Sky Component Research Articles

Estimating surface longwave radiative fluxes from satellites utilizing artificial neural networks

A novel approach for calculating downwelling surface longwave (DSLW) radiation under all sky conditions is presented. The DSLW model (hereafter, DSLW/UMD v2) similarly to its predecessor, DSLW/UMD v1, is driven with a combination of Moderate Resolution Imaging Spectroradiometer (MODIS) level‐3 cloud parameters and information from the European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA‐Interim model. To compute the clear sky component of DSLW a two layer feed‐forward artificial neural network with sigmoid hidden neurons and linear output neurons is implemented; it is trained with simulations derived from runs of the Rapid Radiative Transfer Model (RRTM). When computing the cloud contribution to DSLW, the cloud base temperature is estimated by using an independent artificial neural network approach of similar architecture as previously mentioned, and parameterizations. The cloud base temperature neural network is trained using spatially and temporally co‐located MODIS and CloudSat Cloud Profiling Radar (CPR) and the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) observations. Daily average estimates of DSLW from 2003 to 2009 are compared against ground measurements from the Baseline Surface Radiation Network (BSRN) giving an overall correlation coefficient of 0.98, root mean square error (rmse) of 15.84 W m−2, and a bias of −0.39 W m−2. This is an improvement over an earlier version of the model (DSLW/UMD v1) which for the same time period has an overall correlation coefficient 0.97 rmse of 17.27 W m−2, and bias of 0.73 W m−2.

Correlating Indian measured sky luminance distribution and Indian Design clear sky model with five CIE Standard clear sky models

This paper aims to find out representative CIE (International Commission on Illumination) Standard Clear Sky model(s) for three different seasons - winter solstice, equinox, and summer solstice applicable for prevailing clear sky climatic conditions in India [Roorkee]. Indian measured sky luminance distribution database is available only for Roorkee [29°51′ N; 77°53′ E]. To find out the best match between Indian measured sky luminance distribution and each of five CIE Standard Clear sky models, sky component of spatial illuminance distribution over the working plane of a room was simulated by MATLAB for three different seasons. The effectiveness of using Indian design clear sky model proposed in IS: 2440:1976 was also evaluated side by side. Daylight Coefficient method has been applied for the simulation using Indian sky luminance database. The simulation has been done for the room with eight different window orientations ranging from 0° to 315° with an interval of 45° to generate data for the entire sky vault. To find out the best fit CIE model(s), statistical test of significance (95% confidence level) has been carried out for the spatial illuminance distributions taking results obtained with Indian measured sky luminance distribution data as reference. Analysis revealed that CIE Standard General Clear sky type 15 described as “White-blue sky, turbid with a wide solar corona effect” is the best-fit clear sky model for both summer and equinox seasons and sky type 11 described as “White blue sky with a clear solar corona” is the best-fit clear sky model for winter season at Roorkee. Hence these sky types can be used for daylight prediction instead of Indian Design Clear Sky Model which shows large deviations.

Identifying CIE standard skies using vertical sky component

This paper studies the classification of the International Commission on Illumination (CIE) Standard General Skies using vertical sky component ( VSC) defined as the procedures for identifying the Standard General Skies are proposed. The performance of the recommended sky classification process was evaluated against measured data in terms of percentage error (PE) and root-mean-square error (RMSE). The results revealed that the method produced the RMSE of not more than 20% for most of classified Standard General Skies. The proposed approach provides an alternative for defining CIE Standard General Skies when the measured vertical and horizontal daylight illuminance data are available.

Iluminação natural: a contribuição de suas reflexões no interior do ambiente construído

This article tackles the impact of internal reflections in day-lit built environments. This study shows how the inner space geometry, surface properties, openings percentage, position and distribution affect the values and distribution attributed to the internally reflected component. Such knowledge enables us to establish parameters and relations among the above variables so as to fill a gap in the existing biography. The simplified methods provided and used by existing legislation to assess the day-lit environment do not take into account light's basic components, which affect the internal spaces. Therefore, this fact has a negative influence on the prediction of light's behavior. Daylighting contribution in an environment depends directly on the sky component, on the light reflected on external surfaces, and also on the light reflected on internal surfaces among other aspects; this knowledge is crucial for a good understanding of the day-lit environment. The findings of this study should enable the manipulation of variable values and parameters, which affect the reflected light in order to ensure both the quantity and mainly the quality of the day-lit environment.

OBSERVATIONAL DETERMINATION OF THE TURBULENT AMBIPOLAR DIFFUSION SCALE AND MAGNETIC FIELD STRENGTH IN MOLECULAR CLOUDS

We study the correlation of the velocity dispersion of the coexisting molecules H13CN and H13CO+ and the turbulent energy dissipation scale in the DR21(OH) star-forming region. The down-shift of the H13CO+ spectrum relative to H13CN is consistent with the presence of ambipolar diffusion at dissipation length scales that helps the process of turbulent energy dissipation, but at a different cut-off for ions compared to the neutrals. We use our observational data to calculate a turbulent ambipolar diffusion length scale L'\simeq17 mpc and a strength of B_{pos}\simeq1.7 mG for the plane of the sky component of the magnetic field in DR21(OH).

Daylight in atrium buildings: Geometric shape and vertical sky components

This study investigated the impact of well geometry on vertical sky components in atria with square and rectangular forms under a CIE standard overcast sky. By comparing scale model measurements and analytical theory the vertical sky components, calculated using Radiance, were validated. More simulated data of vertical sky components for a very wide range of atrium geometries are given. From the results the attenuation of the vertical sky component on the wall of a square atrium is explained and some empirical functions are derived. In addition, the results from atrium models with rectangular floor plans show the relationships between the vertical sky component and the plan aspect ratio. Some guidelines for design are presented.

THEFERMIHAZE: A GAMMA-RAY COUNTERPART TO THE MICROWAVE HAZE

The Fermi Gamma-ray Space Telescope reveals a diffuse inverse Compton (IC) signal in the inner Galaxy with a similar spatial morphology to the microwave haze observed by WMAP, supporting the synchrotron interpretation of the microwave signal. Using spatial templates, we regress out π0 gammas, as well as IC and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess toward the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with IC from a hard population of electrons. The morphology and spectrum are consistent with it being the IC counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition, the implied electron spectrum is hard; electrons accelerated in supernova shocks in the disk which then diffuse a few kpc to the haze region would have a softer spectrum. We describe the full-sky Fermi maps used in this analysis and make them available for download.

Spreadsheet-based calculation tool for direct daylight illuminance adaptable for different glazing properties and sky models

The methods normally used to calculate daylight illuminance of rooms are limited in their capacity to model different sky luminance distributions and different window glazing properties. A spreadsheet-based model is presented in this paper using first-principle calculations, which allows for a free choice of sky luminance distribution and glazing material properties. The glazing is specified in terms of material refractive index, absorption coefficient and thickness, and may be either single or double glazed. The calculations need to be repeated for each of a large number of small elements of window area, which is easily achieved using a spreadsheet. Use is illustrated by comparing the calculation of sky components with standard published data, and demonstrating the effect of substituting double for single glazing. The model was developed for educational use, but would also be useful for designers who need to investigate quickly the effect of altering glazing properties and sky conditions, without recourse to specialist software.

Daylighting performance of atriums in subtropical climate

Design of an atrium aims to provide a good quality of daylighting. This paper deals with a parametric study of geometric effect on the daylighting performance of two typical types of atriums, enclosed and linear, in the subtropical climatic area. A case study of the daylighting of two existing atrium buildings is presented. The software, RADIANCE, is employed to simulate daylighting performance. The simulation results are compared with the field measurement and a 1:100 model atrium to validate the RADIANCE simulation. In the study of the geometric effects on daylighting performance in atriums, three combined geometric parameters, plane aspect ratio (PAR ¼ W/L), section aspect ratio (SAR ¼ H/L) and well index (WI ¼ H(W þ L)/2WL), are introduced to determine a proper design geometry. The relationships between the daylight factor (DF is defined as the ratio of the illuminance at a particular point within an enclosure to the simultaneous unobstructed outdoor illuminance under same overcast sky component) and these geometric factors are analysed for the two types of atriums. The results show that, for the enclosed atrium, the DF has an approximately linear relationship with either SAR or WI, while the DF of the linear atrium has an approximately linear correlation with the PAR. The recommended geometric factor may be WI ¼ 1.5 for the enclosed atrium and PAR ¼ 0.5 for the linear atrium if the comfortable DF is required. The results of this will provide a demonstration and general guide for the designers to predict daylighting performance of an atrium building in the subtropical climate region.

Woody-to-total area ratio determination with a multispectral canopy imager

Leaf area index (LAI) - defined as one half of the total green leaf area per unit ground surface area - can be determined by direct or indirect methods. Three major sources of errors exist in indirect LAI measurements: within-shoot clumping, beyond-shoot clumping and non-photosynthetic components. The effect of non-photosynthetic components on LAI measurements can be described by the woody-to-total area ratio, alpha; however, no convenient and efficient indirect methods have been developed to estimate alpha, especially the variations in alpha with zenith angle , alpha(theta). We describe the development and use of a multispectral canopy imager (MCI) to estimate alpha and alpha(theta) by considering the effects of non-random distributions of canopy elements and woody components and the overestimation of needle-to-shoot area ratio on woody components. The MCI, which mainly comprises a near-infrared band camera (Fujifilm IS-1), two visible band cameras (Canon 40D), filters and a pan tilt, was developed to measure clumping index, woody-to-total area ratio and geometric parameters of isolated trees. Two typical sampling plots (Plots 1 and 5) chosen from among 16 permanent forest experiment plots were selected for the estimation of alpha and alpha(theta). The non-random distributions of canopy elements and woody components were estimated separately at eight zenith angles (from 0 degrees to 70 degrees in increments of 10 degrees) using MCI images based on the gap size distribution theory. The visible/near-infrared image pairs captured by the MCI were able to discriminate among sky, leaves, cloud and woody components. Based on three methods of estimation, we obtained woody-to-total area ratios of 0.24, 0.19, 0.19 for Plot 1 and 0.23, 0.18, 0.17 for Plot 5. If clumping effects were ignored, alpha values were overestimated by as much as 21% and 24% at Plots 1 and 5, respectively. We demonstrated that alpha(theta) varied with the zenith angle, with variations in the range of 3-33% at Plot 1 and 2-65% at Plot 5. A new formula for the precise determination of LAI is proposed.

Simple method for determining daylight illuminance in a heavily obstructed environment

Daylighting has long been recognized as an important issue in architecture. In Hong Kong many buildings are constructed close to each other and hence the external environment plays a significant role in building designs. Recently, vertical daylight factor (VDF) has been used as a criterion to justify the provision of natural lighting in buildings. This paper studies the calculation approach of VDF in a heavily obstructed environment. The techniques for determining the sky component and the reflected daylight from surrounding buildings and ground surfaces are described. Calculation tools in the form of simple equations and diagrams through computer-simulation analysis were established. The performance of the proposed method was evaluated against the daylight illuminance obtained by other independent computer simulations. It was found that predictions from the proposed approach were in good agreements with those produced by simulated results. The findings provide architects and building designers with a simple method for estimating the daylight illuminance at early design and planning stage.

Daylight Prediction and Assessment: Theory and Design Practice

Historical developments of interior daylight criteria were influenced by the simplifying imagination of the uniform Lambertian sky luminance on a fictitious hemisphere with a unity radius. Thus, the Sky Component of the Daylight Factor was defined and standardised in many countries, and different methods, tables or graphical tools were prepared for practical window design tasks or for assessing interior daylighting. With a growing need to design sustainable building environments and energy efficient architecture utilising local daylight climate, conditions that are more realistic have to be taken into account. Several novel approaches in criteria and assessment methods (respecting annual changes in sunlight and skylight conditions) are reviewed in this paper indicating future energy conservation measures or directives that will considerably influence architectural science and design practice.

Predicting daylight availability dynamically based on forecasts of a weather observatory

By using measured data of the research class IDMP station, a subset of three sky types and their probability of occurrence has previously been established to represent sky conditions of Hong Kong. This was dubbed the Hong Kong Representative Sky (HKRS). The HKRS allows a better prediction of daylight on the vertical surfaces of buildings than the CIE Overcast Sky. The HKRS was still a static representation. This paper investigates the possibility of using the weather predictions of the observatory to establish dynamically the HKRS. A methodology is proposed, and the results indicate that the approach is feasible. It is established that the weather prediction information issued by the Hong Kong Observatory (HKO) provides a reliable means for estimating probabilities of the sky types at a particular moment. By using this information, a better prediction of vertical sky component (VSC) on building surfaces, with a reduction of 20–30% mean absolute error, can be achieved. Engineers and architects may use this more accurate information to design more dynamically.

Multiscale optical flow probing of dynamics in solar EUV images

Context. Movies of the solar atmosphere reveal motion and variations in brightness. In particular, sequences of coronal images exhibit the plane- of- the- sky component of the velocity combined with other variations in the signal. The present work analyses solar extremeultraviolet images, as recorded by the Extreme ultraviolet Imaging Telescope ( EIT) on board the Solar and Heliospheric Observatory ( SoHO) and by the Transition Region and Coronal Explorer ( TRACE). Aims. Our aim is to simultaneously estimate the apparent motion vector and the variation in brightness from two successive images. Methods. We present a multiscale optical- flow algorithm derived from a local gradient- based technique that estimates the deformation parameters. Our algorithm is symmetric in the sense that it computes the exact same estimation if the two images are swapped. This also regularises the optical flow when two local image patterns do not match, e. g. in case of temporal sub- sampling. Independently our algorithm regularises the optical flow against aperture effects occurring typically along coronal loops. Results. We demonstrate a new di. erential rotation measurement technique and the identification of coronal events as regions exhibiting a significant brightness variation or an outstanding velocity field. Space weather services have motivated this study. The range of potential interests includes, but also extends beyond, on- disc signatures of flares and coronal mass ejection ( CME). It encompasses, for example, studies of bright points and filaments, coronal seismology, and EIT wave investigations.

Defining standard skies for Hong Kong

A CIE International Daylight Monitoring Programme (IDMP) general class station was established in 2000 at the Chinese University of Hong Kong. The station was upgraded to Research Class IDMP Station in April 2003. The study, which includes the sky luminance scan data collected from April 2003 to May 2005, firstly fits the data to the CIE Standard General Sky definitions, which consist of 15 luminance distributions for modeling the sky from the heavily overcast sky to cloudless clear sky. Then the paper proposes a reduced set of CIE general skies (and their probability of occurrence) to represent the sky conditions of Hong Kong. This reduced set will be known in this paper as “Hong Kong Representative Sky” (HKRS). Further, the paper evaluates the sensitivity of vertical sky component (VSC) to differences between the HKRS and the standard CIE Overcast Sky model. Comparing with observed data, it is demonstrated that the HKRS could give better results. A reduction in error of approximately 20–40% could be expected, depending on the orientation of a surface. Using the HKRS, building designers could better predict daylight availability of their design. Energy saving and more sustainable buildings might result.

Optimal subtraction of OH airglow emission: A tool for infrared fiber spectroscopy

We have developed an optimal subtraction algorithm for removing the residuals of the sky background from multi-fiber spectra. In particular, this algorithm is suited to recover the flux of unresolved lines blended with atmospheric line features and works well in case of spectra dominated by the sky component. Our method is based on PSF matching and can compensate both imperfect wavelength calibration residuals and fiber-to-fiber PSF variations. Using simulated fiber spectra of an emission line object, we show how the algorithm improves the accuracy in line fluxes evaluation and retrieves diagnostic lines normally hidden, making them usable for scientific analysis.

Generalization of the Direct Sky Component Calculation to Openings of Arbitrary Tilt Angle

Among the three components of daylight affecting a surface's illumination inside a room, the direct component, due to light flux reaching the surface element directly from the sky, (also called sky factor) is the most significant one. Its estimation must therefore be as accurate as possible when assessing the daylighting performances of a building.To estimate its value for a given surface, analytical methods can be employed, based on the luminance distribution of the sky and the window's geometric properties: dimensions and position in regard to the surface element. However, such methods have always been restricted to vertical and horizontal windows, requiring heavy approximations to be applied whenever a tilted rectangular opening was considered.A generalized method for assessing the sky component is proposed here, extending it to rectangular windows of any tilt angle. As a purely analytical result could not be derived, it is based on an optimized combination of vertical and horizontal windows situations.To validate the methodology, scale model measurements were performed with a sky simulator for two rectangular openings of varying tilt angle (every 15° from vertical to horizontal): the experimental results proved to be in very good agreement with the calculation-based approach.

A Simplified Procedure Using Daylight Coefficient Concept for Sky Component Prediction

The amount of daylight illuminance at a point inside a building is mainly influenced by the luminance levels and patterns of the sky in the direction of views of the surface at that time. The daylight coefficient approach, which considers changes in the luminance of the sky elements, offers a more effective way for computing indoor daylight illuminances. Recently, Kittler et al. have proposed a new range of 15 standard skies including the CIE (Commission Internationale de l'Eclairage) standard clear sky. This paper presents a graphical method to calculate interior illuminance for the CIE standard clear sky based on daylight coefficient concept. The simplified techniques in the form of a nomograph and Waldram diagram were developed and described. The performance of the graphical method was assessed against the results determined using daylight factor method and a lighting simulation program. It was shown that the sky components predicted by the nomograph were in reasonably good agreement with those produced from the other two approaches. The findings provide architects and building designers a reliable and simple alternative that incorporates daylight coefficient concept to estimate the interior daylight illuminance.

Algorithms for calculation of daylight factors in streets

This paper presents algorithms for calculation of sky components and daylight factors in linear building structures using the solid angle projection method. In order to estimate the precision of calculations two sky models; the uniform sky and CIE overcast sky, both giving equal illuminance on the horizontal plane, are used. Daylight factors calculated for CIE overcast sky are also compared to daylight factors measured in a physical scale model of a linear atrium in an artificial sky of the mirror-box type.

Multi-Colour Photometry with GAIA of the Diffuse Sky Background

The report discusses the possible multi-colour photometry of the diffuse sky surface with the Astro and Spectro telescopes of GAIA. The ordinary photometry of faint stars will give good results of the background for at least 500 million spots on the sky. Measurement from about 100 of these spots in a small area could be averaged to obtain a precision of about 0.5 S 10 of GAIA medium-band photometry in each of the 5 million areas. It is concluded that GAIA medium-band photometry is suited to tackle a whole range of astrophysically interesting diffuse sky components. After modelling and subtracting the time-variable Zodiacal Light, GAIA could provide multi-wavelength all sky maps of the low galactic latitude Diffuse Galactic Light and the high latitude Galactic Cirrus Clouds. Possibly also the Extragalactic Background Light could be detected.