Cristian Oprea’s scientific contributions

In the past, long-term recordings of solar radiation energy were not commonly conducted. However, several observations using the Arago-Davy actinometer were made in different parts of the world during the nineteenth century. In this paper, we propose a method to convert actinometric degree data into information on global solar irradiance on a horizontal surface. We utilized hourly actinometric degree data from the Montsouris Observatory in Paris recorded between 1873 and 1877. Three models were tested for estimating solar global irradiance at ground level from actinometric degrees. Despite the quality of the solar irradiance data provided by the Twentieth Century Reanalysis Project version 3 (20CRv3) was subjected to criticisms, these data are used here as a reference, taking into account the lack of similar data for the nineteenth century. One of the main challenges in this study is the fact that the solar irradiance incident on the Arago-Davy actinometer is not global solar irradiance on horizontal surface, GH${G}_{H}$, a quantity which is usually measured and recorded. To address this issue, a proxy value of GH${G}_{H}$, named GH+${G}_{H}^{+}$, is defined by using the first sub-model of the Ferrel-Pouillet model (named FP1). The analysis showed that GH+${G}_{H}^{+}$ is suitable for estimating both hourly and daily averaged values of the global solar irradiance GH${G}_{H}$. However, the model FP1 requires two input parameters, namely the actinometric degree DD and the bright-bulb thermometer temperature tbright-bulb${t}_{{\mathrm{bright}}-{\mathrm{bulb}}}$, while most long-term observations on the globe have data only for DD. This makes the FP1 model less useful in these cases. To address this, a method was proposed that relies only on DD as an input parameter. This method provides hourly and daily averaged irradiance data that are similar to those obtained using the FP1 model with two input parameters. The accuracy of this method is expected to remain the same in colder climates but decrease in warmer climates. The proposed method also provides credible lower and upper bounds for the interval of variation of monthly averaged irradiance. Additionally, at the level of monthly averaged solar irradiance data, the proposed method and the 20CRv3 project seem to support each other.

This paper overviews the development of the solar radiation monitoring network in Romania, stressing the milestones, the current configuration and utility. In 1879, Ștefan C. Hepites, the most prominent figure and pioneer of the Romanian meteorology, founded a meteorological station in his courtyard from Regală Street, in respect of all the recommendations issued by the First International Meteorological Congress held in Vienna in 1873. It is this station where the first solar radiation measurements in Romania started. The station included an Arago actinometer for global solar radiation measurements, and continuous daily observations were performed until the end of 1881. Once the National Meteorological Service of Romania is founded, global radiation was measured within the Observatory of the Meteorological Institute București-Filaret (from 1888 to 1908). Despite dramatic difficulties such as WWII, political changes or economic crisis, the actinometric network developed gradually, aiming to provide useful operational and climate information. The main solar radiation observatories are located in Bucharest-Afumați (from 1949), Constanța (1952), Timișoara (1957), Cluj-Napoca (1957), Iași (1963), Craiova (1971), and Deva (1982), and they monitor the following variables: direct, diffuse, global and reflected solar radiation, total radiation budget, daylight illumination on horizontal surface, other meteorological parameters. Automatic measurements started in 2006 and from 2010 new stations were added to the national network which at present contains 36 stations, well distributed in accordance to the Romania’s geographical conditions.

Fifty-four broadband clear-sky models for computation of global solar irradiance on horizontal surfaces are tested by using measured data from Romania (South-Eastern Europe). The input data to the models consist of surface meteorological data, column integrated data and data derived from satellite measurements. The testing procedure is performed in twenty-one steps for two different sites in Romania. The models accuracy is reported for various sets of input data. No model ranked “the best” for all sets of input data. However, some of the models were ranked among the best for most of the testing steps, and thus performed significantly better than others. These “better” models are, on an equal footing, ESRA3, Ineichen, METSTAT and REST2 (version 8.1). The next “better” models are, on an equal footing, Bird, CEM and Paulescu and Schlett. Details about the accuracy of each model are found in the Electronic Supplementary Content for all testing steps.

Fifty-four broadband models for computation of solar diffuse irradiation on horizontal surface were tested in Romania (South-Eastern Europe). The input data consist of surface meteorological data, column integrated data, and data derived from satellite measurements. The testing procedure is performed in 21 stages intended to provide information about the sensitivity of the models to various sets of input data. There is no model to be ranked “the best” for all sets of input data. However, some of the models performed better than others, in the sense that they were ranked among the best for most of the testing stages. The best models for solar diffuse radiation computation are, on equal footing, ASHRAE 2005 model (ASHRAE 2005) and King model (King and Buckius, Solar Energy 22:297–301, 1979). The second best model is MAC model (Davies, Bound Layer Meteor 9:33–52, 1975). Details about the performance of each model in the 21 testing stages are found in the Electronic Supplementary Material.

Fifty-four broad band models for computation of global and diffuse irradiance on horizontal surface are shortly presented and tested. The input data for these models consist of surface meteorological data, atmospheric column integrated data and data derived from satellite measurements. The testing procedure is performed for two meteorological stations in Romania (South-Eastern Europe). The testing procedure consists of forty-two stages intended to provide information about the sensitivity of the models to various sets of input data. There is no model to be ranked ''the best'' for all sets of input data. Very simple models as well as more complex models may belong to the category of ''good models''. The best models for solar global radiation computation are, on equal-footing, ESRA3, Ineichen, METSTAT and REST2 (version 81). The second best models are, on equal-footing, Bird, CEM and Paulescu & Schlett. The best models for solar diffuse radiation computation are, on equal-footing, ASHRAE2005 and King. The second best model is MAC model. The best models for computation of both global and diffuse radiation are, on equal-footing, ASHRAE 1972, Biga, Ineichen and REST2 (version 81). The second best is Paulescu & Schlett model.

Commencer du mois d’aout 2006 dans le réseau météorologique national de la Roumanie les mesurages du rayonnement solaire global et de la durée effective d’ensoleillement ont été effectués en même temps par les traducteurs et les appareilles classiques. Le system classique pour les mesurages du rayonnement solaire global est représenté par les actinographes bimétalliques Robitzsch. Le system automatique utilise les thermoélectriques pyranometres CM6 B Kipp&Zonen. Le system classique pour les mesurages de la durée effective d’ensoleillement est représenté par les héliographes R. Fuess. Le system automatique utilise les thermoélectriques pyranometres CM6 B Kipp&Zonen. L’actinographe Robitzsch est un system mécanique qui présente une inertie à l’ordre de minutes, pendant que le thermoélectrique pyranometre CM 6B a une inertie à l’ordre de seconds. Ainsi, les valeurs du rayonnement solaire global mensurées par les deux systèmes sont différentes. Cette première comparaison a été effectuée avec les données météorologiques d’année 2007 pour les stations météorologiques Timişoara, Galaţi et Cluj-Napoca. L’analyse des valeurs de la durée effective d’ensoleillement résultantes des les deux systèmes a relevé des différences techniques et méthodologiques. Cette première comparaison a été effectuée avec les données météorologiques d’année 2007 pour la station météorologique Gura Portitei, qui est l’une des plus représentatives stations météorologiques de la Roumanie.