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Remote Sensing of Aerosol, Cloud and Their Interactions

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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Atmospheric Remote Sensing".

Deadline for manuscript submissions: closed (1 November 2023) | Viewed by 17267

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Special Issue Editors

Prof. Dr. Jinming Ge

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Guest Editor

College of Atmospheric Sciences, Lanzhou University, Lanzhou 730000, China
Interests: aerosol and cloud optical properties; radiative effects

Dr. Jing Li

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Guest Editor

School of Physics, Peking University, Beijing 100871, China
Interests: aerosol remote sensing and radiative effects
Special Issues, Collections and Topics in MDPI journals

Dr. Yannian Zhu

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Guest Editor

School of Atmospheric Sciences, Nanjing University, Nanjing 210023, China
Interests: aerosol-cloud-precipitation interactions

Dr. Zeen Zhu

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Guest Editor

Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY 11973, USA
Interests: cloud microphyscis and dynamics; remote sensing; radar technology

Special Issue Information

Dear Colleagues,

Aerosols and clouds play important roles in the Earth’s climate system by modulating the radiative energy balance and the hydrological cycle. The large variability of the cloud/aerosol optical property distributions and the complex aerosol cloud interaction (ACI) lead to considerable discrepancies in the aerosol climate effects among climate models, and are considered as the largest uncertainty for future climate prediction. The widely used remote sensing instruments (e.g., radar and lidar, radiometer and imager) are powerful in detecting aerosols, clouds and radiation with high resolution and global coverage. The development and utilization of the retrieval algorithms for these active and passive sensors during the recent decades provide encouraging and promising opportunities for advancing the understanding of cloud/aerosol properties and for better quantifying the cloud-aerosol radiative forcing effect and further the ACI effect on climate.

This Special Issue calls for original studies with the focus on the novel remote-sensing applications on the aerosol, cloud and radiation observations. The aim is to solicit the state of the art research with the hope to improve the understanding of the cloud/aerosol properties and of the controlling mechanisms related to cloud formation, dissipation, new particle formation and ACI. Manuscripts including, but not limited to, the following topics are encouraged for submission:

Aerosol–cloud interaction;
Aerosol compostions;
Aerosol optical properties;
New particle formation;
Cloud microphysical/macrophyscial properties;
Developemnt of aerosol and cloud properties retreival algorithms;
Validaiton and application of aerosol and cloud remote sensing datasets;
Newly designed remote sensing insturments for cloud/aerosol observation.

Prof. Dr. Jinming Ge
Dr. Jing Li
Dr. Yannian Zhu
Dr. Zeen Zhu
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

cloud
aerosol
radiation
aerosol–cloud interaction
radar
lidar

Published Papers (12 papers)

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Research

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18 pages, 6346 KiB

Open AccessArticle

Cloud Overlap Features from Multi-Year Cloud Radar Observations at the SACOL Site and Comparison with Satellites

by Xuan Yang, Qinghao Li, Jinming Ge, Bo Wang, Nan Peng, Jing Su, Chi Zhang and Jiajing Du

Remote Sens. 2024, 16(2), 218; https://doi.org/10.3390/rs16020218 - 5 Jan 2024

Viewed by 772

Abstract

Cloud overlap, referring to distinct cloud layers occurring over the same location, is essential for accurately calculating the atmospheric radiation transfer in numerical models, which, in turn, enhances our ability to predict future climate change. In this study, we analyze multi-year cloud overlap properties observed from the Ka-band Zenith Radar (KAZR) at the Semi-Arid Climate and Environment Observatory of Lanzhou University’s (SACOL) site. We conduct a series of statistical analyses and determine the suitable temporal-spatial resolution of 1 h with a 360 m scale for data analysis. Our findings show that the cloud overlap parameter and total cloud fraction are maximized during winter-spring and minimized in summer-autumn, and the extreme value of decorrelation length usually lags one or two seasons. Additionally, we find the cloud overlap assumption has distinct effects on the cloud fraction bias for different cloud types. The random overlap leads to the minimum bias of the cloud fraction for Low-Middle-High (LMH), Low-Middle (LM), and Middle-High (MH) clouds, while the maximum overlap is for Low (L), Middle (M), and High (H) clouds. We also incorporate observations from satellite-based active sensors, including CloudSat, Cloud-Aerosol Lidar, and Infrared Pathfinder Satellite Observations (CALIPSO), to refine our study area and specific cases by considering the total cloud fraction and sample size from different datasets. Our analysis reveals that the representativeness of random overlap strengthens and then weakens with increasing layer separations. The decorrelation length varies with the KAZR, CloudSat-CALIPSO, CloudSat, and CALIPSO datasets, measuring 1.43 km, 2.18 km, 2.58 km, and 1.11 km, respectively. For H, MH, and LMH clouds, the average cloud overlap parameter from CloudSat-CALIPSO aligns closely with KAZR. For L, M, and LM clouds, when the level separation of cloud layer pairs are less than 1 km, the representative assumption obtained from different datasets are maximum overlap. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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18 pages, 6223 KiB

Open AccessArticle

Distributions and Direct Radiative Effects of Different Aerosol Types in North China

by Nan Peng, Jing Su, Xinyi Han, Xingzhu Deng, Weiqi Lan and Jinyan Wang

Remote Sens. 2023, 15(23), 5511; https://doi.org/10.3390/rs15235511 - 27 Nov 2023

Viewed by 786

Abstract

Different aerosol types exhibit distinct radiative effects in different regions, attributed to their unique optical characteristics and regional distributions. This study focuses on North China, which is impacted by both natural and anthropogenic aerosols with high concentrations and a variety of aerosol types. While many studies on aerosol direct radiative effects have been conducted in this region, the majority have focused on a specific type of aerosol or overall aerosol, leaving limited research on the direct radiative effects and contributions of different aerosol types. In this study, we use CALIPSO satellite data from 2011 to 2020 to investigate concentrations and distributions of different aerosol types. The results reveal that dust, polluted dust, polluted continental/smoke, and elevated smoke are the dominant aerosol types in North China. Based on the radiative closure experiment, we systematically calculate the radiative effects of different aerosol types and their corresponding contributions to the energy budget by combining satellite data with the Fu–Liou radiative transfer model. The annual average net aerosol direct radiative effect (ADRE) of North China is −6.1 and −13.43 W m⁻² at the TOA and surface, respectively, causing a net warming effect of 7.33 W m⁻² in the atmosphere. For each main aerosol type, dust contributes 93% to the shortwave ADRE in the western dust source region, while polluted dust mainly contributes 31% and 45% of the total ADRE, in Northwest China and North China Plain, respectively. Anthropogenic pollutant aerosols account for 58% of the total ADRE in Northeast China. This study holds great significance in elucidating the dominant aerosol types and their concentrations in North China, comprehending the impacts of different aerosol types on the local energy balance. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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20 pages, 884 KiB

Open AccessArticle

Velocity Dealiasing for 94 GHz Vertically Pointing MMCR with Dual-PRF Technique

by Hai Lin, Jie Wang and Junxiang Ge

Remote Sens. 2023, 15(21), 5234; https://doi.org/10.3390/rs15215234 - 3 Nov 2023

Viewed by 533

Abstract

Velocity aliasing is unavoidable for millimeter-wave cloud radar (MMCR) due to its short wavelength. In the vertically pointing MMCR, a special aliasing state called half-folding will cause the traditional postprocessing dealiasing methods used for weather radar, including the dual-PRF method, to fail. In this paper, we propose a method that applies the dual-PRF technique to spectral dealiasing. By utilizing the property that the true velocity difference between peaks should be the same in both PRFs, our method is able to solve a special case of half-folding caused by multiple peaks, which is ignored by other spectral dealiasing methods. The special case, which we call implicit half-folding, occurs in the presence of multiple peaks in a Doppler power spectrum, where none of the peaks are folded, and they appear to be in the same Nyquist interval, whereas the peaks are actually not in the same Nyquist interval. Observations from a 94 GHz vertically pointing MMCR called TJ-II were used to demonstrate various aliasing cases, including the implicit half-folding case. As a result, our method successfully solved all aliasing cases while the other method failed when the implicit half-folding case occurred. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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18 pages, 8469 KiB

Open AccessArticle

The Vertical Distribution of Ice-Nucleating Particles over the North China Plain: A Case of Cold Front Passage

by Chuan He, Yan Yin, Yi Huang, Xiang Kuang, Yi Cui, Kui Chen, Hui Jiang, Alexei Kiselev, Ottmar Möhler and Jann Schrod

Remote Sens. 2023, 15(20), 4989; https://doi.org/10.3390/rs15204989 - 17 Oct 2023

Viewed by 906

Abstract

Ice-nucleating particles (INPs) are crucial for cloud freezing processes in the atmosphere. Given the limited knowledge about the vertical distribution of INPs and its relation to aerosols in China, we present two aircraft observations of INPs over the North China Plain on 23 October 2019 and 25 October 2019, before and after a cold front passage. We used a well-established method to identify the INPs on a silicon wafer and then performed single-particle chemical composition analysis using an environmental scanning electron microscope-energy dispersive spectrometer (ESEM-EDS). The INP concentrations range from 0.1 to 9.2 L⁻¹ within activation temperatures from −20 to −29 °C. INPs are mostly concentrated within the boundary layer, and their concentration shows a decreasing trend with height (0.5~6 km) before the cold front passage. However, the highest INP concentration always appears at higher altitudes (4~5 km) after the cold front passage. The cold front passage also significantly weakens the correlations between the concentrations of INPs and aerosol particles at different sizes. The activated fraction (AF) of total aerosols increases from 10⁻⁶ to 10⁻⁴ with height from near ground to 6 km, reflecting a better nucleating capacity of the aerosols at higher altitudes. There is no obvious variation in AF after the cold front passage. Chemical analysis reveals that the INPs containing mineral dust components comprise the majority of total INPs during both flights. The proportion of pure mineral dust declines from 52.2% to 43.5% after the cold front passage while the proportion of mixed mineral dust increases from 23.9% to 45.7%, suggesting that an increased probability of aging or coating of INPs is introduced by the cold front during their long-distance transport. In addition, 88% of INPs have a diameter larger than 1 μm. This indicates that larger aerosols (>1 μm) are the major contributors to INPs at high altitudes despite their relatively low abundance. Our results demonstrate a significant impact of transport events on the sources and vertical distribution of INPs in the atmosphere. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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17 pages, 9122 KiB

Open AccessArticle

Optical Properties and Vertical Distribution of Aerosols Using Polarization Lidar and Sun Photometer over Lanzhou Suburb in Northwest China

by Mengqi Li, Xianjie Cao, Zhida Zhang, Hongyu Ji, Min Zhang, Yumin Guo, Pengfei Tian and Jiening Liang

Remote Sens. 2023, 15(20), 4927; https://doi.org/10.3390/rs15204927 - 12 Oct 2023

Viewed by 909

Abstract

To better understand aerosol vertical distribution and radiation effects, the seasonal variation and vertical distribution characteristics of aerosol optical properties were analyzed based on the aerosol extinction coefficient, depolarization ratio and backscatter Ångström exponent derived from the dual-wavelength polarization lidar at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from December 2009 to November 2012. Combining the CE-318 sun photometer, the microphysical, optical and vertical distribution characteristics of aerosol during a dust process were discussed comprehensively. The results revealed that the vertical profiles of the aerosol extinction coefficient and depolarization ratio clearly had seasonal variation characteristics. The aerosol optical depth (AOD) integrating with the aerosol extinction coefficient within 0–2 km in the spring, summer, autumn and winter accounted for 48%, 45%, 56% and 58% of the total AOD, respectively. The non-spherical feature was most distinctive in the spring, followed by the winter, autumn and summer. The particle size of aerosol in the lower layer was larger than that in the upper layer according to the vertical profile of the backscatter Ångström exponent. The cluster analysis of backward trajectory showed SACOL is dominated by dust aerosol in the spring and the mixtures of dust with anthropogenic pollution in the winter. A dust event in April 2010 was selected and the analysis showed that it mainly came from the high-altitude and long-range transportation from the Taklamakan Desert. During this period, the extinction coefficient increased up to 0.9 km⁻¹, the maximum AOD was 2.21 and the SSA ranged from 0.92 to 0.99. The radiation force in the atmosphere reached 126.15 W/m². It can be found that the influence of aerosol on the atmospheric radiation effect cannot be ignored. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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21 pages, 3660 KiB

Open AccessArticle

Spatio-Temporal Variation of Critical Relative Humidity Based on Multiple Datasets

by Weiyuan Zhang, Jiming Li, Sihang Xu, Yang Zhao and Bida Jian

Remote Sens. 2023, 15(17), 4187; https://doi.org/10.3390/rs15174187 - 25 Aug 2023

Viewed by 1082

Abstract

Clouds remain an important source of uncertainty in climate simulations, in large part because subgrid processes are not well represented. Critical relative humidity (RHc) is an important metric for subgrid-scale variability in humidity in cloud parameterization. Based on CloudSat and CALIPSO satellite data, we explored the spatial and temporal distribution characteristics of RHc, assessed the ability of ERA-5 and MERRA-2 reanalysis and CMIP-6 climate models to characterise humidity subgrid variability and further explored the influence of meteorological factors and aerosols. The statistical results showed that there was significant variation in the spatial distribution of RHc, with large variations in both latitude and altitude, as well as more pronounced monthly variations, and that there were differences in monthly variations between regions. Both the reanalysis data and the climate models were able to reproduce similar spatial and temporal distribution patterns but differed significantly in their specific values. The temporal correlations with satellite observations were also relatively poor. In addition, aerosols and meteorological conditions affected the distribution of RHc by influencing the cloud fraction at a certain relative humidity level, indicating that their influence needs to be considered in future parameterization schemes. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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11 pages, 5933 KiB

Open AccessCommunication

A Time-Gated, Time-Correlated Single-Photon-Counting Lidar to Observe Atmospheric Clouds at Submeter Resolution

by Fan Yang, Yong Meng Sua, Alexandros Louridas, Katia Lamer, Zeen Zhu, Edward Luke, Yu-Ping Huang, Pavlos Kollias, Andrew M. Vogelmann and Allison McComiskey

Remote Sens. 2023, 15(6), 1500; https://doi.org/10.3390/rs15061500 - 8 Mar 2023

Cited by 2 | Viewed by 2194

Abstract

Most lidars used for cloud observations have the range resolution of about 10 m, so they are incapable of resolving submeter-scale processes that are crucial to cloud evolution. This article describes a prototype of a ground-based, vertically pointing, time-gated, time-correlated single-photon-counting lidar (referred to as the T2 lidar) developed to explore atmospheric clouds at range resolution two orders of magnitude finer than traditional atmospheric lidars. The T2 lidar emits green-light pulses (532 nm) at a repetition rate of 20.6 kHz and a pulse width of ∼650 ps, which enables the observation of aerosol and cloud layers at heights from a few hundred meters to 7.28 km above the ground level at range resolution down to 10 cm. In addition, a digital delay pulse generator controls the detector to only receive photons for a short period after each laser pulse. This time-gated technique blocks photons arriving from regions outside the target zone, thus significantly reducing the noise level and allowing observation even inside clouds. Initial observations show that the T2 lidar can detect sharp cloud boundaries and fine structures near the cloud base. Such refined measurements of cloud structure could lead to an improved understanding of microphysical processes such as droplet activation, entrainment and mixing, and precipitation. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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18 pages, 6568 KiB

Open AccessArticle

Identification of Supercooled Cloud Water by FY-4A Satellite and Validation by CALIPSO and Airborne Detection

by Xiaohong Xu, Yi Zeng, Xing Yu, Guihua Liu, Zhiguo Yue, Jin Dai, Qiujuan Feng, Pu Liu, Jin Wang and Yannian Zhu

Remote Sens. 2023, 15(1), 126; https://doi.org/10.3390/rs15010126 - 26 Dec 2022

Cited by 2 | Viewed by 1923

Abstract

Cold clouds are the main operation target of artificial precipitation enhancement, and its key is to find a supercooled cloud water area where the catalyst can be seeded to promote the formation of precipitation particles and increase precipitation to the ground. Based on the multi-spectral characteristics of the Fengyun-4A (FY-4A) satellite, a methodology for identifying supercooled cloud water is developed. Superimposed by a cloud top brightness temperature of 10.8 µm, a combination of 0.46 µm, 1.6 µm, and 2.2 µm red–green–blue (RGB) composites are used to identify the cloud phase and to obtain the real-time supercooled cloud water distribution every 5 min and in a 2 km resolution for the whole coverage of China. Based on the RGB composition, the supervised machine learning method K-mean clustering was applied to classify the cloud top phase. The results were validated extensively with Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP). It is worthwhile to highlight that the corresponding hit rate reached 87% over the full disk domain for both the summer and winter seasons. Furthermore, on 29 November 2019, microphysical properties were measured, and the data of supercooled cloud droplets and ice crystals were obtained using YUN-12 transport aircraft in Taiyuan. After simultaneously matching the satellite with an airborne track, the cloud particle image data were obtained near the cloud top and within the clouds during the climb and descending stages of the flight. The phase obtained from the microphysical properties of supercooled cloud droplets and ice crystals was compared with cloud phase results identified by FY-4A and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud phase products. The case study and comparison show that (1) the supercooled water clouds and ice particles identified by FY-4A are in good agreement with those from the airborne measurement at the cloud top and within the cloud and (2) the positions and shapes of water clouds and ice clouds identified by FY-4A correspond well with MODIS cloud phase products. However, there is a small deviation in the extent of ice clouds, which is mainly located in the transition area between ice clouds and water clouds. The extent of ice clouds identified by FY-4A is slightly larger than that of MODIS products. Combined with airborne detection, the comparison shows that the ice clouds identified by the FY-4A satellite are consistent with aircraft detection. The supercooled cloud water identified by FY-4A can meet the needs of the operational precipitation enhancement of cold clouds, improve operational effectiveness, and promote the application of satellite technology for weather modification. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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15 pages, 6857 KiB

Open AccessArticle

Polar Aerosol Vertical Structures and Characteristics Observed with a High Spectral Resolution Lidar at the ARM NSA Observatory

by Damao Zhang, Jennifer Comstock, Hailing Xie and Zhien Wang

Remote Sens. 2022, 14(18), 4638; https://doi.org/10.3390/rs14184638 - 16 Sep 2022

Cited by 2 | Viewed by 1719

Abstract

Aerosol vertical distributions impact both the direct and indirect radiative effects of aerosols. High Spectra Resolution Lidar (HSRL) separates between atmospheric molecular signals and aerosol particle signals and therefore can provide reliable measurements of aerosol properties. Six years of HSRL measurements between 2014 and 2019 from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) atmospheric observatory at Utqiaġvik are used to statistically analyze Arctic aerosol vertical distributions. The annual cycle of aerosol vertical distributions in terms of aerosol particulate backscatter coefficient (

β_{p}

), lidar scattering ratio (SR), and aerosol particulate depolarization ratio (

δ_{p}

) profiles at the wavelength of 532 nm shows that Arctic Haze events are prevalent in later winter and spring at the NSA site. Mineral dust is frequently presented in strong aerosol layers in the spring, fall, and winter seasons. Over the summer season, the NSA site has large aerosol loadings that are dominated by small spherical aerosol particles. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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19 pages, 6693 KiB

Open AccessArticle

Climatology of Dust Aerosols over the Jianghan Plain Revealed with Space-Borne Instruments and MERRA-2 Reanalysis Data during 2006–2021

by Chuan Liu, Zhenping Yin, Yun He and Longlong Wang

Remote Sens. 2022, 14(17), 4414; https://doi.org/10.3390/rs14174414 - 5 Sep 2022

Cited by 9 | Viewed by 1948

Abstract

In recent years, climate change and the intervention of anthropogenic activities have altered the seasonal features of Asian dust storms. This may also cause seasonal variations (including dust occurrence frequency and optical/microphysical properties) in dust aerosols transported to downstream regions. The Jianghan Plain is dramatically influenced by multiple dust sources due to its geographical location in central China. In this study, we focused on the climatology of dust aerosols over the Jianghan Plain based on the 15-year (2006–2021) continuous space-borne observations of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) as well as Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis data. A typical dust event that intrudes the Jianghan Plain was studied in detail. According to the statistical results, dust aerosols frequently intrude into the Jianghan Plain in spring and winter, with occurrence frequencies (under cloud free condition hereafter) exceeding 0.70 and higher altitudes of 4–6 km. The dust occurrence frequency declined to approximately 0.40 in autumn and nearly zero in summer, while the dust plumes were generally located at lower altitudes of 1–3 km. The dust plumes observed in the Jianghan Plain were simultaneously linked to the Taklimakan Desert and Gobi Desert in spring and mainly originated from the Taklimakan Desert in winter and autumn. The dust particles were mainly distributed below 4-km altitude, with the largest dust extinction coefficients and dust mass concentrations in spring. In all seasons, the particle depolarization ratios are 0.1–0.2 below 4-km altitude, suggesting a possible mix with local anthropogenic aerosols. The mean dust column mass concentrations in spring showed an evident declining trend from 210 µg m⁻² in 2006 to 100 µg m⁻² in 2021 in the Jianghan Plain, attributed to the reduced dust activity in the source regions of Asian dust. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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15 pages, 2374 KiB

Open AccessTechnical Note

Impacts of Aerosol Chemical Composition on Cloud Condensation Nuclei (CCN) Activity during Wintertime in Beijing, China

by Quan Liu, Xiaojing Shen, Lei Li, Junying Sun, Zirui Liu, Weibin Zhu, Junting Zhong, Yangmei Zhang, Xinyao Hu, Shuo Liu, Huizheng Che and Xiaoye Zhang

Remote Sens. 2023, 15(17), 4119; https://doi.org/10.3390/rs15174119 - 22 Aug 2023

Cited by 1 | Viewed by 970

Abstract

The cloud condensation nuclei (CCN) activity and aerosol chemical composition were concurrently measured via a scanning mobility CCN analyzer (SMCA) and an Aerodyne Time-of-Flight Aerosol Chemical Speciation Monitor (ACSM), respectively, during wintertime 2022 in Beijing, China. During the observation period, the mean CCN number concentrations ranged from 1345 ± 1270 cm⁻³ at SS = 0.1% to 3267 ± 2325 cm⁻³ at SS = 0.3%. The mean critical activation diameters (D₅₀) at SS = 0.1%, 0.2%, and 0.3% were 172 ± 13 nm, 102 ± 8 nm, and 84 ± 7 nm, corresponding to the average hygroscopicity parameters (κ_CCN) of 0.34, 0.33, and 0.26, respectively. The diurnal variations in D₅₀ suggested that the local primary emissions significantly enhanced D₅₀ at SS = 0.2% and 0.3%, but had less influence on D₅₀ at SS = 0.1% due to the limited size (<150 nm) of particles emitted from primary sources. As PM_2.5 concentration increases, the dominant driver of CCN activity transitions from sulfate to nitrate. At a specific SS, D₅₀ decreased with increases in the degree of internal mixing, implying that the elevated internal mixing degree during atmospheric aging was beneficial to CCN activation. In this study, the commonly used f44 (or O:C) was weakly correlated with κ_org and failed to describe the variations in κ_org. Instead, the variations in κ_org can be well parameterized with the Org/BC ratio. The correlation between κ derived from bulk chemical compositions and CCN measurements was substantially improved when this κ_org scheme was adopted, emphasizing the importance of considering κ_org variations on deriving κ_chem from aerosol chemical composition. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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13 pages, 2026 KiB

Open AccessTechnical Note

A 94 GHz Pulse Doppler Solid-State Millimeter-Wave Cloud Radar

by Hai Lin, Jie Wang, Zhenhua Chen and Junxiang Ge

Remote Sens. 2023, 15(12), 3098; https://doi.org/10.3390/rs15123098 - 13 Jun 2023

Cited by 1 | Viewed by 1595

Abstract

A 94 GHz pulse Doppler solid-state millimeter-wave cloud radar (MMCR), Tianjian-II (TJ-II), has been developed. It reduces the size and cost using a solid-state power amplifier (SSPA) and a single antenna. This paper describes the system design, including hardware and signal processing components. Pulse compression, segmented pulse, and dual pulse repetition frequency (PRF) technologies are employed to overcome the limitations imposed by the low power of the SSPA and the high frequency of 94 GHz. The TJ-II also features a dual-polarization, high-gain antenna for linear depolarization ratio detection and a time-division receive channel to improve channel consistency and save on costs. To achieve high flexibility and low interference in signal transmission and reception, the TJ-II uses software-defined radio technology, including direct digital synthesis, digital downconversion, and bandpass sampling. A series of Doppler power spectrum processing methods are proposed for detecting weak cloud signals and improving scene adaptability. Full article

(This article belongs to the Special Issue Remote Sensing of Aerosol, Cloud and Their Interactions)

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