Increase In Cloud Condensation Nuclei Concentration Research Articles

Simulations of the impact of cloud condensation nuclei and ice-nucleating particles perturbations on the microphysics and radar reflectivity factor of stratiform mixed-phase clouds

Abstract. In this research, we delve into the influence of cloud condensation nuclei (CCN) and ice-nucleating particle (INP) concentrations on the morphology and abundance of ice particles in mixed-phase clouds, emphasizing the consequential impact of ice particle shape, number, and size on cloud dynamics and microphysics. Leveraging the synergy of the Advanced Microphysics Prediction System (AMPS) and the Kinematic Driver (KiD) model, we conducted simulations to capture cloud microphysics across diverse CCN and INP concentrations. The Passive and Active Microwave radiative TRAnsfer (PAMTRA) radar forward simulator further augmented our study, offering insights into how the concentrations of CCN and INPs affect radar reflectivities. Our experimental framework encompassed CCN concentrations ranging from 10 to 5000 cm−3 and INP concentrations from 0.001 to 10 L−1. Central to our findings is the observation that higher INP concentrations yield smaller ice particles, while an increase in CCN concentrations leads to a subtle growth in their dimensions. Consistent with existing literature, our results spotlight oblate-like crystals as dominant between temperatures of −20 and −16 °C. Notably, high-INP scenarios unveiled a significant prevalence of irregular polycrystals. The aspect ratio (AR) of ice particles exhibited a decline with the rise in both CCN and INP concentrations, highlighting the nuanced interrelation between CCN levels and ice particle shape, especially its ramifications on the riming mechanism. The forward-simulated radar reflectivities, spanning from −11.83 dBZ (low INP, 0.001 L−1) to 4.65 dBZ (high INP, 10 L−1), elucidate the complex dynamics between CCN and INPs in determining mixed-phase cloud characteristics. Comparable differences in radar reflectivity were also reported from observational studies of stratiform mixed-phase clouds in contrasting aerosol environments. Our meticulous analysis of KiD-AMPS simulation outputs, coupled with insights into aerosol-driven microphysical changes, thus underscores the significance of this study in refining our ability to understand and interpret observations and climate projections.

Surface impacts and associated mechanisms of a moisture intrusion into the Arctic observed in mid-April 2020 during MOSAiC

Distinct events of warm and moist air intrusions (WAIs) from mid-latitudes have pronounced impacts on the Arctic climate system. We present a detailed analysis of a record-breaking WAI observed during the MOSAiC expedition in mid-April 2020. By combining Eulerian with Lagrangian frameworks and using simulations across different scales, we investigate aspects of air mass transformations via cloud processes and quantify related surface impacts. The WAI is characterized by two distinct pathways, Siberian and Atlantic. A moist static energy transport across the Arctic Circle above the climatological 90th percentile is found. Observations at research vessel Polarstern show a transition from radiatively clear to cloudy state with significant precipitation and a positive surface energy balance (SEB), i.e., surface warming. WAI air parcels reach Polarstern first near the tropopause, and only 1–2 days later at lower altitudes. In the 5 days prior to the event, latent heat release during cloud formation triggers maximum diabatic heating rates in excess of 20 K d-1. For some poleward drifting air parcels, this facilitates strong ascent by up to 9 km. Based on model experiments, we explore the role of two key cloud-determining factors. First, we test the role moisture availability by reducing lateral moisture inflow during the WAI by 30%. This does not significantly affect the liquid water path, and therefore the SEB, in the central Arctic. The cause are counteracting mechanisms of cloud formation and precipitation along the trajectory. Second, we test the impact of increasing Cloud Condensation Nuclei concentrations from 10 to 1,000 cm-3 (pristine Arctic to highly polluted), which enhances cloud water content. Resulting stronger longwave cooling at cloud top makes entrainment more efficient and deepens the atmospheric boundary layer. Finally, we show the strongly positive effect of the WAI on the SEB. This is mainly driven by turbulent heat fluxes over the ocean, but radiation over sea ice. The WAI also contributes a large fraction to precipitation in the Arctic, reaching 30% of total precipitation in a 9-day period at the MOSAiC site. However, measured precipitation varies substantially between different platforms. Therefore, estimates of total precipitation are subject to considerable observational uncertainty.

Microphysical Pathways Active Within Thunderstorms and Their Sensitivity to CCN Concentration and Wind Shear

AbstractThe impact of cloud condensation nuclei (CCN) concentration on microphysical processes within thunderstorms and the resulting surface precipitation is not fully understood yet. In this work, an analysis of the microphysical pathways occurring in these clouds is proposed to systematically investigate and understand these sensitivities. Thunderstorms were simulated using convection‐permitting (1 km horizontal grid spacing) idealized simulations with the ICON model, which included a 2‐moment microphysics parameterization. Cloud condensation nuclei concentrations were increased from 100 to 3,200 CCN/cm3, in five different wind shear environments ranging from 18 to 50 m/s. Large and systematic decreases of surface precipitation (up to 35%) and hail (up to 90%) were found as CCN was increased. Wind shear changes the details, but not the sign, of the sensitivity to CCN. The microphysical process rates were tracked throughout each simulation, closing the mass budget for each hydrometeor class, and collected together into “microphysical pathways,” which quantify the different growth processes leading to surface precipitation. Almost all surface precipitation occurred through the mixed‐phase pathway, where graupel and hail grow by riming and later melt as they fall to the surface. The mixed‐phase pathway is sensitive to CCN concentration changes as a result of changes to the riming rate, which were systematically evaluated. Supercooled water content was almost insensitive to increasing CCN concentration, but decreased cloud drop size led to a large reduction in the riming efficiency (from 0.79 to 0.24) between supercooled cloud drops and graupel or hail, resulting in less surface precipitation.

The numerical study on the sensitivity of different auto-conversion parameterization to CCN concentration

The auto-conversion from cloud droplet to raindrop is a process whereby rain drops formed by collision-coalescence of cloud droplets. As an essential link connecting aerosol-cloud interaction, it significantly influences the changes in cloud morphology and precipitation. In order to explore the sensitivity of auto-conversion schemes to cloud condensation nuclei (CCN) concentration, using the auto-conversion scheme in the Thompson scheme (TH-AU) and Milbrandt-Yau scheme (MY-AU), we set four groups of CCN concentrations to simulate a strong convection process in Ningxia region of China. The results show that: The sensitivity of different auto-conversion schemes to changes in CCN concentrations varies significantly, and the aerosol-induced changes in precipitation and convection strongly depend on the auto-conversion scheme. With the increase of CCN concentration, the mixing ratio of cloud droplets increases, and the particle size decreases, resulting in a decrease in the auto-conversion intensity for the two schemes, which makes more supercooled water participate in the ice phase process. Compared with the TH-AU, the MY-AU has lower auto-conversion intensity at the same CCN concentration, the proportion of supercooled cloud droplets participating in the ice phase process is higher than that in the TH-AU, which leads to the raindrop mixing ratio of 4000–6000 m in MY-AU is lower than that in TH-AU at the same CCN concentration, and the mixing ratio of ice phase particles in MY-AU scheme is higher in the convective mature stage, especially snow and graupel particles, and the graupel particle generation height of MY-AU is lower than that of TH-AU. In terms of dynamic structure, with the increase of CCN concentration, more cloud droplets are activated and frozen which makes the enhancement of updraft mainly occur in the upper layer in both schemes, but the stronger gravitational drag caused by graupel particles in MY-AU may enhance the downdraft in the middle and lower layers, which makes the convection of MY-AU decay early at higher CCN concentration. In addition, changes in microphysical processes also lead to differences in cumulative precipitation and accumulated ground graupel-fall of the two schemes. The cumulative precipitation and the accumulated ground graupel-fall of the MY-AU decrease strongly with the increase of CCN concentration because the warm rain process of MY-AU is strongly inhibited. Compared with MY-AU, the warm rain process of TH-AU is not significantly inhibited, which leads to the cumulative precipitation and the accumulated ground graupel-fall of the TH-AU scheme increases when the CCN concentration is 50–200 cm−3 and slightly decreases when the CCN concentration is 200–10000 cm−3.

Number‐Size Distribution and CCN Activity of Atmospheric Aerosols in the Western North Pacific During Spring Pre‐Bloom Period: Influences of Terrestrial and Marine Sources

AbstractA cruise observation of atmospheric aerosols and gaseous air/seawater compounds was performed in the western North Pacific during the spring pre‐bloom period. The aerosol size distribution and cloud condensation nuclei (CCN) activity were characterized according to the source type: terrestrial/pollution, marine, and mixed. The CCN activity was represented by the aerosol hygroscopicity parameter (ĸCCNC) from the activation diameter (dact) in water‐vapor supersaturation (SS), which is controlled by the composition of particles with sizes around dact. For marine type, whereas a high hygroscopicity parameter value of 0.86 for accumulation mode (ĸaccum) suggests a strong influence of sea salt and sulfate, a lower value of 0.49 in Aitken mode (ĸAit) suggests the contribution of biogenic organic matter (OM). Lower ĸAit and ĸaccum for terrestrial/pollution type (0.12 and 0.62, respectively) suggests greater contributions of OM. Under the influence of marine sources, ĸCCNC at a dact of ∼110 nm (low SS) was not responsive to the chlorophyll a (Chl‐a) concentration, whereas ĸCCNC at ∼50 nm (high SS) correlated negatively with Chl‐a. The hygroscopicity parameter of OM was estimated as 0.13–0.29 considering the aerosol composition. During a new particle formation event, the ĸCCNC at 0.60% SS increased; nucleated particles grew in size to become CCN. Sensitivity analysis indicated that changes in the size distribution and chemical composition of the aerosol contributed to the increase in CCN concentration. This study highlights the high variability in size distribution and CCN activity of off‐coast aerosols from different sources, including marine biological activities.

Examination of aerosol impacts on convective clouds and precipitation in two metropolitan areas in East Asia; how varying depths of convective clouds between the areas diversify those aerosol effects?

Abstract. This study examines the role played by aerosols which act as cloud condensation nuclei (CCN) in the development of clouds and precipitation in two metropolitan areas in East Asia that have experienced substantial increases in aerosol concentrations over the last decades. These two areas are the Seoul and Beijing areas and the examination was done by performing simulations using the Advanced Research Weather Research and Forecasting model as a cloud system resolving model. The CCN are advected from the continent to the Seoul area and this increases aerosol concentrations in the Seoul area. These increased CCN concentrations induce the enhancement of condensation that in turn induces the enhancement of deposition and precipitation amount in a system of less deep convective clouds as compared to those in the Beijing area. In a system of deeper clouds in the Beijing area, increasing CCN concentrations also enhance condensation but reduce deposition. This leads to negligible CCN-induced changes in the precipitation amount. Also, in the system there is a competition for convective energy among clouds with different condensation and updrafts. This competition results in different responses to increasing CCN concentrations among different types of precipitation, which are light, medium and heavy precipitation in the Beijing area. The CCN-induced changes in freezing play a negligible role in CCN-precipitation interactions as compared to the role played by CCN-induced changes in condensation and deposition in both areas.

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.

A numerical study of effects of aerosol particles on the electric activities of a thunderstorm with a 1.5D aerosol-cloud bin model

Numerical simulations are carried out to study the effects of aerosol particles acted as cloud condensation nuclei (CCN) and ice nuclei (IN) on thunderstorm properties. A two-cylinder time-dependent cloud and aerosol interaction model with the spectral bin microphysical parameterization, explicit noninductive electrification and lightning process is utilized to explore the impacts. This model not only uses the high-resolution particle size bins to describe the distributions of aerosol particles and hydrometeors, but also can be applied to study the diffusional growth of aerosol particles to cloud droplets and to track the mass of both CCN and IN in hydrometeors. The simulated cloud properties show that increased CCN concentrations result in more numerous but smaller droplets and form ice particles less efficiently, producing less numbers of graupel and less efficient conversion to raindrops. Increased IN concentrations directly enhance the heterogeneous nucleation, and then boost the subsequent microphysical processes, contributing to the production of increasingly numbers of graupel which eventually pass through the melting layer and then enhance the precipitation. As CCN concentrations increase, the charge density gained by different mass-binned ice particles and total charge density significantly decrease due to reduced large ice particles with greater diameters than 360 μm. In contrast, as IN concentrations are further increased, dramatically increased large ice particles with greater diameters than 360 μm are produced and thus, support the enhanced charge density gained by different mass-binned ice particles and total charge density. Besides, as expected, the reduced (enhanced) lightning frequency is well explained by the reduced (increased) numbers of large ice particles (diameter>360 μm).

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.

Effect of cloud condensation nuclei concentration on a hail event with weak warm rain process in a semi-arid region of China

Although there are several studies on the effects of cloud condensation nuclei (CCN) concentration on convection, it requires more attention. A hail event in semi-arid China is simulated with four different CCN concentration levels, which correspond to the marine area, mid-continental, continental, and polluted background CCN concentrations. It was found that the auto-conversion of cloud water to rainwater is not significant in the simulated convection. The increase in CCN concentration was accompanied by a large number of graupel particles of smaller size, which consequently induced smaller rain droplets. It is shown that increasing CCN concentration changed the dominant average mass-weighted diameters of the graupel, hail particles and rain droplet. The number concentration of rain droplets increased with CCN concentration, corresponding to the significant evaporation during the falling process. Consequently, the maximum of rainfall on the ground did not increase. Additionally, increasing the CCN concentration does not have the same apparent impact on the size of hail particles.

Aircraft measurements of aerosol and trace gas chemistry in the eastern North Atlantic

Abstract. The Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA) investigated properties of aerosols and subtropical marine boundary layer (MBL) clouds. Low subtropical marine clouds can have a large effect on Earth's radiative budget, but they are poorly represented in global climate models. In order to understand their radiative effects, it is imperative to understand the composition and sources of the MBL cloud condensation nuclei (CCN). The campaign consisted of two intensive operation periods (IOPs) (June–July 2017 and January–February 2018) during which an instrumented G-1 aircraft was deployed from Lajes Field on Terceira Island in the Azores, Portugal. The G-1 conducted research flights in the vicinity of the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) atmospheric observatory on Graciosa Island. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and Ionicon proton-transfer-reaction mass spectrometer (PTR-MS) were deployed aboard the aircraft, characterizing chemistry of non-refractory aerosol and trace gases, respectively. The eastern North Atlantic region was found to be very clean, with an average non-refractory submicrometer aerosol mass loading of 0.6 µg m−3 in the summer and 0.1 µg m−3 in the winter, measured by the AMS. Average concentrations of the trace reactive gases methanol and acetone were 1–2 ppb; benzene, toluene and isoprene were even lower, <1 ppb. Mass fractions of sulfate, organics, ammonium and nitrate in the boundary layer were 69 %, 23 %, 7 % and 1 % and remained largely similar between seasons. The aerosol chemical composition was dominated by sulfate and highly processed organics. Particulate methanesulfonic acid (MSA), a well-known secondary biogenic marine species, was detected, with an average boundary layer concentration of 0.021 µg m−3, along with its gas-phase precursor, dimethyl sulfide (DMS). MSA accounted for no more than 3 % of the submicron, non-refractory aerosol in the boundary layer. Examination of vertical profiles of aerosol and gas chemistry during ACE-ENA reveals an interplay of local marine emissions and long-range-transported aged aerosol. A case of transport of biomass burning emissions from North American fires has been identified using back-trajectory analysis. In the summer, the non-refractory portion of the background CCN budget was heavily influenced by aerosol associated with ocean productivity, in particular sulfate formed from DMS oxidation. Episodic transport from the continents, particularly of biomass burning aerosol, periodically increased CCN concentrations in the free troposphere. In the winter, with ocean productivity lower, CCN concentrations were overall much lower and dominated by remote transport. These results show that anthropogenic emissions perturb CCN concentrations in remote regions that are sensitive to changes in CCN number and illustrate that accurate predictions of both transport and regional aerosol formation from the oceans are critical to accurately modeling clouds in these regions.

Cloud condensation nuclei properties of South Asian outflow over the northern Indian Ocean during winter

Abstract. Extensive measurements of cloud condensation nuclei (CCN) and condensation nuclei (CN) concentrations in the South Asian outflow to the northern Indian Ocean were carried out on board an instrumented research vessel, as part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during the winter season (January–February 2018). Measurements include a north–south transect across the South Asian plume over the northern Indian Ocean and an east–west transect over the equatorial Indian Ocean (∼2∘ S), which is far away from the continental sources. South Asian outflow over the northern Indian Ocean is characterized by the high values of CCN number concentration (∼5000 cm−3), low CCN activation efficiency (∼25 %) and a steep increase in CCN concentration with the increase in supersaturation. In contrast, low CCN concentration (∼1000 cm−3) with flat supersaturation spectra was found over the equatorial Indian Ocean. The CCN properties exhibited significant dependence on the geometric mean diameter (GMD) of the aerosol number size distribution, and CCN activation efficiency decreased to low values (<20 %) at the time of new-particle formation events over near-coastal and remote oceanic regions. The analysis of the activation efficiencies for the “similar” aerosol size distributions over the northern Indian Ocean indicated the primary role of aerosol number size distribution on CCN activation efficiency. The dependence of CCN properties and activation efficiency on size-segregated aerosol number concentration, especially during the ultrafine (<100 nm) particle events, is investigated in detail for the first time over the region.

Aerosol Impacts on Mesoscale Convective Systems Forming Under Different Vertical Wind Shear Conditions

AbstractFollowing our previous study of wind shear effect on mesoscale convective system (MCS) organization under a clean atmospheric condition using the Weather Research and Forecasting model coupled with spectral‐bin microphysics, we conduct sensitivity simulations by increasing cloud condensation nuclei concentration to investigate aerosol impacts on MCSs forming under different wind shear conditions. We find that increased aerosols induce stronger updrafts and downdrafts in all MCSs. The stronger updrafts and enlarged convective core area contribute to larger vertical mass fluxes and enhance precipitation. The enhanced updrafts and vertical mass fluxes indicate convective invigoration. Increased updraft speed below 8‐km altitude is attributed to enhanced condensational heating, except for the weak wind shear and strong low‐level shear cases in which the enhanced low‐level convergence is another contributing factor. Interestingly, above 8‐km altitude, we see reduced updraft speed by the increased aerosols due to reduced vertical pressure perturbation gradient force. The accumulated rainfall and mean rain rate are increased with a greater occurrence frequency of heavy rain. Larger rain rate is seen in both convective and stratiform regions. In general, we see a higher frequency of deep clouds in the polluted condition because of invigorated convection, and more stratiform/anvil clouds, but a lower frequency of shallow warm clouds, with a larger significance for more organized MCSs. The consistently invigorated MCSs by aerosols under various wind shear conditions revealed by this study have important implications to weather and climate in warm and humid regions that are influenced by pollution.

The Roles of Mineral Dust as Cloud Condensation Nuclei and Ice Nuclei During the Evolution of a Hail Storm

AbstractAerosols play important roles in the evolution of deep convective systems like hailstorms. In this study, the heterogeneous ice nucleation schemes have been improved in the Weather Research and Forecasting model coupled with a spectral bin microphysics (WRF‐SBM), which considered aerosols acting as ice nuclei (IN). A hail storm occurred around Tianshan mountains, northwestern China, was simulated with updated WRF‐SBM, and the results have been compared with satellite observations. Further, four sensitive simulation tests were conducted with different cloud condensation nuclei (CCN) and IN concentrations to investigate their respective roles during the evolution of the hailstorm. The increase in CCN concentration resulted in larger cloud droplet concentration and cloud water content, as well as enhanced condensational growth, which released more latent heat and led to stronger updraft at lower levels. The increase in IN number almost did not affect warm processes but led to larger ice crystal concentration and enhanced Bergeron process. Larger CCN concentration led to larger supercooled liquid water content, which in turn contributed to the enhanced hail growth by more efficient drop‐ice collisions and led to larger size of hail particles, while larger IN number reduced the size of graupel and suppressed the growth of hailstones. An analysis of the mobility of hail indicated increased frequency of larger hail with stronger sedimentation induced by more CCN. A further three ensemble runs with random perturbations on initial temperature and humidity were performed for each aerosol scenario, and the results suggested the robustness of simulated CCN and IN effects.

Probing the Response of Tropical Deep Convection to Aerosol Perturbations Using Idealized Cloud-Resolving Simulations with Parameterized Large-Scale Dynamics

Abstract A framework is introduced to investigate the indirect effect of aerosol loading on tropical deep convection using three-dimensional limited-domain idealized cloud-system-resolving model simulations coupled with large-scale dynamics over fixed sea surface temperature. The large-scale circulation is parameterized using the spectral weak temperature gradient (WTG) approximation that utilizes the dominant balance between adiabatic cooling and diabatic heating in the tropics. The aerosol loading effect is examined by varying the number of cloud condensation nuclei (CCN) available to form cloud droplets in the two-moment bulk microphysics scheme over a wide range of environments from 30 to 5000 cm−3. The radiative heating is held at a constant prescribed rate in order to isolate the microphysical effects. Analyses are performed over the period after equilibrium is achieved between convection and the large-scale environment. Mean precipitation is found to decrease modestly and monotonically when the aerosol number concentration increases as convection gets weaker, despite the increase in cloud liquid water in the warm-rain region and ice crystals aloft. This reduction is traced down to the reduction in surface enthalpy fluxes as an energy source to the atmospheric column induced by the coupling of the large-scale motion, though the gross moist stability remains constant. Increasing CCN concentration leads to 1) a cooler free troposphere because of a reduction in the diabatic heating and 2) a warmer boundary layer because of suppressed evaporative cooling. This dipole temperature structure is associated with anomalously descending large-scale vertical motion above the boundary layer and ascending motion at lower levels. Sensitivity tests suggest that changes in convection and mean precipitation are unlikely to be caused by the impact of aerosols on cloud droplets and microphysical properties but rather by accounting for the feedback from convective adjustment with the large-scale dynamics. Furthermore, a simple scaling argument is derived based on the vertically integrated moist static energy budget, which enables estimation of changes in precipitation given known changes in surfaces enthalpy fluxes and the constant gross moist stability. The impact on cloud hydrometeors and microphysical properties is also examined, and it is consistent with the macrophysical picture.

A Numerical Study of Aerosol Effects on Electrification with Different Intensity Thunderclouds

Numerical simulations are performed to investigate the effect of varying CCN (cloud condensation nuclei) concentration on dynamic, microphysics, electrification, and charge structure in weak, moderate, and severe thunderstorms. The results show that the response of electrification to the increase of CCN concentration is a nonlinear relationship in different types of thunderclouds. The increase in CCN concentration leads to a significant enhancement of updraft in the weak thunderclouds, while the high CCN concentration in moderate and severe thunderclouds leads to a slight reduction in maximum updraft speed. The increase of the convection promotes the lift of more small cloud droplets, which leads to a faster and stronger production of ice crystals. The production of graupel is insensitive to the CCN concentration. The content of graupel increases from low CCN concentration to moderate CCN concentration, and slightly decreases at high CCN concentration, which arises from the profound enhancement of small ice crystals production. When the intensity of thundercloud increases, the reduction of graupel production will arise in advance as the CCN concentration increases. Charge production tends to increase as the aerosol concentration rises from low to high in weak and moderate thundercloud cases. However, the magnitude of charging rates in the severe thundercloud cases keeps roughly stable under the high CCN concentration condition, which can be attributed to the profound reduction of graupel content. The charge structure in the weak thundercloud at low CCN concentrations keeps as a dipole, while the weak thunderclouds in the other cases (the CCN concentration above 100 cm−3) change from a dipole charge structure to a tripole charge structure, and finally disappear with a dipole. In cases of moderate and severe intensity thunderclouds, the charge structure depicts a relatively complex structure that includes a multilayer charge region.

Numerical Simulation of the Effect of Cloud Condensation Nuclei Concentration on the Microphysical Processes in Typhoon Usagi

The Weather Research and Forecasting model version 3.2.1 with the Lin microphysics scheme was used herein to simulate super typhoon Usagi, which occurred in 2013. To investigate the effect of the concentration of cloud condensation nuclei (CCN) on the development of typhoon Usagi, a control simulation was performed with a CCN concentration of 100 cm−3, together with two sensitivity tests: C10 and C1000, having CCN concentrations of 10 cm−3 and 1000 cm−3, respectively. The path, intensity, precipitation, microphysical processes, and the release of latent heat resulting from the typhoon in all three simulations were analyzed to show that an increase in CCN concentration leads to decreases in intensity and precipitation, an increase of the cloudless area in the eye of the typhoon, a more disordered cloud system, and less latent heat released through microphysical processes, especially the automatic conversion of cloud water into rainwater. Overall, an increase in CCN concentration reduces the total latent heat released during the typhoon suggesting that typhoon modification by aerosol injection may be optimized using numerical simulations to ensure the strongest release of latent heat within the typhoon.

Production of extremely low volatile organic compounds from biogenic emissions: Measured yields and atmospheric implications

Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O3 and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget.

Sensitivity of convection to observed variation in aerosol size distributions and composition at a rural site in the southeastern United States

We present a sensitivity analysis to determine the impact of variations in aerosol physical and chemical characteristics, as observed in the field in the southeastern United States, on convective cloud microphysics. Scenarios reflecting changes in aerosol properties, observed over the Piedmont of Virginia and associated with new particle formation and growth events, were evaluated using the two-dimensional version of the Goddard Cumulus Ensemble Model with detailed spectral-bin microphysics. Two aerosol size distributions represented early and late stages of particle growth, and two aerosol chemical compositions (norpinic acid and ammonium sulfate) represented extremes in aerosol hygroscopicity observed at the field site, for a total of four scenarios. The chosen compositions reflect inferred local changes in aerosol composition over short time scales. Variations in the aerosol size distribution and composition resulted in substantial variation in the total number of cloud condensation nuclei (CCN) produced in the four case studies. Cases with high CCN concentrations developed larger, more vigorous clouds with more precipitation generated by both warm and cold rain processes. Greater numbers of drops were propelled aloft and formed an extensive ice anvil that produced a large area of stratiform rain. Convection was enhanced by increasing aerosols despite decreases in precipitation efficiency. In contrast, lower CCN concentrations developed smaller clouds with suppressed cold rain processes and less total precipitation. The relatively small increase in CCN concentration associated with the increase in aerosol hygroscopicity resulted in an increase in accumulated modeled precipitation of 12 % after 180 min of simulation time in both high and low CCN cases. The increase in accumulated precipitation due to the substantial increase in CCN concentration associated with growth of the aerosol size distribution was 93 % for both aerosol compositions. The timing of the onset of precipitation was not affected by aerosol concentration or composition.

Aerosol Effects on Idealized Supercell Thunderstorms in Different Environments

Abstract Idealized supercell thunderstorms are simulated with the Weather Research and Forecasting (WRF) Model at 15 cloud condensation nuclei (CCN) concentrations (100–10 000 cm−3) using four environmental soundings with different low-level relative humidity (RH) and vertical wind shear values. The Morrison microphysics scheme is used with explicit prediction of cloud droplet number concentration and a variable shape parameter for the raindrop size distribution (results from simulations with a fixed shape parameter are also presented). Changes in the microphysical process rates with CCN concentration are negligible beyond CCN ≈ 3000 cm−3. Changes in cold pool characteristics with CCN concentration are nonmonotonic and highly dependent on the environmental conditions. In moist conditions with moderate vertical wind shear, the cold pool area is nearly constant with respect to CCN concentration, while the area is reduced by 84% and 22% in the soundings with dry RH and large vertical wind shear, respectively. With the exception of the dry RH sounding, domain-averaged precipitation peaks between 500 and 5000 cm−3, after which it remains constant or slowly decreases. For the dry RH sounding, the domain-averaged precipitation monotonically decreases with CCN concentration. Accumulated precipitation is enhanced (by up to 25 mm) in the most polluted cases near the updrafts, except for the dry RH sounding. The different responses for moist and dry soundings are mostly due to increased (decreased) low-level latent cooling from melting hail (evaporating rain) with increasing CCN concentration in the moist soundings. This compensating effect does not exist when the low-level RH is dry.