Authors: Reda I., Andreas A., and Gotseff P. (National Renewable Energy Laboratory, USA)
Title: Using an Absolute Cavity Pyrgeometer to Validate the Calibration of a Transfer Standard Pyrgeometer Outdoors, Independent from the Reference Value of the Atmospheric Longwave Irradiance
Abstract: A unique method of pyrgeometer calibration has been developed to improve the measurement uncertainty [1]. The results of this method yielded irradiance values within ±3 𝑊/𝑚2 of those traceable to the World Infrared Standard Group (WISG). The absolute cavity pyrgeometers (ACPs) and pyrgeometer model passive infrared (PIR) were deployed outdoors, the PIR was placed on a temperature controller like the ACP’s temperature controller. The responsivity of the pyrgeometer is then calculated by cooling its case temperature, as described in the next slide. The irradiance measured by the pyrgeometer was compared against the irradiance measured by ACP95F3. Based on the results, it is possible to achieve an uncertainty of ±3.51 𝑊/𝑚2. These results suggest that this pyrgeometer calibration method might be useful in addressing the international need for a transfer standard pyrgeometer traceable to the International System of Units (SI).

Authors: Suter M., and Buchli J. (Davos Instruments, Switzerland)
Title: The HF- Radiometer in active mode
Abstract: Despite the fact that the Hickey-Frieden (HF) radiometer is normally operated in the passive/pacrad-mode, it is possible to operate it in the active mode, similar to a PMO6. An AHF-radiometer has been operated in the active mode, using a slightly modified Linard-control unit from Davos Instruments.
While the passive mode allowed to take readings of solar irradiance with an almost infinitely high cadence, the active mode usually allowed only one reading every two minutes. Using a new fast, digital controller loop, a measurement cadence of one reading every 30 seconds can be achieved eliminates this limitation of the active mode.
A data set form summer 2021 is presented, the readings of the active HF are compared to different instruments run in parallel at PMOD/WRC, including instruments of the World Standard Group (WSG).

Authors: Suter M., and Finsterle, W. (Davos Instruments and PMOD/WRC, Switzerland)
Title: Diffraction Corrections for CSAR and for everyone
Abstract: Diffraction effects in solar radiometry become more and more important once the absolute accuracy should go beyond 0.1%. Diffraction effects occur at the apertures, and have an impact of roughly 1000 ppm, but can increase by up to 50% depending on the spectral conditions.
The Cryogenic Solar Absolute Radiometer (CSAR) is a candidate for a new primary standard, eventually replacing the WRR. Therefore each measurement must be diffraction corrected according to the current solar spectra.
In order to correct the reading of the CSAR for diffraction losses, diffraction corrections have been calculated for different atmospheric conditions, based on the key parameters: Solar Zenith Angle (SZA), Integrated Water Vapour, and Aerosol Optical Depth (AOD), and an atmosphere model. A software, based on a lookup table has been developed, allowing to quickly determining the diffraction correction based on the atmospheric key parameters
In addition to the efforts made for CSAR, a similar software package could be used to correct normal pyrheliometers. This will be a prerequisite to reach the goals of the AA-class as defined in the ISO 9060:2018.

Authors: Balenzategui J.L., De Lucas J., González-Leiton A., Cuenca J., Molero M., Romero M.C., Fabero F., Sanz M., Escrivano D., Belenguer T., Ibañez F., Silva J.P., Mejuto E. (CIEMAT, Spain)
Title: On the characterization of an AHF cavity radiometer and its traceability to WRR / SI
Abstract: In addition to the comparison against WSG to get traceability to WRR, solar-type cavity radiometers can also be characterized and be directly traced to SI units. This work summarizes different techniques and procedures applied for the characterization of an Eppley AHF cavity radiometer. The approach for characterization and for calculation of the uncertainty is based on the measurement model function of the instrument. Results obtained from calibration and testing (voltmeter measuring electrical magnitudes, area of the precision aperture, resistance of the leads, non-equivalente factor), and by numerical simulation (effective absorptance, scattering and difraction) are presented. Some additional temperature dependences (electrical current injected into cavity heater, sensitivity of the instrument) are also analysed. Resulting corrections obtained up to now for the AHF by characterization are compatible with the WRR factors obtained by this instrument in the last IPCs.

Authors: Mes J., Bergmans T., Partosoebroto A., and Korevaar M. (Kipp & Zonen, Ott hydromet, The Netherlands)
Title: A new Class A pyranometer
Abstract: The development and specifications of a new class-A pyranometer will be presented.

Authors: Barreto A., Cuevas E., García R.D., Carrillo J., Prospero J.M., Illic L., Basart S., Berjón A., Marrero C.L., Hernández Y., Bustos J.J., Nickovic S., and Yela M. (AEMET, Spain)
Title: Long-term characterization of the the Saharan Air layer in the North Atlantic
Abstract:  In this work, we have performed a robust characterization of the seasonal evolution of the atmospheric aerosol extinction and meteorological vertical profiles at a North Atlantic subtropical site over the period 2007 – 2018 using long-term Micropulse Lidar (MPL-3) and radiosondes observations. Different patterns of dust transport over the subtropical North Atlantic have been observed: dust-free (clean) conditions, the summer-Saharan scenario (Summer-SAL) and the winter-Saharan scenario (Winter-SAL). Our results reveal the important role that both dust and water vapour play in the radiative balance within the Summer- and Winter-SAL and the impact of heterogeneous ice nucleation processes through the frequent occurrence of mid-level clouds observed near the SAL top at relatively warm temperatures.

Authors: Ohkubo, K. (Ishikawa Trading Co., Ltd., Japan)
Title: Development and significance of IRS-04 radiometer
Abstract: ISHIKAWA has brought in an IRS-04 type radiometer and is participating in IPC XⅢ. In 2015, we developed the IRS-01 type and participated in IPC XⅡ. We have developed the IRS-04 as a model that overcomes some of the problems discovered at this time. This IRS-04 is designed not only for outdoor measurements, but also for measurements under pseudo-solar light sources. We are also a supplier of standard solar cells. This makes it possible to measure the irradiance of the pseudo-solar light source required for calibration of the reference solar cell in a traceable manner from the WRR. We would like to introduce these contents.

Authors: Khaled, W., Korany, M., El Shahat, A. (Egyptian Meteorological Authority (EMA), Egypt)
Title: Solar Radiation distribution over Egypt using NCEP/CFSR solar data
Abstract: Solar energy is one of the major attractive and environmentally sustainable resource for producing renewable energy using solar photovoltaic (PV) and solar thermal power systems. Solar radiation measurements are mainly obtained from weather stations, but Middle East and North Africa (MENA) suffer from a lack of observation stations and scant spatial distribution. To overcome this problem, the main aim of this study is to use NCEP Climate Forecast System Reanalysis (CFSR) solar dataset as a replacement of ground measurements. Through the course of this study the MENA region has been divided into three domains during the period 1979-2007, to evaluate in a daily basis each domain individually against solar ground stations. The 1st domain in North West Africa contains three ground stations, the statistical results showed that comparison between CFSR and observations have small mean bias 11.56 W/m2 and a high correlation 0.883. The 2nd domain over Egypt has 13 ground stations, the statistical results showed that the comparison between CFSR and observations has very small mean bias 10.053 W/m2 and a high correlation 0.932. The 3rd domain is over Arab peninsula and has 9 stations; the statistical results showed that the comparison between CFSR and observations has a high mean bias 49.56 W/m2 and a small correlation 0.576. it is highly recommended that CFSR can be used as a substitute for the lack of solar observations over 1st and 2nd domains, but it could be used in the 3rd domain with some restrictions as it needs more observations to the area be evaluated very well.
Based on the evaluation of the CFSR with observations, solar empirical models for the MENA’s domains using CFSR meteorological parameters have been developed. These models not calibrated or validated through this study.
Through decadal distribution of solar radiation over MENA; it is noticed that the maximum amount of solar radiation is concentrated over south of MENA domain and the minimum one is over the northern part of MENA. The highest solar radiation average has been received during 2000s decade. The trend line shows an increment in the amount of solar radiation during the period of study. This may be due to the decrement in the total cloud cover over Egypt and Arabian Peninsula, while the reason in NW Africa may be due to decrement of relative humidity. The seasonal distribution revealed that the maximum power of solar energy could be harvested at spring and summer.

Authors: Cerny R. (dlab GmbH, Switzerland)
Title: The digital absolute radiometer DARA and its Relatives
Abstract: The presentation looks back to the last decade of absolute radiometer development for space application from an engineering perspective. A short overview of different missions will be shown, as well as the measurement concept of the DARA instrument. An outlook to the upcoming development will conclude the presentation.

Session 3 - Radiation Networks

Authors: Sasaki S. (Japan Meteorological Agency, Japan)
Title: Regional Radiation Center (RRC) Tokyo in the WMO RA-II (slides only)
Abstract: Japan Meteorological Agency (JMA) has been conducting solar radiation measurements traceable to the international standard scales since the 1930s, and serving as one of Regional Radiation Centers in the WMO RA-II (RRC-Tokyo) since 1965. RRC-Tokyo has participated in the International Pyrheliometer Comparisons (IPCs) continuously since IPC-III and has held the Regional Pyrheliometer Comparisons (RPCs) once every 5 years in the next fiscal year of IPC, in principle. RRC-Tokyo has contributed to maintaining the traceability of the World Radiometric Reference (WRR) for a long time through these activities. The last RPC (RPC-IV) was held from January to February 2017 at Mt. Tsukuba, Japan. This RPC was held as a joint event of RA-II and RA-V, with 5 National Radiation Centers (NRCs) (China, Hong Kong, Korea, New Zealand and Solomon Islands), WRC and RRC-Melbourne. The results of RPC-IV were reported in WMO IOM report No.130.　Recently in Japan, RRC-Tokyo has prepared to operate the Absolute Cavity Pyrgeometer (ACP) for maintaining a reference of infrared radiation. In addition, JMA started observing at Abashiri instead of Sapporo which is one of BSRN stations. Next RPC is planned in 2023 at Mt. Tsukuba, and the traceability of WRR will be disseminated to NRCs by one of our regional standard pyrheliometers which is operated in IPC-XIII. In this presentation, we would like to report about these things.

Authors: Laguarda A., Russo P., and Abal G. (Laboratorio de Energía Solar, Universidad de la República, Uruguay)
Title: Solar Energy Laboratory: Solarimetric network and outdoor calibration of radiometers for use in solar energy applications in Uruguay
Abstract: The Solar Energy Laboratory (LES) is a research laboratory of the Universidad de la República, Uruguay. From the LES we develop knowledge about solar energy through various lines of research. The main ones are the climatological characterization of the global broadband solar resource and the modeling of thermal solar energy applications. This includes the modeling of the solar resource using satellite images, the measurement and modeling of the efficiency of thermal solar energy applications, and the study of the predictability and variability of the resource. In addition, we carry out the administration, maintenance and operation of a Continuous Measurement Network of Solar Irradiance (RMCIS), which currently includes 9 sites across the country. We also offer an outdoor calibration for radiometers for use in solar energy applications. This is the only calibration service of these type in the country and is free of charge for academic purposes. In this presentation I will describe some of these activities. Specifically I will briefly talk about the context of our laboratory, the recent advances in research lines related to solar assessment, about our measurement network and about the radiometers calibration service.

Authors: Gonzalez-Cabrera A., Valdes-Barron M., Estévez-Perez H., Bonifaz-Alfonzo R., Riveros-Rosas D. (Universidad Nacional Autonoma de Mexico, Mexico)
Title: Reference solarimetric network for Mexico
Abstract: "The reference solarimetric network for Mexico is presented; 13 stations have operated around the country since 5 years ago. The geographic locations were decided with a climatic regionalization. The three principal solar parameters are measured (bean, diffuse and global radiation) in all the stations, and spectral parameters in some stations (longwave radiation, UVB, and illuminance).
It should be noted that the solarimetric network is the first in the country that it was installed under solarimetric criteria.
The principal activities and some troubles during the operation are showed.

Authors: Peterson, J., and Vignola, F. (University of Oregon, Solar Radiation Monitoring Lab, USA)
Title: Status of the SRML comprehensive format code
Abstract: The University of Oregon, Solar Radiation Monitoring Lab (SRML) has been working on a Python based code that takes raw data logger files and publishes them to a user-friendly output csv file. The program is nearly complete and a full-scale role out is intended in 2022. The status of the project will be discussed along with some key features of the output files.
A basic outline of the code is as follows:
First the relevant files are loaded. The files are stored in one-month blocks. The time interval of the data is 1-minute. The various stations of the SRML network typically have 5 - 10 sensors measuring the 3 components of light as well as other MET data. Any gaps in the data are replaced with NAN values.
Apply any adjustments to data. These adjustments may be changes to the responsivity of the instrument as well as changes related to systematic bias of the instrument. For example, thermal offset adjustments.
Several of the stations in the network have a two-axis tracker. From the diffuse and direct normal components a calculated GHI component is computed.
The data from the various sensors are stored in a user-friendly output csv file. The file contains supplemental meta data for each sensor, including, an estimated uncertainty, instrument serial numbers, and a daily energy value for the irradiance sensors. Also the thermal offset of each irradiance measurement also reported.
The irradiance measurements are reported in an “Original” format as well as an “Adjusted” format. The original format, is the data from the instrument directly. No adjustments have been made beyond applying a responsivity value to the millivolt signal. The original format, is intended for users in the solar field that want to understand the various biases that may be associated with a make or model of an instrument. The adjusted values have various adjustments applied to them. The two most common adjustments, are thermal offset responses and rotating shadowband radiometers (RSR) adjustments. The adjusted values are intended for users that just want the irradiance value, modelers and PV professionals.
After the output files are generated, a second program is used that performs quality control on the data. Currently the QC process of the data is done manually. However, various forms of automation are intended for the next version of the program.

Authors: Josefsson W., and Carlund T. (SMHI, Sweden)
Title: 99 years of global radiation observations in Stockholm 1922-1920
Abstract: One of longest known observation series of global radiation has been collected in Stockholm, Sweden. Continuous pyranometer measurements are available since July 1922. Through the years the site as well as measurement equipment and data evaluation have changed. Recently, this measurement series have been re-visited. More original data have been digitized and quality checks have been applied. This resulted in a revised data set with several periods of corrected data clearly differing from what have been published earlier. Despite the improved data set, large uncertainties still remain in the early part of the data series. The global radiation measurements in Stockholm indicate an early brightening over the 1920s to the 1940s. From 1950 to the mid-1980s there is a somewhat bumpy dimming period. From mid 1980s the well documented brightening in many regions is very clear also in Stockholm.

Authors: Sepúlveda E., Cordero R., and Jorquera J. (Universidad de Santiago de Chile, Chile)
Title: A Solar Radiation Network for the Atacama Desert
Abstract:The Atacama Desert is one of the most interesting places for the deployment of solar power plants (photovoltaic and concentrating solar power) due to the high surface irradiation that characterizes this region. Conditions such as high altitude, prevailing cloudiness, and relatively low ozone and water vapor columns make this site an interesting place for the development of a solar radiation network. Here, we present a new set of Direct Normal Irradiance (DNI) measurements from 7 sites along a latitudinal transect from 20°S to 33°S in northern Chile. At one of the sites, the Chajnantor plateau (23°S, 68°W; 5100 m altitude), DNI can reach an average daily maximum of 1216 W/m2 during the austral summer.
We have also Inferred Aerosol effects from DNI measurements. Aerosols from urban pollution emitted during the morning in Santiago (~570 m altitude) can reach ""La Parva"" site (33.2°S, 70.2°W; 2600 m altitude), in the nearby Andes, thereby decreasing DNI values by ~20% after noon with respect to values under unpolluted conditions at the same solar zenith angle.

Authors: Stefan W., Becker R., and Doppler L. (Deutscher Wetterdienst, Germany)
Title: Radiometry at the German Weather Service: Networks, Methods and Outcomes
Abstract: "The German Weather Service (DWD) operates a ground network in which global solar and diffuse radiation are recorded at 120 stations. The incoming long-wave radiation is observed at 20 stations. The network is currently re-arranged. From 2024, global and diffuse solar radiation and thermal radiation will be observed at 42 stations using thermopile-based radiometers. The spatial coverage will be obtained by using satellite products combined with the observations at 42 sites, which will serve as ground truth stations.
In addition to the ground network, DWD operates a WMO Regional and National Radiation Centre at the Meteorological Observatory Lindenberg (MOL). Apart from calibration services, comprehensive broadband, spectral and spectroscopic measurements are conducted. The BSRN station in Lindenberg was established in 1994 and provides continuous high accuracy records of the incoming short-wave and long-wave fluxes. Sun photometer measurements were initiated in the mid-80s and were complemented by observations using a star photometer in 1992 to obtain records of the AOD and the precipitable water vapor column during day and night. Moon photometry was incorporated into the AOD observing program in 2015. In addition to the photometric observations at Lindenberg, MOL maintains a small photometric network comprising four stations at locations ranging from the Baltic Sea in the North up to the Zugspitze at 2962 masl in the South. DWD contributes with its aerosol observations to GAW-PFR, AERONET and EuroSkyRad. Apart from the spectral observations, spectroscopic measurements are conducted from the UV over the visible into the near infrared wavelength range. MOL contributes to the National UV-monitoring program SUVMONET and the international EUBREWNET. Finally, MOL measures as the only site worldwide vertical profiles of incoming and outgoing broadband short- and long-wave fluxes from the ground into the stratosphere using a radiosonde system equipped with a four component radiometer. These observations are used to study the interactions between radiation, clouds and aerosols, particularly in relation with meteorology and renewable energy.
The long-term records of global solar radiation, which were initiated at Potsdam in 1937 show the decrease starting in the 1950s caused by the intensified aerosol increase due to anthropogenic activities. This global dimming period is followed in late 1980s by an increase of global radiation due to the decrease in atmospheric aerosol loads as a result of various actions regarding air pollution control. Indeed, aerosol loads stabilized around the turn of the millennium at low levels. However, the increase in global radiation has continued until the present with the highest records in the past three years. This increase might be caused by a decrease in cloud cover or a change towards different cloud types, which is also confirmed by the changes in the incoming long-wave radiation. The latter increases continuously due to increasing air temperature, humidity and greenhouse gases, which is consistent with the climate projections and the observed increase is more pronounced in the cloud-free atmosphere.

Authors: Nyeki S., Gröbner J., Vuilleumier L., Lanconelli C., Driemel A., Maturilli M., Schmithüsen H., and Ohkawara N. (PMOD/WRC, Switzerland)
Title: Extending the Calibration Traceability of Longwave Radiation Time-Series (ExTrac)
Abstract: The Earth's surface radiation budget plays a crucial role in the climate system, and accurately characterising components of the radiation budget is therefore an important task. The Baseline Surface Radiation Network (BSRN) is one of several international networks to coordinate the measurement and archiving of radiation data. Downward and upward longwave radiation (DLR, ULR) measurements are conducted with pyrgeometers traceable to the World Infrared Standard Group (WISG) at PMOD/WRC, Davos. However, several important aspects concerning the traceability of the WISG as well as other instrumental issues remain to be resolved by the research community. The ExTrac project is investigating whether a method can be developed to retrieve the original raw measurement voltage from pyrgeometers only using archived BSRN DLR data. This would avoid the need to recover original data from BSRN stations themselves, which may no longer be available due to information technology issues or the loss of a knowledge-base. The main aim is therefore to prevent the loss of legacy data and ensure their availability for future use when traceability and instrumental issues have been resolved by the research community.

Authors: Sneed, A. (The Libyan Center for Solar Energy Research & Studies, Lybia)
Title: Testing Two Calibration Models Using Silicon-Photodiode Pyranometer (LICOR200) Measurements under the Libyan Geographic and Climatic Conditions: Tripoli City Case
Abstract: his study aims at testing two approaches of calibration for correcting data of total global radiation collected using silicon pyranometer (LI-COR200.PY35515) in order to decide which of either (King and Mayer,1998)’s approach or (Navaro, 1988) linear regression equation better suits the Libyan climatic and geographic conditions. The collection of the data for the period from (5, March to 31, July 2021) was conducted at the AWS yard at the Libyan Center for Solar Energy Research and Studies (32.48* N, 13.26* E and 65 m above M.S.E). The calibration process is based on the measurement from standard pyranometer (CMP11 of KIPP & ZONEN), which was installed beside the silicon pyranometer at the time of testing. The empirical models used for the correction of data from silicon pyranometer are as recommended by King and Myers, which take into consideration Factors affecting the readings from LI-COR200, F(AMa), F(AIO) and the sensor’s temperature, and Navvaro’s linear regression equation.

Session 4 - Solar Energy Applications

Authors: Pavanello, D., Zaaiman W., Galleano R., and Noseda F. (European Commission DG Joint Research Centre - ESTI, Italy)
Title: Snapshots of the 2019 International Spectroradiometer Intercomparison
Abstract: Today’s variety of photovoltaic (PV) technologies imposes new challenges to laboratories and industries to precisely measure the performance of devices, and consequently to accurately estimate the energy yield once installed in a specific location. Spectroradiometry has become a key discipline for metrology applied to PV: spectral irradiance is one of the three parameters according to which solar simulators are classified according to IEC 60904-91; precise spectrum measurements are a key factor in the spectral mismatch calculation2. Finally, energy rating calculations according to IEC 61853 involve spectral irradiance conditions different than the AM1.5G standard spectrum3. To tackle these issues, since 2011 the International Spectroradiometer Interlaboratory Comparison (ISRC) takes place annually in different locations of Europe with the participation of laboratories, research institutes and industry partners to assess spectral measurement capabilities and share good measurement practices and protocols4,5,6. In this paper several results of the 9th ISRC 2019 are presented, looking in particular at the impact on characterization of new technologies like organic devices (OPV), dye-sensitized (DSSC) and perovskites.

Authors:Müllejans, H. (Joint Research Center, Ispra, European Commission)
Title: From Sunlight to Power: The history of achieving a globally harmonized approach to PV measurement
Abstract: The measurement of PV module power and energy rating requires necessarily the determination of the incident solar irradiance. This used to be the major contribution to measurement uncertainty (UC) and therefore efforts have been undertaken since the 1980s to develop and harmonise the measurement of irradiance with PV reference cells.
In the mid 1990s the World Photovoltaic Scale (WPVS) was established. Based on this starting point, the concept was followed up by the European Solar Test Installation (ESTI) enlarging the number of contributing results and applying advanced data analysis methods, namely the determination of the WPVS calibration values as Key Comparison Reference Value (KCRV) usually employed in interlaboratory comparisons between National Metrology Institutes (NMI).
This led to PV reference cells being able to measure the incident solar irradiance with an UC around 0.25%, the lowest worldwide for any PV device. Furthermore secondary calibrations of PV reference cells and PV modules are possible with an UC of 0.5% and 1.0% respectively. The WPVS has led to world-wide consistency in PV module power
measurement and contributes to improved accuracy of PV energy rating.

Session 5 - Solar-Terrestrial Sciences