Cloud Events Research Articles

Can a 12-km GFS Model Simulate the Observed Relationship between Cloud Optical Properties and Extreme Rainfall of Indian Summer Monsoon?

Abstract The increase in the frequency of extreme precipitation in different parts of the world is well documented and a cause for concern in terms of global climate change. Although clouds are the only source of precipitation, there is a lack of knowledge about the type of clouds involved in extreme precipitation events. Satellite- and ground-based observations show that over the central Indian region (19°–26°N, 75°–85°E), about 60% of the extreme precipitation comes from deep convective clouds (DCCs), which have a cloud-top pressure (CTP) of less than 440 hPa and a cloud optical thickness (COT) of over 23. It was also observed that cloud liquid water (CLW) and COT show the highest correlation with extreme precipitation. Furthermore, CLW and COT show the highest contrast between extreme and nonextreme precipitation events. Simulations by a 12-km Global Forecast System (GFS, spectral) model show that the model underestimates the extreme precipitation threshold with increased forecast lead time. The simulation of associated cloud optical parameters is also poor at all lead times in different parts of India. The model also fails to capture the observed relationship between the frequency of extreme precipitation and deep convective clouds without showing any correlation between them at all lead times. This is possibly because the model simulates the observed vertical structure of the apparent heat source poorly at all lead times which leads to poor simulation of the observed relationship between cloud optical properties and extreme rainfall. Significance Statement The occurrence of extreme rainfall events is on the rise across the globe, and India is no exception. Clouds are the only source of rainfall; however, there is a lack of understanding about what type of clouds is causing them. Therefore, it is necessary to understand the relationship between cloud, convection, and extreme rainfall events during the Indian summer monsoon months. Furthermore, it is also important to investigate the skill of the current-generation global models to simulate them. This study highlights how cloud optical properties, convection, and extreme rainfall events are related to each other and also finds that a 12-km GFS model simulates these observed relationships poorly at all lead times leading to the poor simulation of extreme rainfall events.

Interplanetary Causes and Impacts of the 2024 May Superstorm on the Geosphere: An Overview

The recent superstorm of 2024 May 10–11 is the second largest geomagnetic storm in the space age and the only one that has simultaneous interplanetary data (there were no interplanetary data for the 1989 March storm). The May superstorm was characterized by a sudden impulse (SI+) amplitude of +88 nT, followed by a three-step storm main-phase development, which had a total duration of ∼9 hr. The cause of the first storm main phase with a peak SYM-H intensity of −183 nT was a fast-forward interplanetary shock (magnetosonic Mach number M ms ∼ 7.2) and an interplanetary sheath with a southward interplanetary magnetic field component B s of ∼40 nT. The cause of the second storm's main phase with an SYM-H intensity of −354 nT was a deepening of the sheath B s to ∼43 nT. A magnetosonic wave (M ms ∼ 0.6) compressed the sheath to a high magnetic field strength of ∼71 nT. Intensified B s of ∼48 nT were the cause of the third and most intense storm main phase, with an SYM-H intensity of −518 nT. Three magnetic cloud events with B s fields of ∼25–40 nT occurred in the storm recovery phase, lengthening the recovery to ∼2.8 days. At geosynchronous orbit, ∼76 keV to ∼1.5 MeV electrons exhibited ∼1–3 orders of magnitude flux decreases following the shock/sheath impingement onto the magnetosphere. The cosmic-ray decreases at Dome C, Antarctica (effective vertical cutoff rigidity <0.01 GV) and Oulu, Finland (rigidity ∼0.8 GV) were ∼17% and ∼11%, respectively, relative to quiet-time values. Strong ionospheric current flows resulted in extreme geomagnetically induced currents of ∼30–40 A in the subauroral region. The storm period is characterized by strong polar-region field-aligned currents, with ∼10 times intensification during the main phase and equatorward expansion down to ∼50° geomagnetic (altitude-adjusted) latitude.

Frontal effects on the rapid formation of a deep layer of marine fog and cloud in the NW Atlantic

AbstractRapid formation of a deep layer of fog and clouds extending to a height of 2 km was observed over Sable Island off Nova Scotia on 13–14 July 2019. This fog and cloud event was dominantly caused by the rapid intrusion of a trough from a cyclone over NE Canada. The trough quickly moved from SW–W to E–NE and encroached on the coastal waters of Nova Scotia. Due to the warm front in the trough, the air temperature increased up to 4 km in height with increased stability, while the sea‐level pressure at Sable Island dropped by 11 hPa within 12 hours. The moderate to strong winds turned counterclockwise from the N to the SE, converging with the SE winds maintained by the ridge located to the east. There was no indication of an advection‐type fog mechanism because the surface air temperature was not adjusted to the colder sea‐surface temperature that could have resulted in saturation of the surface air. The leading portion of a massive cloud band in the trough passing over Sable Island included precipitation before fog formation. The rain that evaporated in the unsaturated and warm subcloud layer effectively reduced the air temperature and significantly increased the wet‐bulb temperature in this layer. The cloud extended into the moist subcloud layer, while fog formed at 1400 UTC 13 July and lasted until 2327 UTC 13 July (with two brief periods of mist lasting less than two hours). The development of this deep layer of fog and cloud in the NW Atlantic was caused by evolving synoptic conditions through the simultaneous effects of a rapid trough intrusion over coastal waters with a massive cloud band, induced precipitation causing cloud deepening into the subcloud layer, and surface wind convergence forcing vertical mixing in a stable marine boundary layer.

Advanced Controllers for Heat Transfer Fluid Mass Flow Rate Control in Solar Tower Receivers

Efficient control strategies for managing the mass flow rate (MFR) of heat transfer fluids (HTF) during cloud transients in Solar Tower receivers play a pivotal role in optimizing plant profitability and receiver durability. This study focuses on the performance and durability of Solar Tower receivers during cloud transients. It evaluates adaptive feedback and feedforward control methods, which adjust the flow rate of heat transfer fluids based on real-time measurements of direct normal irradiance (DNI), receiver outlet and receiver panel outlet temperatures. The effectiveness of an aggressive all-sky and conservative clear-sky control strategy is explored against a conventional PI controller, emphasizing energy efficiency and receiver longevity. Simulations using a thermal Modelica model resembling a 100 MWel Crescent Dunes-like solar tower plant reveal that both advanced controllers provide precise setpoint tracking, while the PI controller struggles. The conservative controller which has a cloud standby mode prevents overheating during cloud transients by using a clear sky mass flow rate, while the aggressive controller uses the receiver panel outlet temperatures to correct for upstream tube temperature variations allowing for fast tracking correction and disturbance rejection, albeit with slight overshoots. Furthermore, the controllers significantly decrease the creep-fatigue damage accumulated in the receiver panels during cloudy days, due to limiting the increase in wall temperature spikes when cloud events end. Overall, this study underscores the pivotal role of HTF mass flow rate control systems in influencing receiver system failure modes and longevity and offers a new tool in controller design and operation assessment.

Online headspace monitoring of volatile organic compounds using proton transfer reaction-mass spectrometry: Application to the multiphase atmospheric fate of 2,4-hexadienedial

Chemical processes in clouds have been suggested to contribute significantly to the mass of organic aerosol particles in the atmosphere. Experimental and theoretical evidence suggest that organic mass production in clouds can be substantial and depends on the concentration of organic precursor compounds available in the gas phase. The present study aims at studying the aqueous phase reactivity of one of these overlooked precursors, i.e. 2,4-hexadienedial, an important and toxic intermediate in the atmospheric oxidation of aromatic species. Cautious synthesis and purification of 2,4-hexadienedial was performed. Its effective Henry's law constant was measured using a new simple and fast method based on online flow-injection analysis. The reactivity of 2,4-hexadienedial in the aqueous phase relevant to atmospheric conditions was studied, including hydrate formation, photolysis, ∙OH- and SO4∙--oxidation as well as reaction with NH3. The results revealed a low hydration constant compared to other dicarbonyls (Khyd1 = 7 × 10−2) and no dihydrate formation, indicating in an intermediate solubility (KH = 1.0 × 104 M atm−1) and high absorption cross sections (σ278nm > 10−16 cm2 molecule−1). Compared to its gas phase photolysis, its aqueous phase photolysis showed low quantum yields (Φ290–380nm = 0.9 %), and a significant red shift of the absorbance maximum, leading to a fast aqueous photolysis kinetics (Jaq,atm = 8.7 × 10−5 s−1) under atmospheric solar radiation, but no triplet state formation was detected. Radical oxidation experiments revealed extremely rapid oxidation kinetics (k∙OH = 1.10 × 1010 M−1 s−1 and kSO4∙- = 1.4 × 109 M−1 s−1) driven by fast addition of the radicals to the unsaturated bonds. In contrast, the reaction with aqueous NH3 (kNH3 = 2.6 × 10−3 M−1 s−1) was found slower than glyoxal and 2-butenedial, likely due to the hyperconjugation of 2,4-hexadienedial. Using these new data complemented with assumed aqueous phase kinetics (for NO3, 3C* and 1O2 reactions) and previous gas-phase kinetic ones, the multiphase atmospheric fate of 2,4-hexadienedial was established under atmospheric conditions reported from previous field measurements and models. The results revealed a short day lifetime (∼1 h) and a long night lifetime (>12 h). It was shown that daytime atmospheric chemistry of 2,4-hexadienedial can be influenced by aqueous-phase reactivity during cloud events, up to ∼50 % under thick cloud conditions (Liquid Water Content >2000 g/m3), indicating that even a compound of intermediate solubility can be strongly affected by condensed-phase reactivity. Besides its fast aqueous phase reactivity towards ∙OH and photolysis, its daytime condensed-phase reactivity may be driven by reactions with dissolved triplet states (3C*), up to 35 %, highlighting the need to study further the kinetics, the nature and concentrations of dissolved 3C* under various atmospheric conditions. In addition, the molecular properties and atmospheric behavior of 2,4-hexadienedial were found different from those of glyoxal and 2-butenedial, highlighting the need for detailed atmospheric reactivity studies of polyfunctional compounds, in particular unsaturated compounds.

A Method for Projecting Cloud Shadows Onto a Central Receiver Field to Predict Receiver Damage

This work demonstrates methods of mapping high-spatial-resolution direct normal irradiance (DNI) data from satellites, Total Sky Imagers (TSIs), and analogous data sources onto a heliostat field for characterizing the spatial and temporal variation of the incident flux on a central receiver tower during cloud transient events. The mapping methods are incorporated into an optical software module that interfaces with CoPylot–SolarPILOT’s python API– to provide computationally efficient optical simulation of the heliostat field and the solar power tower. Eventually, this optical model will be incorporated into optimization models whereby a plant operator can understand the effects of cloud transient events on overall power production and receiver lifetime due to creep-fatigue damage and therefore make better informed decisions about receiver shutdown events. By more accurately modelling the effects of cloud events on receiver flux maps, this work may determine the magnitude and frequency of thermal cycling on receiver tubes and panels using actual or realistic cloud shapes instead of averaged DNI values–which may undercount the total cycle number. This work may also prevent unnecessary plant shutdowns due to overly precautionary control strategies and characterize the relative impact of various cloud types on receiver life. We plan to eventually integrate this methodology into the System Advisor Model (SAM) to improve performance model accuracy during periods of cloudiness. In this paper, we demonstrate generating DNI maps and mapping them to a solar field in CoPylot using 10 m resolution data from publicly available Sentinel-2 satellite data over the Crescent Dunes plant.

Competition between the source and loss processes of radiation belt source, seed, and relativistic electrons induced by a magnetic cloud event

The radiation belt energetic electrons that are trapped by the geomagnetic field are one kind of space plasma and magnetic fluid. We quantitatively study the competition process between source and loss processes of radiation belt “source” (a few to tens of keV), “seed” (hundreds of keV), and “relativistic” (&amp;gt;1 MeV) electrons when a typical magnetic cloud (MC) event impinged on the magnetosphere on 19–20 July 2016. A very weak geomagnetic storm with SymHmin = −32 nT was driven by this magnetic cloud event. With the MC-driven shock arrival, the relativistic electrons experienced a “one-kick” energization at lower L-shells while having a moderate dropout at higher L-shells. The dropout became pronounced during the weak storm main phase. However, the seed electrons had a slight depletion after the shock and recovered to the pre-event level in the main phase, while the source electrons continued increasing during the entire event. Further analysis demonstrates that the loss processes (magnetopause shadowing and ultralow-frequency waves-driven outward transport) were competing with the source processes (shock-induced energization, substorm ejections, and wave–particle interactions), which are strongly sensitive to electron energy and L-shells. It is found that L*= ∼ 4 and μ = 102–102.5 MeV/G could be typical values at which the source and loss processes arrived at dynamic equilibrium. Our study first provides the “balance lines” in both energy channels and L* of the radiation belt source, seed, and relativistic electrons in response to magnetic cloud events. The quantitative results could be a key factor when investigating MC–magnetosphere coupling.

On the Precursor Environments to Mountain Lee Wave Clouds in Central Iberia under CMIP6 Projections

Mountain lee waves present significant hazards to aviation, often inducing turbulence and aircraft icing. The current study focuses on understanding the potential impact of global climate change on the precursor environments to mountain lee wave cloud episodes over central Iberia. We examine the suitability of several Global Climate Models (GCMs) from CMIP6 in predicting these environments using the ERA5 reanalysis as a benchmark for performance. The dataset is divided into two periods: historical data (2001–2014) and projections for the SSP5–8.5 future climate scenario (2015–2100). The variations and trends in precursor environments between historical data and future climate scenarios are exposed, with a particular focus on the expansion of the Azores High towards the Iberian Peninsula, resulting in increased zonal winds throughout the Iberian Peninsula in the future. However, the increase in zonal wind is insufficient to modify the wind pattern, so future mountain lee wave cloud events will not vary significantly. The relative humidity trends reveal no significant changes. Moreover, the risk of icing precursor environments connected with mountain lee wave clouds is expected to decrease in the future, due to rising temperatures. Our results highlight that the EC-EARTH3 GCM reveals the closest alignment with ERA5 data, and statistically significant differences between the historical and future climate scenario periods are presented, making EC-EARTH3 a robust candidate for conducting future studies on the precursor environments to mountain lee wave cloud events.

Commencement and Interruption of Relativistic Electron Dropout in the Heart of the Outer Radiation Belt Induced by a Magnetic Cloud Event

AbstractWe analyze and discuss the commencement and interruption of the relativistic electron dropout in the heart of the outer radiation belt (L* = 4–5) induced by a typical magnetic cloud (MC) event. This MC event impinged the Earth’s magnetosphere on 31 October 2012 and caused a moderate geomagnetic storm with a special prolonged initial phase lasting for &gt;13 hr. The relativistic electrons phase space density (PSD) dropout commenced at L* &gt; 5 during the initial phase. The PSD dropout penetrated deep beyond the heart of the radiation belt (reached L* &lt; 4) at the onset of the main phase, while it was partially enhanced with a local PSD maximum around L* = 4.5, thus causing the interruption of the PSD dropout. The dropout became pronounced at L* &gt; ∼4.7 while the local PSD maximum was maintained throughout the main phase. During the recovery phase, the dropout totally disappeared at L* &lt; 5.5 with relativistic electron PSD gradually recovering to pre‐event level or higher. Further investigations on solar wind parameters and plasma waves give evidence that (a) persistent high dynamic pressure and the triggered Ultra‐Low Frequency waves contribute to the dropout of electrons to interplanetary space during the initial phase and the onset of the main phase; (b) local acceleration by chorus waves during the main phase and recovery phase could explain the interruption of the dropout. Our study underlines the persistent high dynamic pressure competing with intense chorus waves in triggering the commencement and interruption of the relativistic electrons PSD dropout in the heart of the outer radiation belt.

CLASS Observations of Atmospheric Cloud Polarization at millimeter Wavelengths

The dynamic atmosphere imposes challenges to ground-based cosmic microwave background observation, especially for measurements on large angular scales. The hydrometeors in the atmosphere, mostly in the form of clouds, scatter the ambient thermal radiation and are known to be the main linearly polarized source in the atmosphere. This scattering-induced polarization is significantly enhanced for ice clouds due to the alignment of ice crystals under gravity, which are also the most common clouds seen at the millimeter-astronomy sites at high altitudes. This work presents a multifrequency study of cloud polarization observed by the Cosmology Large Angular Scale Surveyor experiment on Cerro Toco in the Atacama Desert of northern Chile, from 2016–2022, at the frequency bands centered around 40, 90, 150, and 220 GHz. Using a machine-learning-assisted cloud classifier, we made connections between the transient polarized emission found in all four frequencies with the clouds imaged by monitoring cameras at the observing site. The polarization angles of the cloud events are found to be mostly 90° from the local meridian, which is consistent with the presence of horizontally aligned ice crystals. The 90 and 150 GHz polarization data are consistent with a power law with a spectral index of 3.90 ± 0.06, while an excess/deficit of polarization amplitude is found at 40/220 GHz compared with a Rayleigh scattering spectrum. These results are consistent with Rayleigh-scattering-dominated cloud polarization, with possible effects from supercooled water absorption and/or Mie scattering from a population of large cloud particles that contribute to the 220 GHz polarization.

There is no Open Science without infrastructure and science community

This paper focuses on the role of national-level infrastructure in promoting open science principles, with a specific emphasis on Slovenia. It presents the latest measures implemented within the country to facilitate open science practices.The first part of the paper explores the specific measures adopted in Slovenia to promote open science. It highlights the ongoing development of regulations to implement scientific research following open science principles. The Slovenian Scientific Research and Innovation Activities Act (ZZrID) and the Resolution on the Slovenian Scientific Research and Innovation Strategy 2030 (ReZrIS30) strongly emphasise open science and encompass various measures. The paper discusses the accessibility of research data addressed by the ZZrID and the Decree on the implementation of scientific research work in accordance with the principles of open science. Furthermore, it outlines upcoming actions in education and infrastructure solutions, as outlined in the Open Science Action Plan.The second part of the paper delves into the role of the Academic and Research Network of Slovenia (ARNES), the Slovenian national research and education network (NREN). ARNES has played a pivotal role in the construction of two new national data centres dedicated to the permanent storage of research data. The paper analyses the benefits of these national solutions, which enhance infrastructure accessibility, simplify researchers' work, and provide a more efficient and secure environment for research organisations to store their data. The development of data storage services and the construction of data centres take into consideration existing best practices and collaboration with researchers and research organisations. The solutions offered by the data centres will support existinguniversity repositories and align with the objectives outlined in the Open Science Action Plan. The integration of research funding and evaluation services into a centralised platform will be thoughtfully considered, taking into account the requirements of the research community. A key objective is to enable the implementation of services and infrastructure in accordance with OpenAIRE, where ARNES will play a pivotal role. As a mandated organisation in the European Open Science Cloud (EOSC) and being in touch with best practices in the EU area as an NREN, ARNES is well-positioned to contribute to this endeavour.The paper also highlights the broader objective of improving research quality, efficiency, and responsiveness within the national open science ecosystem. It discusses the establishment of the Slovenian Open Science Community (SSOZ) as part of the tripartite European Open Science Cloud (EOSC) event, making Slovenia one of the pioneering countries in the European Union. The SSOZ, along with Slovenian representatives in the EOSC, has already addressed challenges related to the permanent preservation of research data, which will be comprehensively discussed in relation to the new data centres and associated activities.Additionally, there is a growing demand for education on repository usage and infrastructure upgrades. The Slovenian Open Science Community serves as a unifying platform for all major universities and research institutions in Slovenia, fostering collaboration among its members. It encompasses participants from various initiatives, such as the NI4OS project, which aims to promote the European Open Science Cloud (EOSC) at a regional level. Furthermore, the community includes stakeholders from the Coalition for Advancing Research Assessment (CoARA) and similar European and institutional initiatives dedicated to advancing open science principles.Drawing from the experience of the case study, the paper critically highlights how the national infrastructure can simplify researchers' work. It emphasises the role of data centres in fostering sustainable and long-term data storage solutions aligned with the principles of open science. As an NREN, ARNES already manages public research infrastructure and offers solutions that facilitate research, such as data sharing and access to EuroHPC's supercomputing infrastructure. The establishment of new data centres by ARNES will significantly enhance the infrastructure for open science in Slovenia, providing primary conditions for implementing open science principles in research work. The implementation of planned professional training programs focused on utilising the infrastructure, along with workshops on open science organised by the Central Technical Library at the University of Ljubljana specifically for researchers in Slovenia, will serve as essential catalysts for enabling research activities that align with the principles of open science. With the infrastructure in place, these initiatives will empower researchers with the necessary knowledge and skills to embrace open science practices effectively.

Overview and statistical analysis of boundary layer clouds and precipitation over the western North Atlantic Ocean

Abstract. Due to their fast evolution and large natural variability in macro- and microphysical properties, the accurate representation of boundary layer clouds in current climate models remains a challenge. One of the regions with large intermodel spread in the Coupled Model Intercomparison Project Phase 6 ensemble is the western North Atlantic Ocean. Here, statistically representative in situ measurements can help to develop and constrain the parameterization of clouds in global models. To this end, we performed comprehensive measurements of boundary layer clouds, aerosol, trace gases, and radiation in the western North Atlantic Ocean during the NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) mission. In total, 174 research flights with 574 flight hours for cloud and precipitation measurements were performed with the HU-25 Falcon during three winter (February–March 2020, January–April 2021, and November 2021–March 2022) and three summer seasons (August–September 2020, May–June 2021, and May–June 2022). Here we present a statistical evaluation of 16 140 individual cloud events probed by the fast cloud droplet probe and the two-dimensional stereo cloud probe during 155 research flights in a representative and repetitive flight strategy allowing for robust statistical data analyses. We show that the vertical profiles of distributions of the liquid water content and the cloud droplet effective diameter (ED) increase with altitude in the marine boundary layer. Due to higher updraft speeds, higher cloud droplet number concentrations (Nliquid) were measured in winter compared to summer despite lower cloud condensation nucleus abundance. Flight cloud cover derived from statistical analysis of in situ data is reduced in summer and shows large variability. This seasonal contrast in cloud coverage is consistent with a dominance of a synoptic pattern in winter that favors conditions for the formation of stratiform clouds at the western edge of cyclones (post-cyclonic). In contrast, a dominant summer anticyclone is concomitant with the occurrence of shallow cumulus clouds and lower cloud coverage. The evaluation of boundary layer clouds and precipitation in the Nliquid ED phase space sheds light on liquid, mixed-phase, and ice cloud properties and helps to categorize the cloud data. Ice and liquid precipitation, often masked in cloud statistics by a high abundance of liquid clouds, is often observed throughout the cloud. The ACTIVATE in situ cloud measurements provide a wealth of cloud information useful for assessing airborne and satellite remote-sensing products, for global climate and weather model evaluations, and for dedicated process studies that address precipitation and aerosol–cloud interactions.

Detection and analysis of Lhù'ààn Mân' (Kluane Lake) dust plumes using passive and active ground-based remote sensing supported by physical surface measurements

Abstract. There is growing recognition that high-latitude dust (HLD), originating from local drainage-basin flows, is the dominant source for certain important phenomena such as particle deposition on snow/ice. The analysis of such local plumes (including a better exploitation of remote sensing data) has been targeted as a key aerosol issue by the HLD community. The sub-Arctic Lhù'ààn Mân' (Kluane Lake) region in the Canadian Yukon is subject to regular drainage-basin, wind-induced dust plumes. This dust emission site is one of many current and potential proglacial dust sources in the Canadian north. In situ ground-based measurements are, due to constraints in accessing these types of regions, rare. Ground- and satellite-based remote sensing accordingly play an important role in helping to characterize local dust sources in the Arctic and sub-Arctic. We compared ground-based passive and active remote sensing springtime (May 2019) retrievals with microphysical surface-based measurements in the Lhù'ààn Mân' region in order to better understand the potential for ground- and satellite-based remote sensing of HLD plumes. This included correlation analyses between ground-based coarse mode (CM) aerosol optical depth (AOD) retrievals from AERONET AOD spectra, CM AODs derived from co-located Doppler lidar profiles, and OPS (optical particle sizer) surface measurements of CM particle-volume concentration (vc(0)). An automated dust classification scheme was developed to objectively identify local dust events. The classification process helped distinguish lidar-derived CM AODs which covaried with vdust(0) (during recognized dust events) and those that varied at the same columnar scale as AERONET-derived CM AOD (and thus could be remotely sensed). False positive cloud events for dust-induced, high-frequency variations in lidar-derived CM AODs in cloudless atmospheres indicated that the AERONET cloud-screening process was rejecting CM dust AODs. The persistence of a positive lidar ratio bias in comparing the CIMEL/lidar-derived value with a prescribed value obtained from OPS-derived particle sizes coupled with dust-speciation-derived refractive indices led to the suggestion that the prescribed value could be increased to optically derived values of 20 sr by the presence of optically significant dust particles at an effective radius of 11–12 µm. Bimodal CM PSDs (see Appendix B for a glossary) from full-fledged AERONET inversions (the combination of AOD spectra and almucantar radiances) also showed CM peaks at ∼ 1.3 and 5–6.6 µm radius: this, we argue, was associated with springtime Asian dust and Lhù'ààn Mân' dust, respectively. Correlations between the CIMEL-derived fine mode (FM) AOD and FM OPS-derived particle-volume concentrations suggest that remote sensing techniques can be employed to monitor FM dust (which is arguably a better indicator of the long-distance transport of HLD).

Determination of Air Stability Parameter Threshold Value for Cumulonimbus and Thunderstorm Cloud Events at Kualanamu Meteorological Station

Many studies have carried out calculations related to atmospheric lability as a reference in weather forecasts, especially cumulonimbus clouds, and thunderstorms. However, many air lability index values are found to be inappropriate in each region because conditions in each region are different from each other in the region. So it is necessary to use precise index thresholds to determine weather conditions. In the study, observational data and data from Showalter Index (SI), Lifted Index (LI), K Index (KI), Severe Weather Threat Index (SWEAT), and Convective data were used. Available Potential Energy (CAPE) for ten years (2013-2022), then statistical calculations and verification for one year (2022) are carried out. The results obtained are the atmospheric stability index with the best accuracy in predicting the presence of cumulonimbus clouds and thunderstorms at the Kualanamu Meteorological Station, Deli Serdang is the best LI index to predict TS 00 and TS 12, and the best KI index to predict CB 00 and CB 12.

Revealing the chemical characteristics of Arctic low-level cloud residuals – in situ observations from a mountain site

Abstract. The role aerosol chemical composition plays in Arctic low-level cloud formation is still poorly understood. In this study we address this issue by combining in situ observations of the chemical characteristics of cloud residuals (dried liquid cloud droplets or ice crystals) and aerosol particles from the Zeppelin Observatory in Ny-Ålesund, Svalbard (approx. 480 m a.s.l.). These measurements were part of the 1-year-long Ny-Ålesund Aerosol and Cloud Experiment 2019–2020 (NASCENT). To obtain the chemical composition of cloud residuals at molecular level, we deployed a Filter Inlet for Gases and AEROsols coupled to a Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) with iodide as the reagent ion behind a ground-based counterflow virtual impactor (GCVI). The station was enshrouded in clouds roughly 15 % of the time during NASCENT, out of which we analyzed 14 cloud events between December 2019 and December 2020. During the entire year, the composition of the cloud residuals shows contributions from oxygenated organic compounds, including organonitrates, and traces of the biomass burning tracer levoglucosan. In summer, methanesulfonic acid (MSA), an oxidation product of dimethyl sulfide (DMS), shows large contributions to the sampled mass, indicating marine natural sources of cloud condensation nuclei (CCN) and ice nucleating particle (INP) mass during the sunlit part of the year. In addition, we also find contributions of the inorganic acids nitric acid and sulfuric acid, with outstanding high absolute signals of sulfuric acid in one cloud residual sample in spring and one in late summer (21 May and 12 September 2020), probably caused by high anthropogenic sulfur emissions near the Barents Sea and Kara Sea. During one particular cloud event, on 18 May 2020, the air mass origin did not change before, during, or after the cloud. We therefore chose it as a case study to investigate cloud impact on aerosol physicochemical properties. We show that the overall chemical composition of the organic aerosol particles was similar before, during, and after the cloud, indicating that the particles had already undergone one or several cycles of cloud processing before being measured as residuals at the Zeppelin Observatory and/or that, on the timescales of the observed cloud event, cloud processing of the organic fraction can be neglected. Meanwhile, there were on average fewer particles but relatively more in the accumulation mode after the cloud. Comparing the signals of sulfur-containing compounds of cloud residuals with aerosols during cloud-free conditions, we find that sulfuric acid had a higher relative contribution to the cloud residuals than to aerosols during cloud-free conditions, but we did not observe an increase in particulate MSA due to the cloud. Overall, the chemical composition, especially of the organic fraction of the Arctic cloud residuals, reflected the overall composition of the general aerosol population well. Our results thus suggest that most aerosols can serve as seeds for low-level clouds in the Arctic.

In Situ Observation of Multiphase Oxidation-Driven Secondary Organic Aerosol Formation during Cloud Processing at a Mountain Site in Southern China

Secondary organic aerosols (SOAs) account for a large fraction of atmospheric fine particles, but the mechanisms of their formation and evolution processes remain unclear. In this study, a ground-based counterflow virtual impactor was used in combination with an online time-of-flight aerosol chemical speciation monitor to investigate the multiphase (gaseous and aqueous) oxidation processes and the SOA influencing factors during cloud events at a mountain site in southern China. Our results showed that the clouds promoted the formation of low-oxidized oxygenated organic aerosol (LO-OOA) and more-oxidized oxygenated organic aerosol (MO-OOA). At night, ozone (O3) and nitrate radicals (NO3•) oxidized volatile organic compounds (VOCs, mostly biogenic VOCs) to the precursors of LO-OOA and were absorbed in the cloud droplets to form LO-OOA, while the hydroxyl radical (•OH) was the main oxidant for LO-OOA during daytime. In the cloud droplets, LO-OOA was further oxidized by •OH to form MO-OOA. We propose that isoprene can be oxidized to form products in the gas phase and then absorbed by acidic cloud droplets to form 2-methylglyceric acid (2-MG), 2-MG-organosulfate, and 2-MG-organonitrate. This study improves our understanding of SOA formation, driven by multiphase oxidation during cloud events.

Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol.

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO2. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark. In more atmospherically relevant chamber experiments where suspended aqueous aerosol particles are exposed to gas-phase glyoxal and SO2, the formation of detectable amounts of BrC requires an OH radical source and occurs most rapidly after a cloud event. From these observations we infer that this photobrowning is caused by radical-initiated reactions as evaporation concentrates aqueous-phase reactants and aerosol viscosity increases. Positive-mode electrospray ionization mass spectrometric analysis of aerosol-phase products reveals a large number of CxHyOz oligomers that are reduced rather than oxidized (relative to glyoxal), with the degree of reduction increasing in the presence of OH radicals. This again suggests a radical-initiated redox mechanism where photolytically produced aqueous radical species trigger S(IV)-O2 auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions especially if aerosol-phase O2 is depleted. This process may contribute to daytime BrC production and aqueous-phase sulfur oxidation in the atmosphere. The BrC produced, however, is about an order of magnitude less light-absorbing than wood smoke BrC at 365 nm.

In-cloud scavenging of chemically segregated particle types by individual particle observation

While aerosol plays a significant role in the formation of cloud, the in-cloud scavenging of chemically segregated particle types were still insufficiently investigated and the controlling factors remain ambiguous. Several field observations were carried out during 2018–2021 at Mt. Tianjing (1690 m a.s.l.) in southern China, based on ground-based counterflow virtual impactor (GCVI)-single particle aerosol mass spectrometer (SPAMS), to investigate the in-cloud scavenging (activation) of various particle types, including black carbon (BC), organic-containing particles (OC), mineral dust (Dust), metal-containing particles (Metal), potassium-rich particles (K-rich) and sea salt (SS). GCVI was used to sample cloud droplet residual particles (Res) while PM2.5 inlet sampled interstitial particles (Inter) during cloud events and ambient particles (Amb) on sunny days, respectively. Different types of particles were recognized based on spectral characteristics obtained by the SPAMS. Based on the coupled GCVI-SPAMS measurements, the number-based scavenging efficiencies (NSEs) of various particle types could be quantified. Besides, the influence of liquid water content (LWC), PM2.5, particle size, and mixing state on the NSEs were discussed. The NSE of SS was the highest (32.4%) while OC showed the lowest NSE (9.1%). Other particle types shared similar NSEs with all the detected particles, with an average of ∼20–25%. Apart from OC, NSEs of other types of particles increased with the increase of LWC and decreased with the increase of the concentration of PM2.5, suggesting that numerous PM2.5 suppressed particles entering cloud droplets via competing for water vapor. The annual variations of NSEs were generally in accordance with LWC and PM2.5, reflecting the certain role of these environment conditions. The NSEs typically increased with particle size in a range of 0.2–2.0 μm, reflecting the dominant nucleation mechanism during in-cloud scavenging. Particles mixed with sulfate and nitrate were easier to be scavenged than those coated with organic compounds, e.g., the discrepancy reached 11.8% for the K-rich particles.

Clouds in the Vicinity of the Stratopause Observed with Lidars at Midlatitudes (40.5–41°N) in China

Based on long-term lidar (light detection and ranging) observations at Yanqing (40.5°N, 116°E) and Pingquan (41°N, 118.7°E), cloud events occurred in the vicinity of the stratopause above Beijing were reported for the first time. These events occurred with tenuous and sparse layers within the altitude range of 33–65 km, and the maximum VBSC value ranged from 1×10−10m−1sr−1 to 5.5×10−9m−1sr−1. Considering temperature and water vapor measurements from SABER/TIMED, the occurrence mechanism of these lidar-observed cloud events was examined. It was found that some cloud layers resulted from the nucleation of water vapor due to the local meteorological changes in the middle atmosphere, while other lidar-observed clouds could comprise floating clusters of cosmic dust, hydrate droplets, volcanic ash, space traffic exhaust, etc. These cloud events are rare cloud-like phenomena in the middle atmosphere observed by lidars at midlatitudes in China; they differ from NLCs and PSCs in terms of altitude distribution and seasonal variation, and the relevant microphysics processes behind their occurrence are likely meaningful to meteorology at midlatitudes.

Atmospheric Nitrated Phenolic Compounds in Particle, Gaseous, and Aqueous Phases During Cloud Events at a Mountain Site in North China: Distribution Characteristics and Aqueous‐Phase Formation

AbstractNitrated phenolic compounds in the atmosphere are receiving increasing attention due to their light absorption and biological toxicity. In this study, particulate, gaseous, and cloud water samples were simultaneously collected during cloud events at the summit of Mount Tai in northern China in spring, summer, and winter and the contents of 11 nitrated phenolic compounds were determined. The seasonal average concentrations of the total nitrated phenolic compounds in particles were in the range of 7.3–27.1 ng m−3, a little lower than those measured in the gas‐phase (18.3–70.6 ng m−3). Their concentrations in cloud water were at the levels of 168.4–438.5 μg L−1. 4‐Nitrophenol and nitrosalicylic acids were the dominant compounds in particles, while 4‐nitrophenol and 2,4‐dinitrophenol were the most abundant in the gas phase and cloud water samples. During cloud events, most nitrated phenolic compounds were mainly distributed in the particle phase, except dinitrophenols which were mainly distributed in the gas phase in winter. The field‐derived effective Henry's law coefficients were several orders of magnitude higher than their theoretical values in pure water. Moreover, the measured concentrations of particulate nitrated phenolic compounds were substantially greater than the theoretical predictions, especially in spring. The above results indicate that nitrated phenolic compounds were partly formed via aqueous‐phase reactions inside the cloud droplets or on the wet particle surfaces, which changed their distribution patterns. The much higher ratios of 2,4‐dinitrophenol to the sum of 4‐nitrophenol and 5‐nitrosalicylic acid in cloud water than those in particles further confirm the enhanced formation via aqueous processes.