Cloud Coverage Based on All-Sky Imaging and Its Impact on Surface Solar Irradiance

Pfister, Gabriele; McKenzie, R. L.; Liley, Ben; Thomas, A.; Forgan, Bruce; Long, Charles

doi:10.1175/1520-0450(2003)042<1421:ccboai>2.0.co;2

Cited by 250 publications

(179 citation statements)

References 13 publications

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“…Other ground-based sky imaging designs have also been developed (Seiz et al, 2007;Souza-Echer et al, 2006;Calbo and Sabburg, 2008;Cazorla et al, 2008;Heinle et al, 2010;Román et al, 2012;Gauchet et al, 2012) with the most dissimilar design consisting of a downward-pointing camera capturing the sky from a reflection off a spherical mirror (Pfister et al, 2003;Kassianov et al, 2005;Long et al, 2006;Mantelli et al, 2010;Martínez-Chico et al, 2011). Most ground imaging devices follow a relationship between the camera's signal and radiance similar to Eq.…”

Section: Application To Other Imaging Systemsmentioning

confidence: 99%

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

et al. 2016

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Abstract. A method for retrieving cloud optical depth (τ c ) using a UCSD developed ground-based sky imager (USI) is presented. The radiance red-blue ratio (RRBR) method is motivated from the analysis of simulated images of various τ c produced by a radiative transfer model (RTM). From these images the basic parameters affecting the radiance and red-blue ratio (RBR) of a pixel are identified as the solar zenith angle (θ 0 ), τ c , solar pixel angle/scattering angle (ϑ s ), and pixel zenith angle/view angle (ϑ z ). The effects of these parameters are described and the functions for radiance, I λ (τ c , θ 0 , ϑ s , ϑ z ), and RBR(τ c , θ 0 , ϑ s , ϑ z ) are retrieved from the RTM results. RBR, which is commonly used for cloud detection in sky images, provides non-unique solutions for τ c , where RBR increases with τ c up to about τ c = 1 (depending on other parameters) and then decreases. Therefore, the RRBR algorithm uses the measured I meas λ (ϑ s , ϑ z ), in addition to RBR meas (ϑ s , ϑ z ), to obtain a unique solution for τ c . The RRBR method is applied to images of liquid water clouds taken by a USI at the Oklahoma Atmospheric Radiation Measurement (ARM) program site over the course of 220 days and compared against measurements from a microwave radiometer (MWR) and output from the Min et al. (2003)

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Section: Application To Other Imaging Systemsmentioning

confidence: 99%

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

et al. 2016

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“…A study comparing collocated TSI and WSI sky cover retrievals (Long, ARMTR, 2001) [8] shows that in 87% of the retrievals the absolute difference in total sky cover is less than 5%, and 94% of the time the two retrievals are within 10% total sky cover. Another study comparing TSI sky cover retrievals with another sky imager in Lauder, New Zealand [2] showed that in about 80% of the retrievals the absolute difference in total sky cover is less than 10%. Thus, although there exists some uncertainty in fractional sky cover retrievals from sky images, this uncertainty does not appear to be greater than that uncertainty attached to human observations for the HSI and TSI retrievals used here.…”

Section: Cn Longmentioning

confidence: 99%

“…This problem has been recognized previously, for instance by Pfister (JAM. 2003) [2], Sabburg (ACP, 2004) [3]; and Long (JTech, 2006) [4] which briefly described the methodology presented here in broad general terms. In this *Address correspondence to this author at the Atmospheric Radiation Measurement Program Pacific Northwest National Laboratory P.O.…”

Section: Introductionmentioning

confidence: 99%

Correcting for Circumsolar and Near-Horizon Errors in Sky Cover Retrievals from Sky Images

Long¹

2010

TOASCJ

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Fractional sky cover amounts retrieved from sky imagery are overestimated significantly at times due to occurrences of "whitening" near the sun, and near the horizon for low sun, in the images. This phenomenon occurs due to forward scattering of visible light by aerosols and haze, and the intensity range limitations of the detectors of the cameras used to record the sky images. Our results suggest that when the problem occurs, the magnitude of the overestimate is typically on the order of about 10% to 20% fractional sky cover. To help alleviate this problem, a statistical analysis of the time series of the areas in the image near the sun position and along the horizon centered on the solar azimuth angle has been developed. This statistical analysis requires that images be captured frequently, at least once per minute. For times when the overestimation is detected as occurring, a correction is applied to the retrieved sky cover amounts. When the sky cover amount correction is applied, analysis indicates that the result better matches the actual sky conditions present, as noted by visual inspection of the sky images in question. In addition, frequency-of-occurrence histogram comparisons show that the adjusted results improve the agreement with other methodologies and expectations. Thus, the methodology presented here helps produce more accurate fractional sky cover retrievals.

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“…Posteriormente, essas imagens são tratadas por uma função dentro desse mesmo programa, mensurando qualé a razão entre os montantes das cores vermelho e azul em cada pixel da imagem. O valor dessa razão define se há uma nuvem ou não naquela parte da imagem (Pfister et al, 2003). Em geral, a existência de uma nuvem produz setores onde a presença da cor vermelhaé maior que a da cor azul.…”

Section: Methodsunclassified

Taxa de dose eritematosa sob céu encoberto por alto-estrato: estudo de caso

Silva¹

2010

Rev. Bras. Geof.

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ABSTRACT. The erythemal dose rate (EDR) on the Earth's surface depends mainly on the solar zenith angle, column ozone, clouds, aerosols and surface albedo.Clouds are the main atmospheric agent in the modulation of EDR. Therefore, an investigation into the effect of clouds on the ultraviolet radiation (UVR) incidence is an important matter due to the effects of UVR on the biosphere. In this work, a case study is reported where EDR incidences are presented for an overcast-sky scenario of Altostratus (As) in Belo Horizonte (BH, 19.92 • S, 43.94 • O, 858 m, 331 km 2 , Brazil). Although the attenuation yielded by the As layer, the incidence of UVR remained high setting in a threatening weather condition for people in outdoor activities.Keywords: solar ultraviolet radiation, clouds, cloud cover, UV-Index. RESUMO.A taxa de dose eritematosa (TDE) na superfície terrestre devidoà radiação ultravioleta (RUV) emitida pelo Sol depende principalmente doângulo solar de zênite, da coluna de ozônio, da cobertura de nuvens, dos aerossóis em suspensão e do albedo de superfície. As nuvens são o mais importante agente atmosférico na modulação da TDE, e uma investigação detalhada do efeito produzido por elas na incidência de RUV se justifica pelo impacto dessa radiação na biosfera. Neste trabalho, descreve-se um estudo de caso para a incidência de TDE sob céu completamente encoberto por alto-estrato (As) em Belo Horizonte (BH, 19,92 • S, 43,94 • O, 858 m, 331 km 2 , Brasil). Apesar da RUV ser atenuada pela camada de As, isso nãoé suficiente para reduzir a incidência da mesma em termos de TDE a níveis seguros, representando uma ameaçaàs pessoas em atividades ao ar livre.Palavras-chave: radiação solar ultravioleta, nuvens, cobertura de nuvens,Índice UV.Instituto de Estudos Avançados (IEAv), Rodovia dos Tamoios km 5,5, Putim, 12228-001 São José dos Campos, SP, Brasil.

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Cloud Coverage Based on All-Sky Imaging and Its Impact on Surface Solar Irradiance

Cited by 250 publications

References 13 publications

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

Correcting for Circumsolar and Near-Horizon Errors in Sky Cover Retrievals from Sky Images

Taxa de dose eritematosa sob céu encoberto por alto-estrato: estudo de caso

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