well as (d) vertical co-location between aerosol and cloud, on the assessment of&amp;#160;&lt;em&gt;N&lt;/em&gt;&lt;sub&gt;d&lt;/sub&gt;-toaerosol sensitivity (&lt;em&gt;S&lt;/em&gt;) in the context of marine warm (liquid) clouds. Our analysis suggests that&amp;#160;&lt;em&gt;S&lt;/em&gt;&amp;#160;increases remarkably with both cloud base height and cloud geometric thickness (proxies for vertical velocity at cloud base), consistent with stronger aerosol-cloud interactions at larger updraft velocity. In turn, introducing the confounding effect of aerosol&amp;#8211;precipitation interaction can artificially amplify&amp;#160;&lt;em&gt;S&lt;/em&gt;&amp;#160;by an estimated 21 %, highlighting the necessity of removing precipitating clouds from analyses on the Twomey effect. It is noted that the retrieval biases in aerosol and cloud appear to underestimate&amp;#160;&lt;em&gt;S&lt;/em&gt;, in which cloud fraction acts as a key modulator, making it practically difficult to balance the accuracies of aerosol&amp;#8211;cloud retrievals at aggregate
Trofimov H, Post P, Gryspeerdt E, et al., 2022, Meteorological conditions favorable for strong anthropogenic aerosol impacts on clouds, Journal of Geophysical Research: Atmospheres, Vol: 127, ISSN: 2169-897X
Ship-track-like polluted cloud tracks provide a direct way to study aerosol-cloud interactions. Here, we study environmental conditions favorable for pollution tracks' formation. We study polluted cloud tracks forming downwind of localized anthropogenic air pollution hot spots of Norilsk and Cherepovets in Russia and Thompson in Canada. Polluted cloud tracks form on 20%–37% of days with liquid-phase clouds. The large-scale atmospheric circulation largely determines the occurrence of track-favoring conditions. Tracks tend to form in clean and thin clouds under stable and dry conditions that are more often associated with anticyclonic large-scale flow in the studied locations.
Christensen MW, Gettelman A, Cermak J, et al., 2022, Opportunistic experiments to constrain aerosol effective radiative forcing, Atmospheric Chemistry and Physics, Vol: 22, Pages: 641-674, ISSN: 1680-7316
Aerosol–cloud interactions (ACIs) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The nonlinearity 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 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 spatiotemporal 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 datasets 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. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. 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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