We intend to present and overview of ground-based sun photometry for aerosol optical properties retrieval, including:
- the state of the art on basic principles of direct sun and sky radiance measurements and retrieval methods used worldwide
- Calibration procedures for aerosol optical depth
- Global efforts for aerosol optical properties homogenization
- An estimation of the aerosol optical depth uncertainty and the factors affecting it
- Examples of aerosol retrievals, trends and spatiotemporal variability on a global scale.

Currently, global networks retrieving aerosol properties (e.g. aerosol optical depth (AOD)) are using different calibration principles and hierarchy and also there are differences on the post processing of solar direct sun measurements, in order to retrieve AOD (e.g. inclusion of trace gases, cloud elimination , Rayleigh scattering effects). Concerning the calibration, the most common methods are the Langley calibration (with certain limitations) and the traceability to defined (e.g. from WMO) standards. Lately a link of such standards with SI traceable ones helped towards the goal of the transition from "artificial" standards towards SI traceable ones.

Long term measurements of various instruments belonging to different networks showed, in the case of network reference instruments, very good agreement within initatives supported by the world meteorological organization ( e.g. filter radiometer comparisons in Davos, Switzerland from 2000 to 2021 organized by the World Optical depth Research and Calibration Center)  and European infrastructure initiatives such as ones supported by the Calibration center for Atmospheric remote sensing of the EU infrastructure ACTRIS (Aerosol, Clouds and Trace Gases Research Infrastructure).

Recently these aspects have been investigated and communicated through a networking COST action project called Harmonia (International network for harmonization of atmospheric aerosol retrievals from ground based photometers) where more than 100 scientists are participating.

Acknowledgement: The presenter would like to acknowledge funding for the participation at EMS2023  from  the COST Action HARMONIA (International network for harmonisation of atmospheric aerosol retrievals from ground based photometers), CA21119

How to cite: Kazadzis, S., Kouremeti, N., Masoom, A., and Groebner, J. and the Harmonia Core group Team: Aerosol sun photometry state of the art and progress and the COST action Harmonia, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-41, https://doi.org/10.5194/ems2023-41, 2023.

Since four decades the Aerosol Optical Depth (AOD) is retrieved operationally using the photometry technique. Photometers are operated during day pointing the sun (solar photometers) or during night pointing the moon (lunar photometers) or the stars (stellar photometers). There are different networks of photometers, using different kind of instruments and methods of inversion. The three main networks are AERONET, SkyNet and GAW/PFR. HARMONIA (International network for harmonization of atmospheric aerosol retrievals from ground-based photometers) is a COST action that gathers a large panel of actors/stakeholders of photometry and has the ambition to describe the state of the art of the techniques, measurements and inversion methods, pointing out their diversity and suggesting some harmonized standard procedures of measurements and inversion techniques.

One milestone of COST/HARMONIA presented here is to report on recent (2017 – 2022) campaigns using different kind of photometers and methods of inversion: 1) The Nocturnal AOD Intercomparison of Izaña 2007. Two types of lunar photometers and one stellar photometer have been involved. The quality of lunar measurements to the AOD stellar measurements and the lunar exo-atmospheric irradiance model have been evaluated. 2) QUATRAM (QUAlity and TRaceabiliy of Atmospheric aerosol Measurements) were five campaigns (2017-2021) involving three different type of sun photometers (Cimel CE318, Prede POM and PMOD PFR). 3) ANACC (Arctic Night Aerosol Characterization Campaign) was a campaign during the polar night (February 2020) in Ny Ålesund, involving two kinds of lunar photometers, a stellar photometer and a Raman-Lidar. In addition to the instrument intercomparison, this campaign could focus on Arctic Haze and Polar Stratospheric Clouds, whose optical properties could be investigated. 4) SCILLA (Summer Campaign for Intercomparison of Lunar measurements of Lindenberg’s Aerosol) was a nocturnal AOD campaign in Summer 2020, involving lunar photometers of all three types (Cimel, PFR, Prede), two stellar photometers, a Raman lidar, and some COBALD balloon-carried AOD radiosondes. The aim was to estimate the differences of AOD obtained with lunar photometers of the same type and compare them to the differences of AOD obtained from instrument of other types and the stellar photometers. Also, a focus was set on the synergy total column measurements (AOD from photometers) with profiling measurements (LIDAR, COBALD). 5) FRC-V (Fifth Filter Radiometer Comparison) was a WMO solar photometer campaign in Davos. Thirty-two filter radiometers and spectroradiometers from 12 countries participated. 6) The MAPP campaign in September 2017 in Izaña (MAPP: Metrology for aerosol optical properties; a project part of the EURAMET EMPIR program). One aim was to obtain extra-terrestrial solar and lunar spectral irradiance traceable to the International System. The campaign involved the most sophisticated instrumentation for measuring solar and lunar irradiance, including QASUME, the reference spectral radiometer of PMOD-WRC, Fourier transform Spectrometers and more than 30 instruments.

How to cite: Doppler, L., Kazadzis, S., Kouremeti, N., Masoom, A., Barreto, A., Cuevas, E., Toledano, C., Roman, R., Campanelli, M., and Ritter, C.: Learnings of measurement campaigns in photometry instrumentation and inversion methods, a COST/HARMONIA study, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-656, https://doi.org/10.5194/ems2023-656, 2023.

In this study, we analyze the impact of dust on shortwave surface irradiance (global and direct) over a period of five years (2017-2022). We use measurements of global horizontal irradiance (GHI) and direct normal irradiance (DNI) from three stations located in different areas in Cyprus: Agia Marina Xyliatou (35.04N; 33.06E; 535m above sea level), Athalassa (35.14N;33.39E; 158m above sea level) and Larnaca Airport (34.873N; 33.62E; 2m above sea level).

Under clear sky conditions, aerosols are one of the most influential factors on surface solar radiation levels. Aerosols can change surface radiation fluxes by both absorbing and scattering solar radiation. In particular, dust aerosols are known to efficiently absorb solar radiation, especially at lower wavelengths. Cyprus, located in close proximity to both Northern Africa and the Arabian Peninsula, frequently experiences dust events. The island's high number of cloudless days make it an ideal location for studying the radiative effects of dust. Previous studies using satellite based and CAMS related data showed that Aerosols attenuate 5–10% of the annual global horizontal irradiation and 15–35% of the annual direct normal irradiation. Dust is responsible for 30–50% of the overall aerosol attenuation. In addition, optical properties of dust vary, as the origin can be from N. Africa or Middle East. We selected only cloud-free, quality-controlled, excluding any data that fail to pass the BSRN recommended QC tests, measurements for this study.

The dust events are identified using the CIMEL sun-photometers data  from the Aeronet stations in Agia Marina Xyliatou and CUT-TEPAK in Limassol in conjuction with the PollyXT lidar station in Limassol. The origin of the dust airmasses are identified as well by  analyzing the back-trajectories from the Hysplit model. Cloudless and aerosol-free Sky conditions are simulated with the radiative transfer model LibRadtran and the dust radiative effects are estimated as the difference between measured irradiance and the modeled values. This study is important for understanding the impact of dust on surface solar radiation, which has important applications in solar energy production and climate modeling.

Acknowledgments: “This research is performed under the auspices of the Memorandum of Understanding between ERASTOTHENES CoE and The Cyprus Institute. The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EΧcellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu).The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology”. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 856612 and the Cyprus Government (EMME-CARE).

How to cite: Charalampous, G., Fragkos, K., Marenco, F., Derimian, Y., Nisantzi, A., Mamouri, R.-E., Pikridas, M., El Hajj, D., Hadjimitsis, D., Sciare, J., and Kazadzis, S.: Dust impact on surface solar radiation levels in Cyprus, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-493, https://doi.org/10.5194/ems2023-493, 2023.

Atmospheric aerosols modify the incoming solar radiation by scattering and absorption and through their impact on the formation and dissolution of clouds and additionally change their optical properties. Currently, no NWP model can fully account for these very complex effects. Especially during weather conditions affected by high aerosol concentrations, as in the event of Saharan dust outbreaks or wildfires, this can lead to errors in the forecast. One important factor to overcome this shortfall of NWP models, aerosols need to be treated as a prognostic variable.

At Deutscher Wetterdienst (DWD) and Karlsruhe Institute of Technology (KIT) project "PermaStrom" aims to improve radiation forecasts during such events. Using the ICON-ART modeling system emission, transport, and deposition of mineral dust, black carbon from vegetation fires, and sea salt are explicitly simulated. To achieve the project goals and to examine the effect of Saharan dust on solar radiation, accurate and extensive measurements of the Saharan dust in the atmosphere and the ground reaching solar radiation are needed. Within the measurement network of the DWD, we currently have 35 Stations equipped with two secondary standard pyranometers. One to measure the global radiation and one shaded to measure the diffuse incoming solar radiation at the surface with a temporal resolution of 1 minute. At the Meteorological Observatory Lindenberg, we also develop a retrieval algorithm to estimate the AOD at 550 nm from those measurements. With the additional information on the vertical distribution of the dust cloud from Ceilometer measurements in Germany, we obtain a detailed picture of aerosols and radiation.

In our contribution, we will give an overview of how accurate measurements at the surface could help to identify deficiencies in the ICON-ART modeling system and how the development process could benefit. We will focus on the direct and indirect aerosol effects and the benefit of the information of two radiation components.

How to cite: Filipitsch, F., Doppler, L., Wacker, S., and Becker, R.: Aerosol and radiation observations and model forecasts, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-558, https://doi.org/10.5194/ems2023-558, 2023.

Solar energy is an important source of renewable energy, and understanding its variability due to clouds and aerosols is critical for efficient and reliable solar energy systems. The interaction between clouds and aerosols can have a significant impact on surface solar irradiance during broken cloud conditions. Reflections at cloud edges and changing aerosol properties in the vicinity of clouds affect the surface solar irradiance during broken cloud conditions. However, the boundaries between clouds and clear skies with aerosols are not always well defined, and quantifying the frequency and spatial extent of the transition zone between these regions is challenging¹. This study uses a unique dataset of observations from the TROPOS dense pyranometer network to detect and characterize the transition zone between clouds and clear skies with aerosols.

The TROPOS pyranometer network consists of up to 100 individual stations. Datasets from two campaigns are used for this study: 99 stations were distributed over an area less than 10 km² operated during the HOPE² measurement campaign in 2013 in Jülich, Germany; 60 stations were distributed over an area of about 6 km² during the S2VSR³ measurement campaign in 2023 in Oklahoma, USA. The surface solar irradiance is measured at each station with a time resolution of 10 Hz. The transition zone is detected and characterized by applying modified clear sky detection algorithms⁴ to the data. To quantify the spatial characteristics of the transition zones, a method based on optimum averaging/spatio-temporal Kriging is introduced. A main component of this method is the determination of the cloud motion vector. It is determined whether this can be reliably estimated from ground-based measurements, by using cross-correlation techniques, or whether additional information from satellite data is required. First results utilizing the HOPE and S2VSR dataset are presented.

The study aims to quantify the small scale effects of the transition zone on surface solar irradiance and potential photo-voltaic yield. This information is valuable for photo-voltaic site planning and provides scientifically relevant insights into the interaction between clouds and aerosols. Furthermore, the study provides initial considerations for modelling the identified relationships and insights into small-scale surface irradiance variability not captured by current high-resolution satellite observations.

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How to cite: Witthuhn, J., Kalesse-Los, H., and Deneke, H.: Characterising the effect of the transition zone of clouds and aerosol on solar surface irradiance using a dense pyranometer network, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-459, https://doi.org/10.5194/ems2023-459, 2023.

Clouds are one of the most significant factors in the climate system that strongly affect the Earth’s energy budget. Clouds reflect some of the sun's energy back into space, producing a cooling effect at the top of the atmosphere and at the same time trap the longwave radiation producing a warming effect. On average, the cooling effect is stronger than the warming effect, but accurately estimating the effects of clouds on the Earth's energy budget is still a major area of research. The cloud radiative effects (CRE), which is defined as the difference between the radiation under cloudy and cloud-free conditions, are mostly estimated from satellite observations. For accurate estimation from the ground, measurements by a combination of different instruments that provide vertical information about the optical and microphysical cloud and aerosol properties are necessary, to feed this information in radiative transfer models. During the CyCARE campaign, which was a joint initiative between the Cyprus University of Technology (CUT), Limassol and TROPOS, the Leipzig Aerosol and Cloud Remote Observations System (LACROS) operated at the CUT from October 2016 to March 2017. LACROS includes active and passive remote-sensing instruments, such as a PollyXT Raman-polarization lidar to retrieve aerosol vertical distribution, a 35-GHz cloud radar to obtain cloud microphysical properties, a disdrometer to measure precipitation, a Doppler lidar to track aerosol and cloud dynamics, and a microwave radiometer to measure water vapor and liquid water. Using the radiative transfer package LibRadtran the cloudy and clear sky shortwave and longwave radiation fluxes on the surface and the top of atmosphere were calculated during the CyCARE campaign. The aerosol and cloud profiles, along with other microphysical properties of clouds and aerosols were used as input in the radiative transfer model for the above-mentioned calculations. Subsequently, the CRE were obtained as the differences between all sky and clear sky fluxes. Since ERATOSTHENES Centre of Excellence (ERATOSTHENES CoE) is in the process to build up a permanent station named Cyprus Atmospheric Remote Sensing Observatory (CARO), the demonstrated CyCARE campaign depicts the capacity of the Centre to obtain long-term aerosol-cloud-radiation interactions in the sensitive climate area of the Eastern Mediterranean. The CARO constitutes the basic tool to improve the representation of clouds and aerosols in climate models and validate the satellite derived CREs.    

Acknowledgments: “The authors acknowledge the ‘EXCELSIOR’: ERATOSTHENES: EΧcellence Research Centre for Earth Surveillance and Space-Based Monitoring of the Environment H2020 Widespread Teaming project (www.excelsior2020.eu). The ‘EXCELSIOR’ project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 857510, from the Government of the Republic of Cyprus through the Directorate General for the European Programmes, Coordination and Development and the Cyprus University of Technology”.

How to cite: Fragkos, K., Mamouri, R.-E., Fountoulakis, I., Nisantzi, A., Ene, D., Charalampous, G., Papachristopoulou, K., Ansmann, A., Buhl, J., Seifert, P., Hadjimitsis, D., and Kazadzis, S.: The capacity of Eratosthenes Centre of Excellence to estimate cloud radiative effects: the CyCARE campaign, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-409, https://doi.org/10.5194/ems2023-409, 2023.

The Antarctic Peninsula (AP) region is well known for high variability of atmospheric parameters compared to the central part of Antarctica. This area is influenced by high cyclonic activity which results in high cloud cover and strong surface wind. Due to complex orography and the effect of AP mountains, different amount of cloud cover occurs along western and eastern coast. As the cloud cover is one of the most important atmospheric factors affecting the Earth's energy budget, its changes cause significant spatiotemporal variability of incoming solar radiation. Solar radiation is a key driver of many physical and biological processes in Antarctic ecosystems, as well as the main component of surface net radiation. Together with other radiative fluxes and atmospheric transmittance is frequently used in many atmospheric and glaciological simulations. To study annual and seasonal differences in global solar radiation (GR), measurements of shortwave downward fluxes were carried out at J.G. Mendel station (JGM) located on the north-eastern side of the peninsula and King Sejong station (KSJ) on the western side of AP. The GR monitoring was undertaken with class A pyranometers in the period 2011–2016. Furthermore, theoretical intensity of GR was calculated using radiative transfer model and used for estimation of cloud modification factor at both stations. Factors influencing the GR variability were retrieved from ERA5 reanalysis, consisting of the following parameters: surface pressure, surface albedo, water vapor, cloud cover and total ozone column (Ozone Monitoring Instrument). The results showed significantly higher intensity of GR (up to ~30.5 %) and higher values of cloud modification factor (~28.5 %) at JGM station compared to KSJ station. The main factor influencing daily variation in GR was solar zenith angle at both stations (~50 % of variability). The influence of cloud cover, main atmospheric factor, was more pronounced at JGM station (~22 %) compared to KSJ station (~15 %), while water vapor was less important compared to cloud cover (~7 % at JGM and ~17 % at KSJ station respectively).

How to cite: Szymszová, S., Láska, K., Kim, S.-J., and Park, S.-J.: Evaluation of global solar radiation and cloud cover variability in northern Antarctic Peninsula, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-347, https://doi.org/10.5194/ems2023-347, 2023.

Surface observations in Antarctica have always been challenging but cloud observations are particularly scarce due to the lack of observers and instruments and the strict limitation caused by the polar night. This work aims at testing and finding methods that can fill the gap of information on cloud cover, based on solar or terrestrial broadband radiation measurements. In particular, the results from Long et al.[1] method, exploiting solar radiation measurements, and Dürr and Philipona[2] APCADA algorithm and Town et al.[3] methods, both based on terrestrial radiation measurements, will be shown. The methods are chosen to make use of shortwave and longwave radiation components, specially the latter because it can yield information throughout the year and not only during months of daylight, as methods based on shortwave radiation do. Studied data sets are from different coastal sites: Marambio (64°14’50’’S - 56°37’39’’W), Professor Julio Escudero (62°12’57’’S - 58°57’35’’W) and Jang Bogo (74°37’38’’S - 164°14’16’’E). Another analysed data set comprises measurements taken at DomeC (75°05’59’’S – 123°19’57’’E). Since stations can be equipped with different instruments that could record only some of the broadband radiation components, investigating different methods allows to adapt to the actually available data. It also means that is possible to cross-check results when more than one method can be used, like in the case of Marambio. Before data sets are used as input, they undergo quality check controls[4] as recommended by the Baseline Surface Radiation Network[5], when applicable. Preliminary results show that Marambio and Escudero stations thought to be similar, for their latitude and close distance, actually differ from one another when it comes to frequency of occurrence of clear and cloudy sky conditions.

Bibliography

[1] Long et al. (2006): “Estimation of fractional sky cover from broadband shortwave radiometer measurements”, J. Geophys. Res. 111, doi: 10.1029/2005JD006475
[2] Dürr and Philipona (2004): “Automatic cloud amount detection by surface longwave downward radiation measurements”, J. Geophys. Res. 109, doi: 10.1029/2003JD0041823
[3] Town et al. (2007): “Cloud Cover over the South Pole from Visual Observations, Satellite Retrievals, and Surface-Based Infrared Radiation Measurements”, Journal of Climate 20, doi: 10.1175/JCLI4005.1
[4] Long and Shi (2008): “An automated quality assessment and control algorithm for surface radiation measurements”, Open Atm. Science J. 2, doi: 10.2174/1874282300802010023
[5] https://bsrn.awi.de/

How to cite: Frangipani, C., Ahn, S.-H., Choi, T., Cordero, R., Gulisano, A. M., Lupi, A., Mazzola, M., Ochoa, H. A., Rowe, P., and Vitale, V.: Cloud cover estimation at various sites in Antarctica using different methods based on broadband radiation measurements, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-476, https://doi.org/10.5194/ems2023-476, 2023.

How to cite: Hieronymus, M., Neuhauser, C., Kern, M., Rautenhaus, M., Oertel, A., and Westermann, R.: Multi-parameter sensitivities along warm conveyor belt trajectories: A visual analysis using Met.3D, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-508, https://doi.org/10.5194/ems2023-508, 2023.

Satellite observations reveal the glaciation of supercooled stratiform liquid phase clouds by anthropogenic aerosols acting as ice-nucleating particles downwind of industrial aerosol point sources. Glaciation events are observed downwind of oil refineries, coal-fired power plants, cement, metal smelting and processing, chemical plants, and other anthropogenic air pollution sources in Europe, Asia, North America and Australia. Glaciation leads to snowfall, reduced cloud cover and reduced back-scattering of solar radiation to space. The polluted areas with decreased cloud cover are plume-shaped, with a distinctive head pointing towards the pollution source, similar to aerosol-polluted cloud tracks in liquid-water clouds (Toll et al 2019 Nature https://doi.org/10.1038/s41586-019-1423-9). Moreover, heavily polluted areas derived by simulating aerosol dispersion from strong anthropogenic aerosol point sources overlap with the areas of glaciation, snowfall, and decreased cloud cover strikingly well. These lines of evidence suggest that anthropogenic aerosols are the cause of the observed glaciation events.

Glaciation-induced snowfall downwind of aerosol sources is observed using ground-based precipitation radars, and tracks of snow are also seen on the ground in satellite imagery. Thus, at least locally, anthropogenic ice nucleating particles could significantly affect precipitation amounts. At multiple aerosol sources, glaciation events are more frequent than polluted tracks in liquid-phase clouds. Thus, at least locally at some aerosol sources in the middle and high latitudes, the warming effect induced by aerosols acting as ice-nucleating particles likely exceeds the cooling effect induced by aerosols acting as cloud condensation nuclei. Further research is needed to quantify the global radiative forcing by anthropogenic ice nucleating particles.

How to cite: Toll, V., Rahu, J., Keernik, H., Trofimov, H., Voormansik, T., Manshausen, P., Hung, E., Michelson, D., Christensen, M., Post, P., Junninen, H., Lohmann, U., Watson-Parris, D., Stier, P., Donaldson, N., Storelvmo, T., Kulmala, M., Murray, B., and Bellouin, N.: Ice nucleation by anthropogenic aerosols downwind of industrial point sources of air pollution, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-345, https://doi.org/10.5194/ems2023-345, 2023.