Condensation Nuclei Concentrations Research Articles

Isotopic Equilibration in Convective Downdrafts

AbstractWhile it is becoming increasingly clear that stable water isotopologues are important tools in the study of atmospheric convection, many questions remain on how different processes affect them. This work is focused on water vapor isotopes in precipitation‐driven downdrafts. Using idealized simulations, the contributions of rain evaporation and liquid‐vapor equilibration processes are analyzed. It is shown that rain evaporation tends to deplete water vapor isotopes throughout the downdraft's descent, whereas equilibration has a neutral or depleting effect in the first half of the downdraft's life cycle and an enriching one in the second half. The total contribution of the two processes is then discussed, and it is argued that equilibration has an overall small effect compared to rain evaporation. Finally, using two sensitivity experiments, it is shown that the role played by equilibration varies significantly in regimes with different concentrations of cloud condensation nuclei.

Simulating the radiative forcing of oceanic dimethylsulfide (DMS) in Asia based on machine learning estimates

Abstract. Dimethylsulfide (DMS) emitted from seawater is a key precursor to new particle formation and acts as a regulator in Earth's warming climate system. However, DMS's effects are not well understood in various ocean regions. In this study, we estimated DMS emissions based on a machine learning method and used the GEOS-Chem global 3D chemical transport model coupled with the TwO Moment Aerosol Sectional (TOMAS) microphysics scheme to simulate the atmospheric chemistry and radiative effects of DMS. The contributions of DMS to atmospheric SO42- aerosol and cloud condensation nuclei (CCN) concentrations along with the radiative effects over the Asian region were evaluated for the first time. First, we constructed novel monthly resolved DMS emissions (0.5∘×0.5∘) for the year 2017 using a machine learning model; 4351 seawater DMS measurements (including the recent measurements made over the Chinese seas) and 12 relevant environment parameters were selected for model training. We found that the model could predict the observed DMS concentrations with a correlation coefficient of 0.75 and fill the values in regions lacking observations. Across the Asian seas, the highest seasonal mean DMS concentration occurred in March–April–May (MAM), and we estimate the annual DMS emission flux of 1.25 Tg (S), which is equivalent to 15.4 % of anthropogenic sulfur emissions over the entire simulation domain (which covered most of Asia) in 2017. The model estimates of DMS and methane sulfonic acid (MSA), using updated DMS emissions, were evaluated by comparing them with cruise survey experiments and long-term online measurement site data. The improvement in model performance can be observed compared with simulation results derived from the global-database DMS emissions. The relative contributions of DMS to SO42- and CCN were higher in remote oceanic areas, contributing 88 % and 42 % of all sources, respectively. Correspondingly, the sulfate direct radiative forcing (DRF) and indirect radiative forcing (IRF) contributed by DMS ranged from −200 to −20 mW m−2 and from −900 to −100 mW m−2, respectively, with levels varying by season. The strong negative IRF is mainly over remote ocean regions (−900 to −600 mW m−2). Generally, the magnitude of IRF derived by DMS was twice as large as its DRF. This work provides insights into the source strength of DMS and the impact of DMS on climate and addresses knowledge gaps related to factors controlling aerosols in the marine boundary layer and their climate impacts.

Characterizing the hygroscopicity of growing particles in the Canadian Arctic summer

Abstract. The impact of aerosols on clouds is a well-studied, although still poorly constrained, part of the atmospheric system. New particle formation (NPF) is thought to contribute 40 %–80 % of the global cloud droplet number concentration, although it is extremely difficult to observe an air mass from NPF to cloud formation. NPF and growth occurs frequently in the Canadian Arctic summer atmosphere, although only a few studies have characterized the source and properties of these aerosols. This study presents cloud condensation nuclei (CCN) concentrations measured on board the CCGS Amundsen in the eastern Canadian Arctic Archipelago from 23 July to 23 August 2016 as part of the Network on Climate and Aerosols: Addressing Uncertainties in Remote Canadian Environments (NETCARE). The study was dominated by frequent ultrafine particle and/or growth events, and particles smaller than 100 nm dominated the size distribution for 92 % of the study period. Using κ-Köhler theory and aerosol size distributions, the mean hygroscopicity parameter (κ) calculated for the entire study was 0.12 (0.06–0.12, 25th–75th percentile), suggesting that the condensable vapours that led to particle growth were primarily slightly hygroscopic, which we infer to be organic. Based on past measurement and modelling studies from NETCARE and the Canadian Arctic, it seems likely that the source of these slightly hygroscopic, organic, vapours is the ocean. Examining specific growth events suggests that the mode diameter (Dmax) had to exceed 40 nm before CCN concentrations at 0.99 % supersaturation (SS) started to increase, although a statistical analysis shows that CCN concentrations increased 13–274 cm−3 during all ultrafine particle and/or growth times (total particle concentrations >500 cm−3, Dmax<100 nm) compared with background times (total concentrations <500 cm−3) at SS of 0.26 %–0.99 %. This value increased to 25–425 cm−3 if the growth times were limited to times when Dmax was also larger than 40 nm. These results support past results from NETCARE by showing that the frequently observed ultrafine particle and growth events are dominated by a slightly hygroscopic fraction, which we interpret to be organic vapours originating from the ocean, and that these growing particles can increase the background CCN concentrations at SS as low as 0.26 %, thus pointing to their potential contribution to cloud properties and thus climate through the radiation balance.

Dilution of Boundary Layer Cloud Condensation Nucleus Concentrations by Free Tropospheric Entrainment During Marine Cold Air Outbreaks

AbstractRecent aircraft measurements over the northwest Atlantic enable an investigation of how entrainment from the free troposphere (FT) impacts cloud condensation nucleus (CCN) concentrations in the marine boundary layer (MBL) during cold‐air outbreaks (CAOs), motivated by the role of CCN in mediating transitions from closed to open‐cell regimes. Observations compiled over eight flights indicate predominantly far lesser CCN concentrations in the FT than in the MBL. For one flight, a fetch‐dependent MBL‐mean CCN budget is compiled from estimates of sea‐surface fluxes, entrainment of FT air, and hydrometeor collision‐coalescence, based on in‐situ and remote‐sensing measurements. Results indicate a dominant role of FT entrainment in reducing MBL CCN concentrations, consistent with satellite‐observed trends in droplet number concentration upwind of CAO cloud‐regime transitions over the northwest Atlantic. Relatively scant CCN may widely be associated with FT dry intrusions, and should accelerate cloud‐regime transitions where underlying MBL air is CCN‐rich, thereby reducing regional albedo.

Investigation of aerosol effects on a deep convection case in Southeast China with budget analysis of hydrometeor mass and latent heat

It remains an open question which microphysical processes determine the sign of aerosol effects on deep convective rain. This lack of knowledge prevents us from understanding the uncertainties of the aerosol effects on deep convection. We try to answer this question through a cloud-resolving model (CRM) simulation on a summer storm that occurred in Southeast China, by analyzing both column hydrometeor mass and latent heat budget and quantifying the contribution of each microphysical process to the rain variations with aerosol. Our results show that warm-phase microphysical processes have stronger contributions (52% −99%) to the rain variations than ice-phase microphysical processes (1%−48%) when gradually increasing cloud condensation nuclei (CCN) concentration from 125 cm−3 (clean environment) to 16,000 cm−3 (highly polluted environment). Condensation and evaporation of cloud droplets are identified as two crucial microphysical processes, both of which are enhanced with increased aerosol loading. The competition between them (i.e., net condensation) essentially determines the sign of aerosol effects on surface rain. We also explore aerosol effects on updraft velocity from the perspective of buoyancy and latent heat budget, it reveals that the largely enhanced convective updrafts below 7 km are attributed to the increase of condensation heating. While the competition between enhanced condensation heating and suppressed deposition heating restricts the aerosol-induced convective invigoration above 7 km. This study highlights the dominant role of net condensation in the aerosol-imposed invigoration of deep convection under highly polluted environments, suggesting that an explicit treatment of vapor diffusion processes is necessary to simulate aerosol effects on deep convection more accurately.

Earth System Model Aerosol–Cloud Diagnostics (ESMAC Diags) package, version 1: assessing E3SM aerosol predictions using aircraft, ship, and surface measurements

Abstract. An Earth system model (ESM) aerosol–cloud diagnostics package is developed to facilitate the routine evaluation of aerosols, clouds, and aerosol–cloud interactions simulated by the Energy Exascale Earth System Model (E3SM) from the US Department of Energy (DOE). The first version focuses on comparing simulated aerosol properties with aircraft, ship, and surface measurements, which are mostly measured in situ. The diagnostics currently cover six field campaigns in four geographical regions: eastern North Atlantic (ENA), central US (CUS), northeastern Pacific (NEP), and Southern Ocean (SO). These regions produce frequent liquid- or mixed-phase clouds, with extensive measurements available from the Atmospheric Radiation Measurement (ARM) program and other agencies. Various types of diagnostics and metrics are performed for aerosol number, size distribution, chemical composition, cloud condensation nuclei (CCN) concentration, and various meteorological quantities to assess how well E3SM represents observed aerosol properties across spatial scales. Overall, E3SM qualitatively reproduces the observed aerosol number concentration, size distribution, and chemical composition reasonably well, but it overestimates Aitken-mode aerosols and underestimates accumulation-mode aerosols over the CUS and ENA regions, suggesting that processes related to particle growth or coagulation might be too weak in the model. The current version of E3SM struggles to reproduce the new particle formation events frequently observed over both the CUS and ENA regions, indicating missing processes in current parameterizations. The diagnostics package is coded and organized in a way that can be extended to other field campaign datasets and adapted to higher-resolution model simulations.

Effects of cloud condensation nuclei concentration on the evolution of severe convective storms

Idealized three-dimensional storm simulations were created to investigate the effects of varying initial cloud condensation nuclei (CCN) concentrations (CCNCs) ahead of the severe convective storm using the Weather Research and Forecasting (WRF) model with the National Severe Storms Laboratory (NSSL) double-moment microphysics scheme. The simulations showed that increased initial CCNC ahead of the storm (CCN mainly entrains from the boundary layer) increased the rain- and hailfall intensities. Multiple strong centers of rain- and hailfall appeared at the surface along the moving direction of the convective storm. The increased initial CCNC ahead of the storm played an important role in the warm cloud processes. As the CCNC increased, a large amount of CCN enter the cloud and was activated, which decreased the equivalent sphere radius of the cloud droplets and increased the content of cloud droplets. Thus, the equivalent sphere radius of the raindrops increased due to the enhanced collection process of cloud droplets by raindrops. Furthermore, the collection processes of supercooled cloud droplets and raindrops by hail were also enhanced as the CCNC increased, resulting in an increase in the content of hail. These processes further influenced the cold cloud processes. The increase in CCNC ahead of the storm accelerated the downdraft by the cloud microphysical processes and formed a stronger cold pool and front outflow. These thermodynamic and dynamic processes were favorable for increasing the updraft and thus enhancing the cloud microphysical processes. Therefore, the increase in the CCNC ahead of the storm created a positive feedback mechanism to the thermodynamic and dynamic processes by the cloud microphysical processes, which promoted the development and maintenance of the storm and increased rain- and hailfall intensities at the surface.

Oceanic emissions of dimethyl sulfide and methanethiol and their contribution to sulfur dioxide production in the marine atmosphere

Abstract. Oceanic emissions of dimethyl sulfide (CH3SCH3, DMS) have long been recognized to impact aerosol particle composition and size, the concentration of cloud condensation nuclei (CCN), and Earth's radiation balance. The impact of oceanic emissions of methanethiol (CH3SH, MeSH), which is produced by the same oceanic precursor as DMS, on the volatile sulfur budget of the marine atmosphere is largely unconstrained. Here we present direct flux measurements of MeSH oceanic emissions using the eddy covariance (EC) method with a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToFMS) detector and compare them to simultaneous flux measurements of DMS emissions from a coastal ocean site. Campaign mean mixing ratios of DMS and MeSH were 72 ppt (28–90 ppt interquartile range) and 19.1 ppt (7.6–24.5 ppt interquartile range), respectively. Campaign mean emission fluxes of DMS (FDMS) and MeSH (FMeSH) were 1.13 ppt m s−1 (0.53–1.61 ppt m s−1 interquartile range) and 0.21 ppt m s−1 (0.10–0.31 ppt m s−1 interquartile range), respectively. Linear least squares regression of observed MeSH and DMS flux indicates the emissions are highly correlated with each other (R2=0.65) over the course of the campaign, consistent with a shared oceanic source. The campaign mean DMS to MeSH flux ratio (FDMS:FMeSH) was 5.5 ± 3.0, calculated from the ratio of 304 individual coincident measurements of FDMS and FMeSH. Measured FDMS:FMeSH was weakly correlated (R2=0.15) with ocean chlorophyll concentrations, with FDMS:FMeSH reaching a maximum of 10.8 ± 4.4 during a phytoplankton bloom period. No other volatile sulfur compounds were observed by PTR-ToFMS to have a resolvable emission flux above their flux limit of detection or to have a gas-phase mixing ratio consistently above their limit of detection during the study period, suggesting DMS and MeSH are the dominant volatile organic sulfur compounds emitted from the ocean at this site. The impact of this MeSH emission source on atmospheric budgets of sulfur dioxide (SO2) was evaluated by implementing observed emissions in a coupled ocean–atmosphere chemical box model using a newly compiled MeSH oxidation mechanism. Model results suggest that MeSH emissions lead to afternoon instantaneous SO2 production of 2.5 ppt h−1, which results in a 43 % increase in total SO2 production compared to a case where only DMS emissions are considered and accounts for 30% of the instantaneous SO2 production in the marine boundary layer at the mean measured FDMS and FMeSH. This contribution of MeSH to SO2 production is driven by a higher effective yield of SO2 from MeSH oxidation and the shorter oxidation lifetime of MeSH compared to DMS. This large additional source of marine SO2 has not been previously considered in global models of marine sulfur cycling. The field measurements and modeling results presented here demonstrate that MeSH is an important contributor to volatile sulfur budgets in the marine atmosphere and must be measured along with DMS in order to constrain marine sulfur budgets. This large additional source of marine–reduced sulfur from MeSH will contribute to particle formation and growth and CCN abundance in the marine atmosphere, with subsequent impacts on climate.

Understanding aerosol microphysical properties from 10 years of data collected at Cabo Verde based on an unsupervised machine learning classification

Abstract. The Cape Verde Atmospheric Observatory (CVAO), which is influenced by both marine and desert dust air masses, has been used for long-term measurements of different properties of the atmospheric aerosol from 2008 to 2017. These properties include particle number size distributions (PNSD), light-absorbing carbon (LAC) and concentrations of cloud condensation nuclei (CCN) together with their hygroscopicity. Here we summarize the results obtained for these properties and use an unsupervised machine learning algorithm for the classification of aerosol types. Five types of aerosols, i.e., marine, freshly formed, mixture, moderate dust and heavy dust, were classified. Air masses during marine periods are from the Atlantic Ocean and during dust periods are from the Sahara Desert. Heavy dust was more frequently present during wintertime, whereas the clean marine periods were more frequently present during springtime. It was observed that during the dust periods CCN number concentrations at a supersaturation of 0.30 % were roughly 2.5 times higher than during marine periods, but the hygroscopicity (κ) of particles in the size range from ∼ 30 to ∼ 175 nm during marine and dust periods were comparable. The long-term data presented here, together with the aerosol classification, can be used as a basis to improve our understanding of annual cycles of the atmospheric aerosol in the eastern tropical Atlantic Ocean and on aerosol-cloud interactions and it can be used as a basis for driving, evaluating and constraining atmospheric model simulations.

Maritime Aerosol and CCN Profiles Derived From Ship‐Based Measurements Over Eastern North Pacific During MAGIC

AbstractAtmospheric aerosols are widely recognized to give rise to a substantial radiative forcing to the climate by scattering and absorbing radiation and by modifying the microphysical, lifetime, and radiative properties of clouds. During the Marine ARM GPCI Investigation of Clouds (MAGIC) over the Eastern North Pacific (ENP), the ship‐based measured cloud condensation nuclei (CCN) concentration at 0.2% supersaturation (NCCN,0.2) and condensation nuclei concentration (NCN) had mean values of 116.7 and 219.4 cm−3, with the highest concentrations found closest to LA due to an increase in aerosol sources. Moving westward, both NCCN,0.2 and NCN gradually decreased until stabilizing near 100 cm−3 and 200 cm−3, respectively. Using the methods proposed by Ghan and Collins (2004), https://doi.org/10.1175/1520-0426(2004)021<0387:uoisdt>2.0.co;2 and Ghan et al. (2006), https://doi.org/10.1029/2004jd005752, NCCN,0.2 profiles are retrieved using the surface measured NCCN,0.2 as a constraint. For coupled conditions, correlations between the retrieved NCCN,0.2 profiles and cloud‐droplet number concentration (NC) increase from 0.26 at the surface to 0.38 near cloud base, particularly true for non‐drizzling clouds. Although the correlations are lower than expected, the percentage increase (46.2%) is encouraging. Finally, the relationships between cloud breakup (CB) and the stratocumulus to cumulus transition (SCT) with environmental conditions and associated aerosols are also studied. The decreased NCN trend east of CB is mainly caused by precipitation scavenging, while the increased NCN trend west of CB is strongly associated with the increased surface wind speed and fewer drizzle events. A further study is needed using high‐resolution models to simulate these events.

Classification of aerosol-cloud interaction regimes over Tokyo

Previous studies on the aerosol indirect effect have demonstrated that there are three regimes (aerosol-limited, updraft-limited, and transitional regimes). However, this classification of regimes has not been widely used in field observations as it requires measurements of updraft or supersaturation near the cloud base (Smax), which are not easy to perform. In this study, we attempted to classify the regimes using the effective supersaturation (Seff) which is relatively easy to estimate by matching cloud droplet number concentration (Nc) and supersaturation spectra of cloud condensation nuclei. Parcel model simulations were performed to examine the suitability of Seff as a proxy for Smax. Further, 35 low-level cloud cases observed around 458 m over Tokyo were classified into three regimes. In the aerosol-limited regime, Nc increased more rapidly with an increment of aerosols than in the other two regimes, indicating that the regime classification using Seff was sufficiently robust. The aerosol-limited regime in Tokyo appeared frequently when northerly winds prevailed, indicating the impact of local pollution sources around Tokyo on the aerosol cloud interaction. The results imply that anthropogenic aerosol emissions affect the climate of Tokyo.

Indications of a Decrease in the Depth of Deep Convective Cores with Increasing Aerosol Concentration during the CACTI Campaign

AbstractAn aerosol indirect effect on deep convective cores (DCCs), by which increasing aerosol concentration increases cloud-top height via enhanced latent heating and updraft velocity, has been proposed in many studies. However, the magnitude of this effect remains uncertain due to aerosol measurement limitations, modulation of the effect by meteorological conditions, and difficulties untangling meteorological and aerosol effects on DCCs. The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) campaign in 2018–19 produced concentrated aerosol and cloud observations in a location with frequent DCCs, providing an opportunity to examine the proposed aerosol indirect effect on DCC depth in a rigorous and robust manner. For periods throughout the campaign with well-mixed boundary layers, we analyze relationships that exist between aerosol variables (condensation nuclei concentration > 10 nm, 0.4% cloud condensation nuclei concentration, 55–1000-nm aerosol concentration, and aerosol optical depth) and meteorological variables [level of neutral buoyancy (LNB), convective available potential energy, midlevel relative humidity, and deep-layer vertical wind shear] with the maximum radar-echo-top height and cloud-top temperature (CTT) of DCCs. Meteorological variables such as LNB and deep-layer shear are strongly correlated with DCC depth. LNB is also highly correlated with three of the aerosol variables. After accounting for meteorological correlations, increasing values of the aerosol variables [with the exception of one formulation of aerosol optical depth (AOD)] are generally correlated at a statistically significant level with a warmer CTT of DCCs. Therefore, for the study region and period considered, increasing aerosol concentration is mostly associated with a decrease in DCC depth.

Chemical composition of different size ultrafine particulate matter measured by nanoparticle chemical ionization mass spectrometer

New particle formation (NPF) is the primary source of nanoparticles and contributes a large number of concentrations of cloud condensation nuclei. In recent years, field campaigns and laboratory experiments have been conducted to promote cognition of the mechanism for NPF and its following growth processes. The chemical composition measurement of nanoparticles could help deepen understanding of the initial step of particulate matter formation. In this work, we developed a nanoparticle chemical ionization mass spectrometer to measure nanoparticles' chemical compositions during their initial growth stage. Meanwhile, a non-radioactive ion source was designed for aerosol charging and chemical ionization. Time of flight mass spectrometer coupled with integrated aerosol size selection and collection module would guarantee the picogram level detection limit and high-resolution ability to measure the matrix of ambient samples. The performance of this equipment was overall evaluated, including the transmission efficiency and collection efficiency of custom-built nano differential mobility analyzer, chemical ionization efficiency, and mass resolution of the mass spectrometer. The high sensitivity measurement of ammonium sulfate and methylammonium sulfate aerosols with diameters ranging from 10 to 25 nm could guarantee the application of this instrument in the ambient measurement.

Importance of aerosols and shape of the cloud droplet size distribution for convective clouds and precipitation

Abstract. The predictability of deep moist convection is subject to large uncertainties resulting from inaccurate initial and boundary data, the incomplete description of physical processes, or microphysical uncertainties. In this study, we investigate the response of convective clouds and precipitation over central Europe to varying cloud condensation nuclei (CCN) concentrations and different shape parameters of the cloud droplet size distribution (CDSD), both of which are not well constrained by observations. We systematically evaluate the relative impact of these uncertainties in realistic convection-resolving simulations for multiple cases with different synoptic controls using the new icosahedral non-hydrostatic ICON model. The results show a large systematic increase in total cloud water content with increasing CCN concentrations and narrower CDSDs, together with a reduction in the total rain water content. This is related to a suppressed warm-rain formation due to a less efficient collision–coalescence process. It is shown that the evaporation at lower levels is responsible for diminishing these impacts on surface precipitation, which lies between +13 % and −16 % compared to a reference run with continental aerosol assumption. In general, the precipitation response was larger for weakly forced cases. We also find that the overall timing of convection is not sensitive to the microphysical uncertainties applied, indicating that different rain intensities are responsible for changing precipitation totals at the ground. Furthermore, weaker rain intensities in the developing phase of convective clouds can allow for a higher convective instability at later times, which can lead to a turning point with larger rain intensities later on. The existence of such a turning point and its location in time can have a major impact on precipitation totals. In general, we find that an increase in the shape parameter can produce almost as large a variation in precipitation as a CCN increase from maritime to polluted conditions. The narrowing of the CDSD not only decreases the absolute values of autoconversion and accretion but also decreases the relative role of the warm-rain formation in general, independent of the prevailing weather regime. We further find that increasing CCN concentrations reduce the effective radius of cloud droplets in a stronger manner than larger shape parameters. The cloud optical depth, however, reveals a similarly large increase with larger shape parameters when changing the aerosol load from maritime to polluted. By the frequency of updrafts as a function of height, we show a negative aerosol effect on updraft strength, leading to an enervation of deep convection. These findings demonstrate that both the CCN assumptions and the CDSD shape parameter are important for quantitative precipitation forecasting and should be carefully chosen if double-moment schemes are used for modeling aerosol–cloud interactions.

Estimating cloud condensation nuclei concentrations from CALIPSO lidar measurements

Abstract. We present a novel methodology to estimate cloud condensation nuclei (CCN) concentrations from spaceborne CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations) lidar measurements. The algorithm utilizes (i) the CALIPSO-derived backscatter and extinction coefficient, depolarization ratio, and aerosol subtype information; (ii) the normalized volume size distributions and refractive indices from the CALIPSO aerosol model; and (iii) the MOPSMAP (modelled optical properties of ensembles of aerosol particles) optical modelling package. For each CALIPSO height bin, we first select the aerosol-type specific size distribution and then adjust it to reproduce the extinction coefficient derived from the CALIPSO retrieval. The scaled size distribution is integrated to estimate the aerosol number concentration, which is then used in the CCN parameterizations to calculate CCN concentrations at different supersaturations. To account for the hygroscopicity of continental and marine aerosols, we use the kappa parameterization and correct the size distributions before the scaling step. The sensitivity of the derived CCN concentrations to variations in the initial size distributions is also examined. It is found that the uncertainty associated with the algorithm can range between a factor of 2 and 3. Our results are comparable to results obtained using the POLIPHON (Polarization Lidar Photometer Networking) method for extinction coefficients larger than 0.05 km−1. An initial application to a case with coincident airborne in situ measurements for independent validation shows promising results and illustrates the potential of CALIPSO for constructing a global height-resolved CCN climatology.

On the role of aerosols in the production of orographically-induced extreme rainfall in near-maritime environments

The present study aims at disentangling the role of marine aerosols in extreme precipitation events of orographic nature over near-maritime locations. Being the hazardous character of these events mainly associated to runoffs and river floodings, their destructive potential makes them subjects of special interest. In pursuit of a deep understanding on the development mechanisms of such events, this study reproduces the conditions and development of an event ranging from 20 to 27 of December 2009 that took place in Southern Spain. During this event, a series of Atlantic fronts swept the southernmost part of the Iberian Peninsula which, along with water vapor entrainment in the form of an atmospheric river, produced rainfall accumulations up to 900 mm in some specific mountainous locations. With this purpose, a number of simulations are conducted to study the microphysical processes and aerosol-cloud interactions governing the rain production. The WRF model is utilized with two different configurations of the Morrison double-moment microphysics scheme: (1) with prescribed type and concentrations of aerosols (PA); and (2) a setup in which a chemistry module is coupled to the meteorological core (Interactive Aerosols, IA), allowing for the consideration of prognostic aerosol concentrations and other relevant aspects such as the hygroscopicity and dry size of the different chemical species. According to the validation performed with a database constructed from measurements of rain gauges, results show that using prognostic aerosols concentrations for natural aerosols, namely sea salt and dust, improves the model's skill for reproducing extreme rainfall with respect to the setup using the default aerosols distribution prescribed in the WRF-alone simulations. Additionally, IA simulations increase windward precipitation and reduce leeward rainfall, thus largely amplifying the spatial variability of precipitation along the direction of the moisture flux. Besides, it is observed that this effect is independent of the model resolution. Consequently, it is concluded that a realistic aerosols concentration, along with a detailed treatment of both the size and hygroscopic properties of aerosols, have a crucial impact on the microphysical processes involved in this type of orographically-induced extreme precipitation events. Specifically, under a regime of low concentration of cloud condensation nuclei originating from natural aerosols, the formed cloud droplets are more prone to grow to precipitating size, thus enhancing the auto-conversion rate.

Aerosol–cloud–precipitation interactions during a Saharan dust event – A summertime case‐study from the Alps

AbstractChanges in the ambient aerosol concentration are known to affect the microphysical properties of clouds. Especially regarding precipitation formation, increasing aerosol concentrations are assumed to delay the precipitation onset, but may increase precipitation rates via convective invigoration and orographic spillover further downstream. In this study, we analyse the effect of increased aerosol concentrations on a heavy precipitation event observed in summer 2017 over northeastern Switzerland, an event which was considerably underestimated by the operational weather forecast model. Preceding the precipitation event, Saharan dust was advected towards the Alps, which could have contributed to increased precipitation rates north of the Alpine ridge. To investigate the potential impact of the increased ambient aerosol concentrations on surface precipitation, we perform a series of sensitivity simulations using the Consortium for Small‐scale Modeling (COSMO) model with different microphysical parametrizations and prognostic aerosol perturbations. The results show that the choice of the microphysical parametrization scheme in terms of a one‐ or two‐moment scheme has the relatively largest impact on surface precipitation rates. In the one‐moment scheme, surface precipitation is strongly reduced over the Alpine ridge and increased further downstream. Simulated changes in surface precipitation in response to aerosol perturbations remain smaller in contrast to the impact of the microphysics scheme. Elevated cloud condensation nuclei (CCN) concentrations lead to increased cloud water and decreased cloud ice mass, especially in regions of high convective activity south of the Alps. These altered cloud properties indeed increase surface precipitation further downstream, but the simulated change is too small to explain the observed heavy precipitation event. Additional ice‐nucleating particles (INPs) increase cloud ice mass, but only trigger local changes in downstream surface precipitation. Thus, increased aerosol number concentrations during the Saharan dust outbreak are unlikely to have caused the heavy precipitation event in summer 2017.

Impact of dry intrusion events on the composition and mixing state of particles during the winter Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA)

Abstract. Long-range transport of continental emissions has a far-reaching influence over remote regions, resulting in substantial change in the size, morphology, and composition of the local aerosol population and cloud condensation nuclei (CCN) budget. Here, we investigate the physicochemical properties of atmospheric particles collected on board a research aircraft flown over the Azores during the winter 2018 Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA) campaign. Particles were collected within the marine boundary layer (MBL) and free troposphere (FT) after long-range atmospheric transport episodes facilitated by dry intrusion (DI) events. Chemical and physical properties of individual particles were investigated using complementary capabilities of computer-controlled scanning electron microscopy and X-ray spectromicroscopy to probe particle external and internal mixing state characteristics. Furthermore, real-time measurements of aerosol size distribution, cloud condensation nuclei (CCN) concentration, and back-trajectory calculations were utilized to help bring into context the findings from offline spectromicroscopy analysis. While carbonaceous particles were found to be the dominant particle type in the region, changes in the percent contribution of organics across the particle population (i.e., external mixing) shifted from 68 % to 43 % in the MBL and from 92 % to 46 % in FT samples during DI events. This change in carbonaceous contribution is counterbalanced by the increase in inorganics from 32 % to 57 % in the MBL and 8 % to 55 % in FT. The quantification of the organic volume fraction (OVF) of individual particles derived from X-ray spectromicroscopy, which relates to the multi-component internal composition of individual particles, showed a factor of 2.06 ± 0.16 and 1.11 ± 0.04 increase in the MBL and FT, respectively, among DI samples. We show that supplying particle OVF into the κ-Köhler equation can be used as a good approximation of field-measured in situ CCN concentrations. We also report changes in the κ values in the MBL from κMBL, non-DI=0.48 to κMBL, DI=0.41, while changes in the FT result in κFT, non-DI=0.36 to κFT, DI=0.33, which is consistent with enhancements in OVF followed by the DI episodes. Our observations suggest that entrainment of particles from long-range continental sources alters the mixing state population and CCN properties of aerosol in the region. The work presented here provides field observation data that can inform atmospheric models that simulate sources and particle composition in the eastern North Atlantic.

Impact of Giant Sea Salt Aerosol Particles on Precipitation in Marine Cumuli and Stratocumuli: Lagrangian Cloud Model Simulations

Abstract The impact of giant sea salt aerosols released from breaking waves on rain formation in marine boundary layer clouds is studied using large-eddy simulations (LES). We perform simulations of marine cumuli and stratocumuli for various concentrations of cloud condensation nuclei (CCN) and giant CCN (GCCN). Cloud microphysics are modeled with a Lagrangian method that provides key improvements in comparison to previous LES of GCCN that used Eulerian bin microphysics. We find that GCCN significantly increase precipitation in stratocumuli. This effect is strongest for low and moderate CCN concentrations. GCCN are found to have a smaller impact on precipitation formation in cumuli. These conclusions are in agreement with field measurements. We develop a simple parameterization of the effect of GCCN on precipitation, accretion, and autoconversion rates in marine stratocumuli. Significance Statement Breaking sea waves release salt particles into the atmosphere. Cloud droplets formed on these salt particles can grow larger than droplets formed on other smaller particles. Therefore, sea salt particles can be important for rain formation over oceans. To investigate this effect, we performed idealized computer simulations of stratocumulus and cumulus clouds. Sea salt particles were modeled with an unprecedented precision thanks to the use of an emerging modeling method. In our simulations sea salt particles significantly enhance rain formation in stratocumuli, but not in cumuli. Our study has implications for climate models, because stratocumuli are important for Earth’s energy budget and for rain enhancement experiments.

Ambient aerosol properties in the remote atmosphere from global-scale in situ measurements

Abstract. In situ measurements of aerosol microphysical, chemical, and optical properties were made during global-scale flights from 2016–2018 as part of the Atmospheric Tomography Mission (ATom). The NASA DC-8 aircraft flew from ∼ 84∘ N to ∼ 86∘ S latitude over the Pacific, Atlantic, Arctic, and Southern oceans while profiling nearly continuously between altitudes of ∼ 160 m and ∼ 12 km. These global circuits were made once each season. Particle size distributions measured in the aircraft cabin at dry conditions and with an underwing probe at ambient conditions were combined with bulk and single-particle composition observations and measurements of water vapor, pressure, and temperature to estimate aerosol hygroscopicity and hygroscopic growth factors and calculate size distributions at ambient relative humidity. These reconstructed, composition-resolved ambient size distributions were used to estimate intensive and extensive aerosol properties, including single-scatter albedo, the asymmetry parameter, extinction, absorption, Ångström exponents, and aerosol optical depth (AOD) at several wavelengths, as well as cloud condensation nuclei (CCN) concentrations at fixed supersaturations and lognormal fits to four modes. Dry extinction and absorption were compared with direct in situ measurements, and AOD derived from the extinction profiles was compared with remotely sensed AOD measurements from the ground-based Aerosol Robotic Network (AERONET); this comparison showed no substantial bias. The purpose of this work is to describe the methodology by which ambient aerosol properties are estimated from the in situ measurements, provide statistical descriptions of the aerosol characteristics of different remote air mass types, examine the contributions to AOD from different aerosol types in different air masses, and provide an entry point to the ATom aerosol database. The contributions of different aerosol types (dust, sea salt, biomass burning, etc.) to AOD generally align with expectations based on location of the profiles relative to continental sources of aerosols, with sea salt and aerosol water dominating the column extinction in most remote environments and dust and biomass burning (BB) particles contributing substantially to AOD, especially downwind of the African continent. Contributions of dust and BB aerosols to AOD were also significant in the free troposphere over the North Pacific. Comparisons of lognormally fitted size distribution parameters to values in the Optical Properties of Aerosols and Clouds (OPAC) database commonly used in global models show significant differences in the mean diameters and standard deviations for accumulation-mode particles and coarse-mode dust. In contrast, comparisons of lognormal parameters derived from the ATom data with previously published shipborne measurements in the remote marine boundary layer show general agreement. The dataset resulting from this work can be used to improve global-scale representation of climate-relevant aerosol properties in remote air masses through comparison with output from global models and assumptions used in retrievals of aerosol properties from both ground-based and satellite remote sensing.