Comparing 2D and 3D Solar Radiation Modeling in Urban Areas

The use of solar radiation in the urban environment is becoming increasingly important for the sustainable development of cities and human societies. Several factors influence the distribution of solar radiation in urban areas, including urban morphology and the physical properties of urban materials. Most of these factors can be modeled with a relatively high accuracy using 2D and 3D solar radiation models. In this paper, the r.sun and v.sun solar radiation models are used to calculate solar radiation for the city of Košice in Eastern Slovakia to assess the accuracy of both approaches for vertical surfaces frequently found in urban areas. The results were validated by pyranometer measurements. The results showed relatively good estimates by the 3D v.sun model and poor estimates by the 2D r.sun model. This can be attributed to an improper representation of vertical surfaces by a digital surface model, which has a strong impact on solar resource assessments. We found that 3D city models prepared in level of detail 2 (LoD2) are not always adequate in case of complex buildings with morphological structures, such as terraces. These cast shadows on facades especially when solar altitude is high and, thus, assessments, even by a 3D model, are inaccurate.

LAMOST medium-resolution spectral survey of Galactic nebulae (LAMOST-MRS-N): subtraction of geocoronal Hα emission

We introduce a method of subtracting geocoronal Hα emissions from the spectra of LAMOST medium-resolution spectral survey of Galactic nebulae (LAMOST-MRS-N). The flux ratios of the Hα sky line to the adjacent OH λ6554 single line do not show a pattern or gradient distribution in a plate. More interestingly, the ratio is well correlated to solar altitude, which is the angle of the Sun relative to the Earthʼs horizon. It is found that the ratio decreases from 0.8 to 0.2 with the decreasing solar altitude from –17 to –73 degree. Based on this relation, which is described by a linear function, we can construct the Hα sky component and subtract it from the science spectrum. This method has been applied to the LAMOST-MRS-N data, and the contamination level of the Hα sky to nebula is reduced from 40% to less than 10%. The new generated spectra will significantly improve the accuracy of the classifications and the measurements of physical parameters of Galactic nebulae.

Definition of Characteristics of Instantaneous Diurnal Activity of Human Body Affected by Solar Radiation in the Circadian Region of Optical Spectrum during Daytime

The article reviews the impact of daily changes in characteristics of solar radiation in the circadian region of optical spectrum on diurnal (circadian) activity of human body with changes of solar altitude angle. Analytical expressions, allowing to theoretically estimation instantaneous melatonin content in human blood plasma and dependence of instantaneous human activity during daytime on solar altitude angle, were obtained.

Analytic Representation of Illuminance on the Earth Surface with Different Types and Conditions of Cloud Cover over the 11-Year Solar Activity Cycle

The article describes the analytical expression approximating experimental data on daily natural illuminance on the Earth surface with different types and conditions of cloud cover over the 11-year solar activity cycle within the solar altitude angle range of 0 ° to 90 °. The values of the direct, diffused, and total illuminance on the Earth surface with different types and conditions of the cloud cover and, to some extent, of the substrate were defined and summarised in tables. The data presented may be used as part of the visual perception studies, light and engineering calculations, architecture and construction practice, etc.

Experimental comparison of summer thermal performance of green roof (GR), double skin roof (DSR) and cool roof (CR) in lightweight rooms in subtropical climate

Subtropical climate is characterized by high solar altitude angle in summer which causes the roof get more heat through solar radiation. GR, DSR, and CR all can decrease solar radiation heat gain of the roof. However, few researches have been done to the comparison of the thermal performance of these three roofs, especially in subtropical climate. In this study, four rooms were built separately with GR, DSR, CR, and ordinary roof (OR). The experiment was done from July 23 to August 4. Results showed that stabilities of the indoor air temperature of the four rooms were: DSR room > GR room > CR room > OR room. The GR, CR, and DSR can reduce the external surface temperature by 13.7°C, 12.0°C, and 4.8°C during the day while bring a temperature rise of 2.3°C, 1.9°C, and 0.9°C at night. Correlation analysis results showed that the internal surface heat flux of GR and DSR were negative correlated with weather factors while internal surface heat flux of OR and CR were positive correlated with weather factors. This study can give support to the selection between GR, DSR, and CR.

Diurnal Cycle Model of Lake Ice Surface Albedo: A Case Study of Wuliangsuhai Lake

Ice surface albedo is an important factor in various optical remote sensing technologies used to determine the distribution of snow or melt water on the ice, and to judge the formation or melting of lake ice in winter, especially in cold and arid areas. In this study, field measurements were conducted at Wuliangsuhai Lake, a typical lake in the semi-arid cold area of China, to investigate the diurnal variation of the ice surface albedo. Observations showed that the diurnal variations of the ice surface albedo exhibit bimodal characteristics with peaks occurring after sunrise and before sunset. The curve of ice surface albedo with time is affected by weather conditions. The first peak occurs later on cloudy days compared with sunny days, whereas the second peak appears earlier on cloudy days. Four probability density distribution functions—Laplace, Gauss, Gumbel, and Cauchy—were combined linearly to model the daily variation of the lake ice albedo on a sunny day. The simulations of diurnal variation in the albedo during the period from sunrise to sunset with a solar altitude angle higher than 5° indicate that the Laplace combination is the optimal statistical model. The Laplace combination can not only describe the bimodal characteristic of the diurnal albedo cycle when the solar altitude angle is higher than 5°, but also reflect the U-shaped distribution of the diurnal albedo as the solar altitude angle exceeds 15°. The scale of the model is about half the length of the day, and the position of the two peaks is closely related to the moment of sunrise, which reflects the asymmetry of the two peaks of the ice surface albedo. This study provides a basis for the development of parameterization schemes of diurnal variation of lake ice albedo in semi-arid cold regions.

Geometrical Assessment of Sunlit and Shaded Area of Urban Trees Based on Aligned Orthographic Views

To quantify the ecosystem services of trees in urban environments, it is necessary to assess received direct solar radiation of each tree. While the Sky View Factor (SVF) is suitable for assessing the total incoming short- and longwave radiation fluxes, its information is limited to specific points in space. For a spatial analysis, it is necessary to sample the area for SVF. A new geometrical method, Area View Factor (AVF), for the calculation of sunlit areas is proposed. AVF is the ratio of the unhidden, projected surface of an object to the whole projected surface of an object in a complex environment. Hereby, a virtual, orthographic camera is oriented in accordance to the sun’s position in the 3D model domain. The method is implemented in the microscale model SkyHelios, utilizing efficient rendering techniques to assess AVF of all urban trees in parallel. The method was applied to Rieselfeld in Freiburg, Germany. The assessed sunlit area is compared to the SVF at the top of each tree and solar altitude angle, revealing a strong relationship between sunlit areas to solar altitude angles. This study shows that AVF is an efficient methodology to assess received direct radiation of urban trees. Based on AVF, it is possible to identify urban areas with shaded and sunlit trees, but it can also be applied to other objects in complex environments. Therefore, AVF is applicable for urban architecture or energetic research questions.

A Theoretical Optimum Tilt Angle Model for Solar Collectors from Keplerian Orbit

Solar energy has been extensively used in industry and everyday life. A more suitable solar collector orientation can increase its utilization. Many studies have explored the best orientation of the solar collector installation from the perspective of data analysis and local-area cases. Investigating the optimal tilt angle of a collector from the perspective of data analysis, or guiding the angle of solar collector installation, requires an a priori theoretical tilt angle as a support. However, none of the current theoretical studies have taken the real motion of the Sun into account. Furthermore, a complete set of theoretical optimal tilt angles for solar energy is necessary for worldwide locations. Therefore, from the view of astronomical mechanics, considering the true orbit of the Sun, a mathematical model that is universal across the globe is proposed: the Kepler motion model is constructed from the solar orbit and transformed into the local Earth coordinate system. After that, the calculation of the optimal tilt angle solution is given. Finally, several examples are shown to demonstrate the variation of the optimal solar angle with month and latitude. The results show that for daily fixed solar collectors, the altitude angle of the collector should be about 6° above the noon solar altitude angle in summer and 6° lower in winter. For annual fixed collectors, the tilt angle should be slightly higher than the latitude. In summary, this study demonstrates that when a location is specified, this model can be used to calculate the theoretical optimum tilt angle of solar collectors for that position.

Deriving plasma parameters of large coronal magnetic loops using J-burst observation from LOFAR

<p>Solar type J radio bursts are the signatures of electron beams travelling along closed magnetic loops in the solar corona. Type J bursts provide diagnostics for observing and understanding coronal loops geometry and electron beams dynamics. Due to the observational limitations, large loops around 1 solar radius in height are ill-defined. Whilst J-bursts at meter-wavelengths are well suited for the analysis of coronal loops at these solar altitudes, applying standard empirical solar plasma density distributions have limitations as they are designed for flux tubes extending into the solar wind and do not capture the curvature of such coronal loops.</p><p>We analysed over 20 type J bursts observed by the LOw-Frequency ARray (LOFAR) on the 10th of April 2019. Using a reference height, we derived the ambient plasma density models that varied along the ascending leg of coronal loops, and also with solar altitude. By estimating the density scale height, we inferred physical parameters of large coronal magnetic loops, roughly 0.7 to 1.5 solar radii above the photosphere. These coronal loops had temperatures around 2 MK and pressures around  5 dyn cm<sup>-2</sup> . We then inferred the minimum magnetic field strength of these closed loops to be around 0.3 G. These large coronal loops' plasma conditions are significantly different to smaller coronal loops and loops that extend out into the solar wind.</p>

MODELAGEM ESPAÇO-TEMPORAL DE ILHAS DE CALOR URBANA E CORRELAÇÃO COM VARIÁVEIS- MONTES CLAROS, MG, BRASIL

As ilhas de calor urbana (ICU) é um aumento da temperatura superficial (TS) e atmosférica, atingindo metrópoles e cidades médias do mundo. Alterações de uso da terra, clima e topografia modulam o fenômeno, compreender relação desses fatores possibilita identificar as causas de ICU. A pesquisa tem foco nesse âmbito, com análise TS, formação de ICU e relação com outras variáveis. O estudo foi na sexta maior cidade do estado de Minas Gerais (Montes Claros), em zona de clima AW (Köppen). A modelagem de TS foi a partir da banda termal Landsat 5 e 8 durante meses agosto de 1991, 2001, 2011 e 2018. Análise estatística multivariada foi aplicada entre informações de TS, variáveis de uso da terra e geomorfométricas. Entre 1991 a 2018 a TS aumentou, convertendo faixas de 22 a 26 °C para > 28 °C, com formação e aumento de 333% de ICU (ICU= > 4 °C média/período). Fatores naturais de radiação solar, altitude, índice de circulação do vento, e face de exposição de vertente influenciam na dinâmica de TS. Porém, o aumento de TS tem melhor relação com densidade urbana e com baixa cobertura vegetal. A redução do problema de ICU deve focar no planejamento de uso da terra e aumento de áreas verdes.