Cloud Properties Research Articles

Long-term variability in immersion-mode marine ice-nucleating particles from climate model simulations and observations

Abstract. Ice-nucleating particles (INPs) in the Southern Ocean (SO) atmosphere have significant impacts on cloud radiative and microphysical properties. Yet, INP prediction skill in climate models remains poorly understood, in part because of the lack of long-term measurements. Here we show, for the first time, how model-simulated INP concentrations compare with year-round INP measurements during the Macquarie Island Cloud Radiation Experiment (MICRE) campaign from 2017–2018. We simulate immersion-mode INP concentrations using the Energy Exascale Earth System Model version 1 (E3SMv1) by combining simulated aerosols with recently developed deterministic INP parameterizations and the native classical nucleation theory (CNT) for mineral dust in E3SMv1. Because MICRE did not collect aerosol measurements of super-micron particles, which are more effective ice nucleators, we evaluate the model's aerosol fields at other high-latitude sites using long-term in situ observations of dust and sea spray aerosol. We find that the model underestimates dust and overestimates sea spray aerosol concentrations by 1 to 2 orders of magnitude for most of the high-latitude sites in the Southern Hemisphere. We next compare predicted INP concentrations with concentrations of INPs collected on filter samples (typically for 2 or 3 d) and processed offline using the Colorado State University ice spectrometer (IS) in immersion freezing mode. We find that when deterministic parameterizations for both dust and sea spray INPs are used, simulated INPs are within a factor of 10 of observed INPs more than 60 % of the time during summer. Our results also indicate that the E3SM's current treatment of mineral dust immersion freezing in the SO is impacted by compensating biases – an underprediction of dust amount was compensated by an overprediction of its effectiveness as INPs. We also perform idealized droplet freezing experiments to quantify the implications of the time-dependent behavior assumed by the E3SM's CNT-parameterization and compare with the ice spectrometer observations. We find that the E3SM CNT 10 s diagnostic used in this study is a reasonable approximation of the exact formulation of CNT, when applied to ice spectrometer measurements in low-INP conditions similar to Macquarie Island. However, the linearized 10 s diagnostic underestimates the exact formula by an order of magnitude or more in places with high-INP conditions like the Sahara. Overall, our findings suggest that it is important to correct the biases in E3SM's simulated dust life cycle and update E3SM's INP parameterizations. INP prediction errors of 2 to 3 orders of magnitude can have considerable impacts on the simulated cloud and radiative properties in global climate models. On comparing INP concentrations during MICRE against ship-based campaigns, Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) and Antarctic Circumnavigation Expedition (ACE), we find that INPs from the latter are significantly higher only in regions closer to Macquarie Island. This alludes to the fact that physical, chemical and biological processes affecting INP concentrations as stimulated by the island could be partly responsible for the high INP concentrations observed at Macquarie Island during the MICRE campaign. Therefore, improvements to both aerosol simulation and INP parameterizations are required to adequately simulate INPs and their cloud impacts in E3SM. It will be helpful to include a parallel measurement of the size-resolved aerosol composition and explore opportunities for long-term measurement platforms in future field campaigns studying INP sources in remote marine regions.

Marine Cold‐Air Outbreak Snowfall in the North Atlantic: A CloudSat Perspective

AbstractThis study analyzes the influence of marine cold‐air outbreaks (MCAO) on snowfall and cloud properties in the North Atlantic Ocean using CloudSat observations. Comparing reanalysis‐determined MCAO conditions (low‐level instability) against “non‐CAO” conditions, we find that MCAO conditions are associated with predominantly light snowfall rates (<2 mm day−1 liquid water equivalent) whereas non‐CAO conditions are more frequently associated with higher snowfall rates. Near cold‐air sources, such as sea ice or cold continents, MCAO‐forced snowfall rates tend to be more frequent and more intense. Additionally, 76% of snowing clouds identified during MCAO conditions are shallow (mean cloud top height <3 km) stratocumulus, whereas 44% (43%) of clouds in non‐CAO conditions are deeper nimbostratus (stratocumulus). With greater boundary layer instability (stronger MCAO conditions), CloudSat observes higher cloud‐top heights, reflecting a deepening boundary layer and the presence of two distinct cloud modes during MCAO conditions.

Observational Investigation of the Effect of Turbulence on Microphysics and Precipitation in Warm Marine Boundary Layer Clouds

AbstractTurbulence is omnipresent in the marine boundary layer (MBL) and several mechanisms have been proposed regarding its role on MBL cloud microphysics. In particular, past modeling studies suggested that increasing turbulence broaden the droplet size distribution (DSD) and leads to precipitation initiation, this suggested relationship has not been evaluated yet using long‐term observations. Here, comprehensive retrievals from long‐term ground‐based observations are used to explore the impact of turbulence on microphysics and precipitation in drizzling MBL clouds. Our analysis supports that broader DSD and larger droplets are observed with strong turbulence, but interestingly, the precipitation is reduced instead of being enhanced. This suppression mechanism is mainly due to the entrainment and mixing process in which the entrained dry air evaporates cloud water. This study provides observational evidence to show the complex effect of turbulence on MBL cloud properties and implies the potential influence of turbulence on the clouds lifetime.

Regional Characteristics of Cloud Properties over the Loess Plateau

As an important meteorological element, clouds play an important role in the radiative transfer process and atmospheric and water circulation. The Loess Plateau is the largest arid and semi-arid area in China, with a fragile ecological environment. However, few scholars have studied the spatial and temporal variations in cloud properties in the Loess Plateau. Therefore, in this study, cloud properties in the Loess Plateau were analyzed at the annual, seasonal, and diurnal scales based on Himawari-8 cloud products. The results show that cloud frequency (CF), cloud optical thickness (COT) and cloud effective radius (CER) show obvious spatial discrepancies in the Loess Plateau. Regions with high CF and COT values are mainly concentrated in the southern part of the Loess Plateau. In general, areas with high CER values also have low COT values. The highest CF values are observed in summer, and the highest COT values mainly appear in autumn. However, the highest CER values mainly appear in spring and winter. In terms of the diurnal variation, the CF is high at midday and low in the morning and afternoon, while the diurnal variation in COT values is the opposite: there are high COT values in the morning and afternoon and low values at midday. The CER values show an increasing trend from morning to afternoon and reach a maximum at 17:00 BJT. High CF values in the southern Loess Plateau and in summer relate to surface water and heat conditions; the vegetation cover, total column water vapor and temperature values are relatively high in this area. High COT values in the southern Loess Plateau are associated with sufficient water vapor levels and high levels of aerosol optical thickness. However, high CER levels in the northern Loess Plateau and in spring and winter may be caused by a higher nucleation rate related to the colder temperature. Moreover, more factors could influence CER, i.e., water vapor and aerosols, but they show complex relationships with the CER which need further explored.

Oxidized organic molecules in the tropical free troposphere over Amazonia.

New particle formation (NPF) in the tropical free troposphere (FT) is a globally important source of cloud condensation nuclei, affecting cloud properties and climate. Oxidized organic molecules (OOMs) produced from biogenic volatile organic compounds are believed to contribute to aerosol formation in the tropical FT, but without direct chemical observations. We performed in situ molecular-level OOMs measurements at the Bolivian station Chacaltaya at 5240 m above sea level, on the western edge of Amazonia. For the first time, we demonstrate the presence of OOMs, mainly with 4-5 carbon atoms, in both gas-phase and particle-phase (in terms of mass contribution) measurements in tropical FT air from Amazonia. These observations, combined with air mass history analyses, indicate that the observed OOMs are linked to isoprene emitted from the rainforests hundreds of kilometers away. Based on particle-phase measurements, we find that these compounds can contribute to NPF, at least the growth of newly formed nanoparticles, in the tropical FT on a continental scale. Thus, our study is a fundamental and significant step in understanding the aerosol formation process in the tropical FT.

Aerosol first indirect effect of African smoke at the cloud base of marine cumulus clouds over Ascension Island, southern Atlantic Ocean

Abstract. The interactions between aerosols and clouds are among the least understood climatic processes and were studied over Ascension Island. A ground-based UV polarization lidar was deployed on Ascension Island, which is located in the stratocumulus-to-cumulus transition zone of the southeastern Atlantic Ocean, to infer cloud droplet sizes and droplet number density near the cloud base of marine boundary layer cumulus clouds. The aerosol–cloud interaction (ACI) due to the presence of smoke from the African continent was determined during the monsoonal dry season. In September 2016, a cloud droplet number density ACIN of 0.3 ± 0.21 and a cloud effective radius ACIr of 0.18 ± 0.06 were found, due to the presence of smoke in and under the clouds. Smaller droplets near the cloud base makes them more susceptible to evaporation, and smoke in the marine boundary layer over the southeastern Atlantic Ocean will likely accelerate the stratocumulus-to-cumulus transition. The lidar retrievals were tested against more traditional radar–radiometer measurements and shown to be robust and at least as accurate as the lidar–radiometer measurements. The lidar estimates of the cloud effective radius are consistent with previous studies of cloud base droplet sizes. The lidar has the large advantage of retrieving both cloud and aerosol properties using a single instrument.

Across-track extension of retrieved cloud and aerosol properties for the EarthCARE mission: the ACMB-3D product

Abstract. The narrow cross section of cloud and aerosol properties retrieved by L2 algorithms that operate on data from EarthCARE's nadir-pointing sensors is broadened across-track by an algorithm that is described and demonstrated here. This scene construction algorithm (SCA) consists of four components. At its core is a radiance-matching procedure that works with measurements made by EarthCARE's Multi-Spectral Imager (MSI). In essence, an off-nadir pixel gets filled with retrieved profiles that are associated with a (nearby) nadir pixel whose MSI radiances best match those of the off-nadir pixel. The SCA constructs a 3D array of cloud and aerosol (and surface) properties for entire frames that measure ∼6000 km along-track by 150 km across-track (i.e., the MSI's full swath). Constructed domains out to ∼15 km across-track on both sides of nadir are used explicitly downstream as input for 3D radiative transfer models that predict top-of-atmosphere (TOA) broadband solar and thermal fluxes and radiances. These quantities are compared to commensurate measurements made by EarthCARE's Broadband Radiometer (BBR), thus facilitating a continuous closure assessment of the retrievals. Three 6000 km ×200 km frames of synthetic EarthCARE observations were used to demonstrate the SCA. The main conclusion is that errors in modelled TOA fluxes that stem from use of 3D domains produced by the SCA are expected to be less than ±5 W m−2 and rarely larger than ±10 W m−2. As such, the SCA, as purveyor of information needed to run 3D radiative transfer models, should help more than hinder the radiative closure assessment of EarthCARE's L2 retrievals.

WISDOM Project – XV. Giant molecular clouds in the central region of the barred spiral galaxy NGC 5806

ABSTRACT We present high spatial resolution (≈24 pc) Atacama Large Millimeter/sub-millimeter Array 12CO(2-1) observations of the central region of the nearby barred spiral galaxy NGC 5806. NGC 5806 has a highly structured molecular gas distribution with a clear nucleus, a nuclear ring, and offset dust lanes. We identify 170 spatially and spectrally resolved giant molecular clouds (GMCs). These clouds have comparable sizes (Rc) and larger gas masses, observed linewidths (σobs, los), and gas mass surface densities than those of clouds in the Milky Way disc. The size–linewidth relation of the clouds is one of the steepest reported so far ($\sigma _{\mathrm{obs,los}}\propto R_{\mathrm{c}}^{1.20}$ ), the clouds are on average only marginally bound (with a mean virial parameter 〈αvir〉 ≈ 2), and high velocity dispersions are observed in the nuclear ring. These behaviours are likely due to bar-driven gas shocks and inflows along the offset dust lanes, and we infer an inflow velocity of ≈120 km s−1 and a total molecular gas mass inflow rate of ≈5 M⊙ yr−1 into the nuclear ring. The observed internal velocity gradients of the clouds are consistent with internal turbulence. The number of clouds in the nuclear ring decreases with azimuthal angle downstream from the dust lanes without clear variation of cloud properties. This is likely due to the estimated short lifetime of the clouds (≈6 Myr), which appears to be mainly regulated by cloud–cloud collision and/or shear processes. Overall, it thus seems that the presence of the large-scale bar and gas inflows to the centre of NGC 5806 affect cloud properties.

Direct Radiative Effects of Aerosols on Numerical Weather Forecasts—A Comparison of Two Aerosol Datasets in the NCEP GFS

Abstract This study compares aerosol direct radiative effects on numerical weather forecasts made by the NCEP Global Forecast System (GFS) with two different aerosol datasets, the Optical Properties of Aerosols and Clouds (OPAC) and MERRA-2 aerosol climatologies. The underestimation of aerosol optical depth (AOD) by OPAC over northwest Africa, central to East Africa, the Arabian Peninsula, Southeast Asia, and the Indo-Gangetic Plain, and overestimation in the storm-track regions in both hemispheres are reduced by MERRA-2. Surface downward shortwave (SW) and longwave (LW) fluxes and the top-of-the-atmosphere SW and outgoing LW fluxes from model forecasts are compared with CERES satellite observations. Forecasts made with OPAC aerosols have large radiative flux biases, especially in northwest Africa and the storm-track regions. These biases are also reduced in the forecasts made with MERRA-2 aerosols. The improvements from MERRA-2 are most noticeable in the surface downward SW fluxes. GFS medium-range weather forecasts made with the MERRA-2 aerosols demonstrated slightly improved forecast accuracy of sea level pressure and precipitation over the Indian and East Asian summer monsoon region. A stronger Africa easterly jet is produced, associated with a low pressure over the east Atlantic Ocean and west of northwest Africa. Impacts on large-scale skill scores such as 500-hPa geopotential height anomaly correlation are generally positive in the Northern Hemisphere and the Pacific and North American regions in both the winter and summer seasons.

CLIMCAPS—A NASA Long‐Term Product for Infrared + Microwave Atmospheric Soundings

AbstractThe Community Long‐term Infrared Microwave Combined Atmospheric Product System (CLIMCAPS) spans more than two decades of soundings from infrared and microwave instrument pairs on Aqua, Suomi‐NPP, and NOAA‐20. CLIMCAPS builds on three decades of research within the National Aeronautics and Space Administration Atmospheric Infrared Sounder Science Team (AST) to retrieve profiles of atmospheric temperature, water vapor, ozone, and many other trace gases as well as cloud and surface properties. In this paper, we highlight four innovative aspects of the AST retrieval methodology that allows the CLIMCAPS record (2002 to present) to bridge technology differences, maintain global coverage and stay updated with fast reprocessing times. CLIMCAPS improves on the AST method with rigorous optimal estimation uncertainty quantification for improved understanding of the signal‐to‐noise in sounding retrievals to allow their use in scientific research.

Effects of Circulation on Tropical Cloud Feedbacks in High‐Resolution Simulations

AbstractUncertainty in the response of clouds to global warming remains a significant barrier to reducing uncertainty in climate sensitivity. A key question is the extent to which the dynamic component—that which is due to changes in circulation rather than changes in the thermodynamic properties of clouds—contributes to the total cloud feedback. Here, simulations with a range of cloud‐resolving models are analyzed to quantify the impact of circulation changes on tropical cloud feedbacks. The dynamic component of the cloud feedback is substantial for some models and is controlled both by sea surface temperature (SST) induced changes in circulation and nonlinearity in the climatological relationship between clouds and circulation. We find notable inter‐model differences in the extent to which ascending regions narrow or expand in response to a change in SST, which we link to differences in the longwave and shortwave dynamic components across models. The diversity of changes in ascent area is coupled to intermodel differences in non‐radiative diabatic heating in ascending regions.

CLASSY. VI. The Density, Structure, and Size of Absorption-line Outflows in Starburst Galaxies

Galaxy formation and evolution are regulated by the feedback from galactic winds. Absorption lines provide the most widely available probe of winds. However, since most data only provide information integrated along the line of sight, they do not directly constrain the radial structure of the outflows. In this paper, we present a method to directly measure the gas electron density in outflows (n e ), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling factor, and sizes/masses). We also estimate the distance (r n ) from the starburst at which the observed densities are found. We focus on 22 local star-forming galaxies primarily from the COS Legacy Archive Spectroscopic SurveY (CLASSY). In half of them, we detect absorption lines from fine-structure excited transitions of Si ii (i.e., Si ii*). We determine n e from relative column densities of Si ii and Si ii*, given Si ii* originates from collisional excitation by free electrons. We find that the derived n e correlates well with the galaxy’s star formation rate per unit area. From photoionization models or assuming the outflow is in pressure equilibrium with the wind fluid, we get r n ∼ 1–2r * or ∼5r *, respectively, where r * is the starburst radius. Based on comparisons to theoretical models of multiphase outflows, nearly all of the outflows have cloud sizes large enough for the clouds to survive their interaction with the hot wind fluid. Most of these measurements are the first ever for galactic winds detected in absorption lines and, thus, will provide important constraints for future models of galactic winds.

Near-Cloud Atmospheric Ingredients for Deep Convection Initiation

Abstract A lack of routine environmental observations located near deepening cumulus congestus clouds limits verification of important theorized and simulated updraft–environment interaction processes occurring during deep convection initiation (CI). We analyze radiosonde profiles collected during several hundred CI events near a mountain range in central Argentina during the CACTI field campaign. Statistical analyses illustrate environmental conditions supporting radar-observed CI outcomes that span a spectrum of convective cell depths, widths, and durations, as well as events lacking precipitating convection. Tested environmental factors include a large variety of sounding-derived measurements of CAPE, CIN, moisture, terrain-relative winds, vertical shear, and lifted parcel properties, with supplemental model reanalysis of background larger-scale vertical motion. CAPE and CIN metrics do not consistently differentiate CI success from failure. Only a few environmental factors contain consistent monotonic relationships among the spectrum of cloud depths achieved during CI: (i) the depth and strength of background ascent, and (ii) the component of low-level flow oriented parallel to the ridgeline. These metrics suggest that the ability of the surrounding flow to lift parcels to their LFC and terrain-modified flow are consistently relevant processes for CI. Low- to midlevel relative humidity strongly discriminated between CI and non-CI events, likely reflecting entrainment-driven dilution processes. However, we could not confidently conclude that relative humidity similarly discriminated robust from marginal CI events. Circumstantial evidence was found linking cell width, an important cloud property governing the probability of CI, to LCL height, boundary layer depth, depth and magnitude of the CIN layer, and ambient wind shear.

Large Interferometer For Exoplanets (LIFE)

Context.Terrestrial exoplanets in the habitable zone are likely a common occurrence. The long-term goal is to characterize the atmospheres of dozens of such objects. The Large Interferometer For Exoplanets (LIFE) initiative aims to develop a space-based mid-infrared (MIR) nulling interferometer to measure the thermal emission spectra of such exoplanets.Aims.We investigate how well LIFE could characterize a cloudy Venus-twin exoplanet. This allows us to: (1) test our atmospheric retrieval routine on a realistic non-Earth-like MIR emission spectrum of a known planet, (2) investigate how clouds impact retrievals, and (3) further refine the LIFE requirements derived in previous Earth-centered studies.Methods.We ran Bayesian atmospheric retrievals for simulated LIFE observations of a Venus-twin exoplanet orbiting a Sun-like star located 10 pc from the observer. The LIFESIMnoise model accounted for all major astrophysical noise sources. We ran retrievals using different models (cloudy and cloud-free) and analyzed the performance as a function of the quality of the LIFE observation. This allowed us to determine how well the atmosphere and clouds are characterizable depending on the quality of the spectrum.Results.At the current minimal resolution (R= 50) and signal-to-noise (S/N= 10 at 11.2 μ m) requirements for LIFE, all tested models suggest a CO2-rich atmosphere (≥30% in mass fraction). Further, we successfully constrain the atmospheric pressure-temperature (P–T) structure above the cloud deck (P–Tuncertainty ≤ ± 15 K). However, we struggle to infer the main cloud properties. Further, the retrieved planetary radius (Rpl), equilibrium temperature (Teq), and Bond albedo (AB) depend on the model. Generally, a cloud-free model performs best at the current minimal quality and accurately estimatesRpl,Teq, andAB. If we consider higher quality spectra (especiallyS/N= 20), we can infer the presence of clouds and pose first constraints on their structure.Conclusions.Our study shows that the minimal R and S/N requirements for LIFE suffice to characterize the structure and composition of a Venus-like atmosphere above the cloud deck if an adequate model is chosen. Crucially, the cloud-free model is preferred by the retrieval for low spectral qualities. We thus find no direct evidence for clouds at the minimalRandS/Nrequirements and cannot infer the thickness of the atmosphere. Clouds are only constrainable in MIR retrievals of spectra withS/N≥ 20. The model dependence of our retrieval results emphasizes the importance of developing a community-wide best-practice for atmospheric retrieval studies.

Sea2Cloud: From Biogenic Emission Fluxes to Cloud Properties in the Southwest Pacific

Abstract The goal of the Sea2Cloud project is to study the interplay between surface ocean biogeochemical and physical properties, fluxes to the atmosphere, and ultimately their impact on cloud formation under minimal direct anthropogenic influence. Here we present an interdisciplinary approach, combining atmospheric physics and chemistry with marine biogeochemistry, during a voyage between 41° and 47°S in March 2020. In parallel to ambient measurements of atmospheric composition and seawater biogeochemical properties, we describe semicontrolled experiments to characterize nascent sea spray properties and nucleation from gas-phase biogenic emissions. The experimental framework for studying the impact of the predicted evolution of ozone concentration in the Southern Hemisphere is also detailed. After describing the experimental strategy, we present the oceanic and meteorological context including provisional results on atmospheric thermodynamics, composition, and flux measurements. In situ measurements and flux studies were carried out on different biological communities by sampling surface seawater from subantarctic, subtropical, and frontal water masses. Air–Sea-Interface Tanks (ASIT) were used to quantify biogenic emissions of trace gases under realistic environmental conditions, with nucleation observed in association with biogenic seawater emissions. Sea spray continuously generated produced sea spray fluxes of 34% of organic matter by mass, of which 4% particles had fluorescent properties, and which size distribution resembled the one found in clean sectors of the Southern Ocean. The goal of Sea2Cloud is to generate realistic parameterizations of emission flux dependences of trace gases and nucleation precursors, sea spray, cloud condensation nuclei, and ice nuclei using seawater biogeochemistry, for implementation in regional atmospheric models.

Organic aerosols in the inland Tibetan Plateau: New insights from molecular tracers

Aerosols affect the radiative forcing of the global climate and cloud properties. Organic aerosols are among the most important, yet least understood, components of the sensitive Tibetan Plateau atmosphere. Here, the concentration of and the seasonal and diurnal variations in biomass burning and biogenic aerosols, and their contribution to organic aerosols in the inland Tibetan Plateau were investigated using molecular tracers. Biomass burning tracers including levoglucosan and its isomers, and aromatic acids showed higher concentrations during winter than in summer. Molecular tracers of primary and secondary biogenic organic aerosols were more abundant during summer than those in winter. Meteorological conditions were the main factors influencing diurnal variations in most organic molecular tracers during both seasons. According to the tracer-based method, we found that biogenic secondary organic aerosols (38.5 %) and fungal spores (14.4 %) were the two dominant contributors to organic aerosols during summer, whereas biomass burning (15.4 %) was an important aerosol source during winter at remote continental background site. Results from the positive matrix factor source apportionment also demonstrate the importance of biomass burning and biogenic aerosols in the inland Tibetan Plateau. During winter, the long-range transport of biomass burning from South Asia contributes to organic aerosols. In contrast, the precursors, biogenic secondary organic aerosols, and fungal spores from local emissions/long-range transport are the major sources of organic aerosols during summer. Further investigation is required to distinguish between local emissions and the long-range transport of organic aerosols. In-depth insights into the organic aerosols in the Tibetan Plateau are expected to reduce the uncertainties when evaluating aerosol effects on the climate system in the Tibetan Plateau.

An Intercomparison of Large‐Eddy Simulations of a Convection Cloud Chamber Using Haze‐Capable Bin and Lagrangian Cloud Microphysics Schemes

AbstractRecent in situ observations show that haze particles exist in a convection cloud chamber. The microphysics schemes previously used for large‐eddy simulations of the cloud chamber could not fully resolve haze particles and the associated processes, including their activation and deactivation. Specifically, cloud droplet activation was modeled based on Twomey‐type parameterizations, wherein cloud droplets were formed when a critical supersaturation for the available cloud condensation nuclei (CCN) was exceeded and haze particles were not explicitly resolved. Here, we develop and adapt haze‐capable bin and Lagrangian microphysics schemes to properly resolve the activation and deactivation processes. Results are compared with the Twomey‐type CCN‐based bin microphysics scheme in which haze particles are not fully resolved. We find that results from the haze‐capable bin microphysics scheme agree well with those from the Lagrangian microphysics scheme. However, both schemes significantly differ from those from a CCN‐based bin microphysics scheme unless CCN recycling is considered. Haze particles from the recycling of deactivated cloud droplets can strongly enhance cloud droplet number concentration due to a positive feedback in haze‐cloud interactions in the cloud chamber. Haze particle size distributions are more realistic when considering solute and curvature effects that enable representing the complete physics of the activation process. Our study suggests that haze particles and their interactions with cloud droplets may have a strong impact on cloud properties when supersaturation fluctuations are comparable to mean supersaturation, as is the case in the cloud chamber and likely is the case in the atmosphere, especially in polluted conditions.

Integrated efficient radiative transfer model named Dayu for simulating the imager measurements in cloudy atmospheres.

Rapid radiative transfer models are crucial to remote sensing and data assimilation. An integrated efficient radiative transfer model named Dayu, which is an updated version of the Efficient Radiative Transfer Model (ERTM) is developed to simulate the imager measurements in cloudy atmospheres. In Dayu model, the Optimized alternate Mapping Correlated K-Distribution model (OMCKD) which is predominant in dealing with the overlap of multiple gaseous lines is employed to efficiently calculate the gaseous absorption. The cloud and aerosol optical properties are pre-calculated and parameterized by the particle effective radius or length. Specifically, the ice crystal model is assumed as a solid hexagonal column, of which the parameters are determined based on massive aircraft observations. For the radiative transfer solver, the original 4-stream Discrete ordinate aDding Approximation (4-DDA) is extended to 2N-DDA (2N is the number of streams) which can calculate not only the azimuthally dependent radiance in the solar spectrum (including solar and infrared spectra overlap) but also the azimuthally averaged radiance in the thermal infrared spectrum through a unified adding method. Then the accuracy and efficiency of Dayu model are evaluated by comparing it with the benchmark model, i.e., Line-By-Line Radiative Transfer Model (LBLRTM) and DIScrete Ordinate Radiative Transfer (DISORT). Under the standard atmospheric profile, the maximum relative biases between Dayu model with 8-DDA / 16-DDA and the benchmark model (OMCKD with 64-stream DISORT) are 7.63% / 2.62% at solar channels but decreased to 2.66% / 1.39% at spectra-overlapping channel (3.7 μm). The computational efficiency of Dayu model with 8-DDA / 16-DDA is approximately three / two orders of magnitude higher than that of the benchmark model. At thermal infrared channels, the brightness temperature (BT) differences between Dayu model with 4-DDA and the benchmark model (LBLRTM with 64-stream DISORT) are bounded by 0.65K. Compared to the benchmark model, Dayu model with 4-DDA improves the computational efficiency by five orders of magnitude. In the application to the practical Typhoon Lekima case, the simulated reflectances and BTs by Dayu model have a high consistency with the imager measurements, demonstrating the superior performance of Dayu model in satellite simulation.

Satellite observations of smoke–cloud–radiation interactions over the Amazon rainforest

Abstract. The Amazon rainforest routinely experiences intense and long-lived biomass burning events that result in smoke plumes that cover vast regions. The spatial and temporal extent of the plumes and the complex pathways through which they interact with the atmosphere have proved challenging to measure for purposes of gaining a representative understanding of smoke impacts on the Amazonian atmosphere. In this study, we use multiple collocated satellite sensors on board AQUA and TERRA platforms to study the underlying smoke–cloud–radiation interactions during the diurnal cycle. An 18-year time series for both morning and afternoon overpasses is constructed, providing collocated measurements of aerosol optical depth (AOD; column-integrated aerosol extinction), cloud properties, top-of-atmosphere radiative fluxes, precipitation, and column water vapour content from independent sources. The long-term time series reduces the impact of interannual variability and provides robust evidence that smoke significantly modifies the Amazonian atmosphere. Low loadings of smoke (AOD ≤ 0.4) enhance convective activity, cloudiness, and precipitation, but higher loadings (AOD > 0.4) strongly suppress afternoon convection and promote low-level cloud occurrence. Accumulated precipitation increases with convective activity but remains elevated under high smoke loadings, suggesting fewer but more intense convective cells. Contrasting morning and afternoon cloud responses to smoke are observed, in line with recent simulations. Observations of top-of-atmosphere radiative fluxes support the findings and show that the response of low-level cloud properties and cirrus coverage to smoke results in a pronounced and consistent increase in top-of-atmosphere outgoing radiation (cooling) of up to 50 W m−2 for an AOD perturbation of +1.0. The results demonstrate that smoke strongly modifies the atmosphere over the Amazon via widespread changes to the cloud field properties. Rapid adjustments work alongside instantaneous radiative effects to drive a stronger cooling effect from smoke than previously thought, whilst contrasting morning and afternoon responses of liquid and ice water paths highlight a potential method for constraining aerosol impacts on climate. Increased drought susceptibility, land use change, and deforestation will have important and widespread impacts on the region over the coming decades. Based on this analysis, we anticipate that further increases in anthropogenic fire activity will associated with an overall reduction in regional precipitation and a negative forcing (cooling) on the Earth's energy budget.

Estimating the Radiance in Atmosphere-Ocean Systems With Different Atmosphere Models by Discrete Spherical Harmonics Method

Abstract The radiative transfer in atmosphere-ocean systems with different atmosphere models is evaluated by the discrete spherical harmonics method. Four standard atmosphere models, namely, Tropical, Mid-Latitude Summer, Mid-Latitude Winter, and U.S. Standard (1976) limited to a height of 16 km are considered above an ocean. Two monochromatic radiations are considered according to the preponderance of the interaction they present with the participating medium, namely, λ=0.55 μm for scattering by particles and λ=16.8 μm for absorption and emission by water vapor and carbon dioxide. The absorption by the atmospheric gases considered is analyzed by the statistical narrow-band correlated-k method. The optical properties of aerosols and water clouds considered are calculated by Lorenz-Mie theory. The results obtained by the proposed discrete spherical harmonics method are in agreement with those of the literature and demonstrate the efficiency and accuracy of the developed radiative transfer code. The effects of the governing parameters of the system are investigated and show that the presence of the ocean contributes to increasing the upward radiation fluxes in the atmosphere. The presence of aerosols in the atmosphere leads to downward radiance curves at ground level that show significant peaks around the zenith angle of observation θ=0 deg. Additionally, the presence of the cloud in the atmosphere creates a discontinuity in the radiation flux curves at the height of the cloud.