| 1. |
- Bulatovic, Ines, et al.
(author)
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Aerosol Indirect Effects in Marine Stratocumulus : The Importance of Explicitly Predicting Cloud Droplet Activation
- 2019
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:6, s. 3473-3481
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Journal article (peer-reviewed)abstract
- Climate models generally simulate a unidirectional, positive liquid water path (LWP) response to increasing aerosol number concentration. However, satellite observations and large-eddy simulations show that the LWP may either increase or decrease with increasing aerosol concentration, influencing the overall magnitude of the aerosol indirect effect (AIE). We use large-eddy simulation to investigate the LWP response of a marine stratocumulus cloud and its dependence on different parameterizations for obtaining cloud droplet number concentration (CDNC). The simulations confirm that the LWP response is not always positiveregardless of CDNC treatment. However, the AIE simulated with the model version with prescribed CDNC is almost 3 times larger compared to the version with prognostic CDNC. The reason is that the CDNC in the prognostic scheme varies in time due to supersaturation fluctuations, collection, and other microphysical processes. A substantial spread in simulated AIE may thus arise simply due to the CDNC treatment. Plain Language Summary Our poor understanding of aerosol-cloud-radiation interactions (aerosol indirect effects) results in a major uncertainty in estimates of anthropogenic aerosol forcing. In climate models, the cloud water response to an increased aerosol number concentration may be especially uncertain as models simplify, or do not account for, processes that affect the cloud droplet number concentration and the total amount of cloud water. In this study, we employ large-eddy simulation to explore how different model descriptions for obtaining the number concentration of cloud droplets influences the cloud water response of a marine stratocumulus cloud and thus the simulated aerosol indirect effect. Our simulations show a qualitatively similar cloud water response regardless of model description: the total amount of cloud water increases first and then decreases with increasing aerosol concentration. However, the simulated aerosol indirect effect is almost 3 times as large when the number concentration of cloud droplets is prescribed compared to when it is dependent on the calculated supersaturation and other microphysical processes such as collisions between cloud droplets. Our findings show that a relatively simple difference in the treatment of the number concentration of cloud droplets in climate models may result in a significant spread in the simulated aerosol indirect effect.
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| 2. |
- Bulatovic, Ines
(author)
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Investigating aerosol effects on stratocumulus clouds through large-eddy simulation
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Clouds have a large impact on Earth’s radiative budget by reflecting, absorbing and re-emitting radiation. They thus play a critical role in the climate system. Nevertheless, cloud radiative effects in a changing climate are highly uncertain. Atmospheric aerosol particles are another factor affecting Earth’s climate but the magnitude of their influence is also associated with high uncertainty. Therefore, an accurate representation of aerosol-cloud interactions in models is critical for having confidence in future climate projections. This thesis investigates aerosol impacts on cloud microphysical and radiative properties through numerical modelling, more specifically large-eddy simulation (LES). Moreover, the thesis investigates how the simulated cloud response to changes in the aerosol population depends on the model description of different processes. Mixed-phase stratocumulus (MPS) clouds are especially problematic to simulate for models on all scales. These clouds consist of a mixture of supercooled water and ice in the same volume and are therefore potentially thermodynamically unstable. MPS clouds over the central (north of 80° N) Arctic Ocean are particularly sensitive to aerosol changes due to the relatively clean atmospheric conditions in this region. At the same time, the clouds also have an important impact on the Arctic surface radiative budget. Therefore, this thesis mostly focuses on Arctic MPS clouds.Simulations of a typical subtropical marine stratocumulus cloud showed that the aerosol-cloud forcing depends on the model treatment for calculating the cloud droplet number concentration (CDNC). The simulated change in the top of the atmosphere shortwave radiation due to increased aerosol number concentrations was almost three times as large when the CDNC was prescribed compared to when the CDNC was prognostic. Simulations of a central Arctic summertime low-level MPS cloud confirmed that the chemical composition and the size of aerosol particles both can play an important role in determining the efficiency of an aerosol to act as cloud condensation nuclei - and thus influence cloud properties. However, the hygroscopicity of the aerosol particle was only important if the particles were small in size (i.e., if they correspond to the Aitken mode size) or if they were close to hydrophobic. Further, it was also found that Aitken mode particles can significantly change microphysical and radiative properties of central Arctic MPS if the concentration of larger particles (i.e., corresponding to the accumulation mode) is less than approximately 10-20 cm-3. One of the most recent research expeditions in the central Arctic (in the summer of 2018) was characterized by a high occurrence of multiple cloud layers. Namely, the boundary layer structure consisted of two MPS, one located close to the surface and one at the top of the boundary layer. Large-eddy simulations of an observed case with this particular cloud structure showed that the two-layer boundary-layer clouds are persistent unless the aerosol number concentrations are low (< 5 cm-3) or the wind speed is high (≥ 8.5 m s-1). In the model, low aerosol numbers led to a dissipation of the upper cloud layer while the lower cloud layer dissipated if the wind speed was strong. Changes in the optical thickness and cloud emissivity of each individual cloud layer of the two-layer cloud structure were found to substantially impact the surface radiative fluxes.
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| 3. |
- Bulatovic, Ines, et al.
(author)
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Large-eddy simulation of a two-layer boundary-layer cloud system from the Arctic Ocean 2018 expedition
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Other publication (other academic/artistic)abstract
- Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all seasons and play a critical role in the Arctic energy budget. Previous observations in the central (north of 80° N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (altitudes ~300-400m). However, recent observations from the summer of 2018 (during The Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) Arctic Ocean 2018 (AO2018) expedition) instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed during MOCCHA AO2018 as well as the sensitivity of the cloud layers to different micro- and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the boundary layer caused by a low cloud (located within the first few hundred meters) capped by a lower temperature inversion, and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the boundary layer. The investigated cloud structure is persistent unless there are low aerosol number concentrations (< 5 cm-3), which cause the upper cloud layer to dissipate, or high large-scale wind speeds (³ 8.5 m s-1), which erode the lower inversion and the related cloud layer. These types of changes in cloud structure lead to a substantial reduction of the incoming net longwave radiation at the surface due to a lower emissivity or higher altitude of the remaining cloud layer. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic.
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| 4. |
- Bulatovic, Ines, 1991-, et al.
(author)
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Large-eddy simulation of a two-layer boundary-layer cloud system from the Arctic Ocean 2018 expedition
- 2023
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In: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 23:12, s. 7033-7055
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Journal article (peer-reviewed)abstract
- Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all seasons and play a critical role in the Arctic energy budget. Previous observations in the central (north of 80∘ N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (BL; altitudes ∼ 300 to 400 m). However, recent observations from the summer of 2018 instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed in the summer of 2018 and the sensitivity of the cloud layers to different micro- and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the BL caused by a low cloud (located within the first few hundred meters) capped by a lower-altitude temperature inversion and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the BL. The simulated cloud structure is persistent unless there are low aerosol number concentrations (≤ 5 cm−3), which cause the upper cloud layer to dissipate, or high large-scale wind speeds (≥ 8.5 m s−1), which erode the lower inversion and the related cloud layer. The changes in cloud structure alter both the short- and longwave cloud radiative effect at the surface. This results in changes in the net radiative effect of the modeled cloud system, which can impact the surface melting or freezing. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic.
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| 5. |
- Bulatovic, Ines, et al.
(author)
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The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic - a simulation study supported by observational data
- 2021
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In: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 21:5, s. 3871-3897
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Journal article (peer-reviewed)abstract
- The potential importance of Aitken mode particles (diameters similar to 25-80 nm) for stratiform mixed-phase clouds in the summertime high Arctic (> 80 degrees N) has been investigated using two large-eddy simulation models. We find that, in both models, Aitken mode particles significantly affect the simulated microphysical and radiative properties of the cloud and can help sustain the cloud when accumulation mode concentrations are low (< 10-20 cm(-3)), even when the particles have low hygroscopicity (hygroscopicity parameter - kappa = 0.1). However, the influence of the Aitken mode decreases if the overall liquid water content of the cloud is low, either due to a higher ice fraction or due to low radiative cooling rates. An analysis of the simulated supersaturation (ss) statistics shows that the ss frequently reaches 0.5 % and sometimes even exceeds 1 %, which confirms that Aitken mode particles can be activated. The modelling results are in qualitative agreement with observations of the Hoppel minimum obtained from four different expeditions in the high Arctic. Our findings highlight the importance of better understanding Aitken mode particle formation, chemical properties and emissions, particularly in clean environments such as the high Arctic.
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| 6. |
- Christiansen, Sigurd, et al.
(author)
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Influence of Arctic Microlayers and Algal Cultures on Sea Spray Hygroscopicity and the Possible Implications for Mixed-Phase Clouds
- 2020
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In: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-897X. ; 125:19
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Journal article (peer-reviewed)abstract
- As Arctic sea ice cover diminishes, sea spray aerosols (SSA) have a larger potential to be emitted into the Arctic atmosphere. Emitted SSA can contain organic material, but how it affects the ability of particles to act as cloud condensation nuclei (CCN) is still not well understood. Here we measure the CCN-derived hygroscopicity of three different types of aerosol particles: (1) Sea salt aerosols made from artificial seawater, (2) aerosol generated from artificial seawater spiked with diatom species cultured in the laboratory, and (3) aerosols made from samples of sea surface microlayer (SML) collected during field campaigns in the North Atlantic and Arctic Ocean. Samples are aerosolized using a sea spray simulation tank (plunging jet) or an atomizer. We show that SSA containing diatom and microlayer exhibit similar CCN activity to inorganic sea salt with a κ value of ∼1.0. Large-eddy simulation (LES) is then used to evaluate the general role of aerosol hygroscopicity in governing mixed-phase low-level cloud properties in the high Arctic. For accumulation mode aerosol, the simulated mixed-phase cloud properties do not depend strongly on κ, unless the values are lower than 0.4. For Aitken mode aerosol, the hygroscopicity is more important; the particles can sustain the cloud if the hygroscopicity is equal to or higher than 0.4, but not otherwise. The experimental and model results combined suggest that the internal mixing of biogenic organic components in SSA does not have a substantial impact on the cloud droplet activation process and the cloud lifetime in Arctic mixed-phase clouds.
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