There is a large range of future aerosol emissions scenarios explored in the Shared Socioeconomic Pathways (SSPs), with plausible pathways spanning a range of possibilities from large global ...reductions in emissions by 2050 to moderate global increases over the same period. Diversity in emissions across the pathways is particularly large over Asia. Rapid reductions in anthropogenic aerosol and precursor emissions between the present day and the 2050s lead to enhanced increases in global and Asian summer monsoon precipitation relative to scenarios with weak air quality policies. However, the effects of aerosol reductions do not persist to the end of the 21st century for precipitation, when instead the response to greenhouse gases dominates differences across the SSPs. The relative magnitude and spatial distribution of aerosol changes are particularly important for South Asian summer monsoon precipitation changes. Precipitation increases here are initially suppressed in SSPs 2-4.5, 3-7.0, and 5-8.5 relative to SSP1-1.9 when the impact of remote emission decreases is counteracted by continued increases in South Asian emissions.
Using an interactive aerosol‐climate model we find that absorbing anthropogenic aerosols, whether coexisting with scattering aerosols or not, can significantly affect the Indian summer monsoon ...system. We also show that the influence is reflected in a perturbation to the moist static energy in the sub‐cloud layer, initiated as a heating by absorbing aerosols to the planetary boundary layer. The perturbation appears mostly over land, extending from just north of the Arabian Sea to northern India along the southern slope of the Tibetan Plateau. As a result, during the summer monsoon season, modeled convective precipitation experiences a clear northward shift, coincidently in general agreement with observed monsoon precipitation changes in recent decades particularly during the onset season. We demonstrate that the sub‐cloud layer moist static energy is a useful quantity for determining the impact of aerosols on the northward extent and to a certain degree the strength of monsoon convection.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The global aerosol extinction from the CALIOP space lidar was used to compute
aerosol optical depth (AOD) over a 9-year period (2007–2015) and
partitioned between the boundary layer (BL) and the free ...troposphere (FT)
using BL heights obtained from the ERA-Interim archive. The results show that
the vertical distribution of AOD does not follow the diurnal cycle of the BL
but remains similar between day and night highlighting the presence of a
residual layer during night. The BL and FT contribute 69 and 31 %,
respectively, to the global tropospheric AOD during daytime in line with
observations obtained in Aire sur l'Adour (France) using the Light Optical
Aerosol Counter (LOAC) instrument. The FT AOD contribution is larger in the
tropics than at mid-latitudes which indicates that convective transport
largely controls the vertical profile of aerosols. Over oceans, the FT AOD
contribution is mainly governed by long-range transport of aerosols from
emission sources located within neighboring continents. According to the
CALIOP aerosol classification, dust and smoke particles are the main aerosol
types transported into the FT. Overall, the study shows that the fraction of
AOD in the FT – and thus potentially located above low-level clouds – is
substantial and deserves more attention when evaluating the radiative effect
of aerosols in climate models. More generally, the results have implications
for processes determining the overall budgets, sources, sinks and transport
of aerosol particles and their description in atmospheric models.
A modal aerosol module (MAM) has been developed for the Community Atmosphere Model version 5 (CAM5), the atmospheric component of the Community Earth System Model version 1 (CESM1). MAM is capable of ...simulating the aerosol size distribution and both internal and external mixing between aerosol components, treating numerous complicated aerosol processes and aerosol physical, chemical and optical properties in a physically-based manner. Two MAM versions were developed: a more complete version with seven lognormal modes (MAM7), and a version with three lognormal modes (MAM3) for the purpose of long-term (decades to centuries) simulations. In this paper a description and evaluation of the aerosol module and its two representations are provided. Sensitivity of the aerosol lifecycle to simplifications in the representation of aerosol is discussed. Simulated sulfate and secondary organic aerosol (SOA) mass concentrations are remarkably similar between MAM3 and MAM7. Differences in primary organic matter (POM) and black carbon (BC) concentrations between MAM3 and MAM7 are also small (mostly within 10%). The mineral dust global burden differs by 10% and sea salt burden by 30-40% between MAM3 and MAM7, mainly due to the different size ranges for dust and sea salt modes and different standard deviations of the log-normal size distribution for sea salt modes between MAM3 and MAM7. The model is able to qualitatively capture the observed geographical and temporal variations of aerosol mass and number concentrations, size distributions, and aerosol optical properties. However, there are noticeable biases; e.g., simulated BC concentrations are significantly lower than measurements in the Arctic. There is a low bias in modeled aerosol optical depth on the global scale, especially in the developing countries. These biases in aerosol simulations clearly indicate the need for improvements of aerosol processes (e.g., emission fluxes of anthropogenic aerosols and precursor gases in developing countries, boundary layer nucleation) and properties (e.g., primary aerosol emission size, POM hygroscopicity). In addition, the critical role of cloud properties (e.g., liquid water content, cloud fraction) responsible for the wet scavenging of aerosol is highlighted.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The potential importance of Aitken mode particles (diameters
∼ 25–80 nm) for stratiform mixed-phase clouds in the
summertime high Arctic (>80∘ 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 – κ=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.
Stratiform mixed‐phase clouds (MPCs), which contain both supercooled liquid and ice, play a key role in the energy balance of the Arctic and are a major contributor to surface precipitation. As ...Arctic shipping is projected to increase with climate change, these clouds may frequently be exposed to local aerosol perturbations of up to 15,000 cm−3. Yet little consensus exists within the community regarding the key feedback mechanisms induced in MPCs perturbed by ship exhaust, or aerosol in general. Here we show that many known processes identified in the warm‐phase stratocumulus regime can be extrapolated to the MPC regime. However, their effect may be compensated, or even undermined, by the following two most relevant processes unique to the MPC regime: (i) increased cloud glaciation via immersion freezing due to cloud condensation nuclei (CCN) induced cloud top radiative cooling and (ii) the continued cycling of ice nucleating particles (INPs) through the cloud and subcloud layer.
Key Points
Increases in cloud top radiative cooling lead to increased immersion freezing rates near cloud top
Feedback mechanisms involving the ice phase reduce, if not suppress, changes to the cloud liquid water path triggered by ship exhaust
Changes in cloud condensation nuclei concentrations of 100 cm−3 were sufficient to shift the cloud state beyond its background variability
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Twomey effect describes the radiative forcing associated with a change in cloud albedo due to an increase in anthropogenic aerosol emissions. It is driven by the perturbation in cloud droplet ...number concentration (ΔNd,ant) in liquid-water clouds and is currently understood to exert a cooling effect on climate. The Twomey effect is the key driver in the effective radiative forcing due to aerosol–cloud interactions which also comprises rapid adjustments. These adjustments are essentially the responses of cloud fraction and liquid water path to ΔNd,ant and thus scale approximately with it. While the fundamental physics of the influence of added aerosol particles on the droplet concentration (Nd) is well described by established theory at the particle scale (micrometres), how this relationship is expressed at the large scale (hundreds of kilometres) ΔNd,ant remains uncertain. The discrepancy between process understanding at particle scale and insufficient quantification at the climate-relevant large scale is caused by co-variability of aerosol particles and vertical wind and by droplet sink processes. These operate at scales on the order of 10s of metres at which only localized observations are available and at which no approach exists yet to quantify the anthropogenic perturbation. Different atmospheric models suggest diverse magnitudes of the Twomey effect even when applying the same anthropogenic aerosol emission perturbation. Thus, observational data are needed to quantify and constrain the Twomey effect. At the global scale, this means satellite data. There are three key uncertainties in determining ΔNd,ant, namely the quantification (i) of the cloud-active aerosol – the cloud condensation nuclei concentrations (CCN) at or above cloud base –, (ii) of Nd, as well as (iii) the statistical approach for inferring the sensitivity of Nd to aerosol particles from the satellite data. A fourth uncertainty, the anthropogenic perturbation to CCN concentrations, is also not easily accessible from observational data. This review discusses deficiencies of current approaches for the different aspects of the problem and proposes several ways forward: In terms of CCN, retrievals of optical quantities such as aerosol optical depth suffer from a lack of vertical resolution, size and hygroscopicity information, the non-direct relation to the concentration of aerosols, the impossibility to quantify it within or below clouds, and the problem of insufficient sensitivity at low concentrations, in addition to retrieval errors. A future path forward can include utilizing colocated polarimeter and lidar instruments, ideally including high spectral resolution lidar capability at two wavelengths to maximize vertically resolved size distribution information content. In terms of Nd, a key problem is the lack of operational retrievals of this quantity, and the inaccuracy of the retrieval especially in broken-cloud regimes. As for the Nd – to – CCN sensitivity, key issues are the updraught distributions and the role of Nd sink processes, for which empirical assessments for specific cloud regimes are currently the best solutions. These considerations point to the conclusion that past studies using existing approaches have likely underestimated the true sensitivity and, thus, the radiative forcing due to the Twomey effect.
In situ measurements of Arctic clouds frequently show that ice
crystal number concentrations (ICNCs) are much higher than the number of
available ice-nucleating particles (INPs), suggesting that ...secondary ice production (SIP) may be active. Here we use a Lagrangian parcel model (LPM) and a
large-eddy simulation (LES) to investigate the impact of three SIP mechanisms
(rime splintering, break-up from ice–ice collisions and drop shattering) on
a summer Arctic stratocumulus case observed during the Aerosol-Cloud Coupling And
Climate Interactions in the Arctic (ACCACIA) campaign. Primary ice alone
cannot explain the observed ICNCs, and drop shattering is ineffective in the
examined conditions. Only the combination of both rime splintering (RS) and
collisional break-up (BR) can explain the observed ICNCs, since both of these
mechanisms are weak when activated alone. In contrast to RS, BR is currently
not represented in large-scale models; however our results indicate that
this may also be a critical ice-multiplication mechanism. In general, low
sensitivity of the ICNCs to the assumed INP, to the cloud condensation nuclei
(CCN) conditions and also to the choice of BR parameterization is found.
Finally, we show that a simplified treatment of SIP, using a LPM constrained
by a LES and/or observations, provides a realistic yet computationally
efficient way to study SIP effects on clouds. This method can eventually
serve as a way to parameterize SIP processes in large-scale models.
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.
Atmospheric models often fail to correctly reproduce the
microphysical structure of Arctic mixed-phase clouds and underpredict ice
water content even when the simulations are constrained by observed ...levels
of ice nucleating particles. In this study we investigate whether ice
multiplication from breakup upon ice–ice collisions, a process missing in
most models, can account for the observed cloud ice in a stratocumulus cloud
observed during the Arctic Summer Cloud Ocean Study (ASCOS) campaign. Our results indicate
that the efficiency of this process in these conditions is weak; increases
in fragment generation are compensated for by subsequent enhancement of
precipitation and subcloud sublimation. Activation of collisional breakup
improves the representation of cloud ice content, but cloud liquid remains
overestimated. In most sensitivity simulations, variations in ice habit and
prescribed rimed fraction have little effect on the results. A few
simulations result in explosive multiplication and cloud dissipation;
however, in most setups, the overall multiplication effects become
substantially weaker if the precipitation sink is enhanced through cloud-ice-to-snow autoconversion. The largest uncertainty stems from the
correction factor for ice enhancement due to sublimation included in the
breakup parameterization; excluding this correction results in rapid
glaciation, especially in simulations with plates. Our results indicate that
the lack of a detailed treatment of ice habit and rimed fraction in most
bulk microphysics schemes is not detrimental for the description of the
collisional breakup process in the examined conditions as long as cloud-ice-to-snow autoconversion is considered.
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