Solar Blue Light Radiation Enhancement during Mid to Low Solar Elevation Periods under Cloud Affected Skies (original) (raw)
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Cloud cover and horizontal plane eye damaging solar UV exposures
International Journal of Biometeorology, 2004
The spectral UV and the cloud cover were measured at intervals of 5 min with an integrated cloud and spectral UV measurement system at a sub-tropical Southern Hemisphere site for a 6-month period and solar zenith angle (SZA) range of 4.7 to approximately 80. The solar UV spectra were recorded between 280 nm and 400 nm in 0.5 nm increments and weighted with the action spectra for photokeratitis and cataracts in order to investigate the effect of cloud cover on the horizontal plane biologically damaging UV irradiances for cataracts (UVBE cat ) and photokeratitis (UVBE pker ). Eighty five percent of the recorded spectra produced a measured irradiance to a cloud free irradiance ratio of 0.6 and higher while 76% produced a ratio of 0.8 and higher. Empirical non-linear expressions as a function of SZA have been developed for all sky conditions to allow the evaluation of the biologically damaging UV irradiances for photokeratitis and cataracts from a knowledge of the unweighted UV irradiances.
The influence of clouds on surface UV erythemal irradiance
Atmospheric Research, 2003
The purpose of this study is to examine the effect of clouds on the ultraviolet erythemal irradiance. The study was developed at three stations in the Iberian Peninsula: Madrid and Murcia, using data recorded in the period , and Zaragoza, using data recorded in 2001. In order to determine the cloud effect on ultraviolet erythemal irradiance, we considered a cloud modification factor defined as the ratio between the measured values of ultraviolet erythemal irradiance and the corresponding clear-sky ultraviolet erythemal irradiance, which would be expected for the same time period and atmospheric conditions. The dependence of this cloud modification factor on total cloud amount, cloud type and solar elevation angle was investigated. The results suggest that the effect of cloud on ultraviolet erythemal irradiance can be parameterized in a simple way in terms of the cloud amount. Our results suggest that the same cloud modification factor model can be used at the three analysed locations estimating the ultraviolet erythemal irradiance with mean bias deviation (MBD) in the range of the expected experimental errors. This cloud modification factor is lower than that associated to the whole solar spectral range, indicating that the attenuation for the ultraviolet erythemal irradiance is lower than that associated to other solar spectral ranges. The cloud modification factor for ultraviolet erythemal irradiance presents dependence with solar elevation, with opposite dependencies with solar elevation for overcast and partial cloud cover conditions, a fact that can be explained in terms of the influence of reflection-enhancement of the ultraviolet irradiance in the last case. Concerning the influence of cloud type, a limited study of two cloud categories, low and medium level and high level, indicated that for overcast conditions, lower clouds presents an attenuation of ultraviolet erythemal irradiance 20% greater than that associated to high level clouds. D
Atmospheric Research, 2007
This research examines the influence of cloud on the cataract effective UV (UV Cat ) irradiances on a horizontal plane over an extended period of 12 months that included the range of cloud conditions, solar zenith angle (SZA) and ozone conditions experienced over that time. The data were collected at five minute intervals. Cloud modification factors were determined from the influence of clouds on the global broadband solar radiation and these were applied to the cloud free cataract effective UV to evaluate the UV Cat irradiances on a horizontal plane for all cloud conditions. A comparison of the measured and calculated UV Cat irradiances for the 2004 data set in the range of SZA of 70 o or less provided an R 2 value of 0.85. The data in the first 6 months of 2005 for an SZA of 70 o or less that were at a different time to that when the technique was developed provided an R 2 value of 0.83 for the comparison of the measured and calculated UV Cat irradiances.
Variation of the enhanced biologically damaging solar UV due to clouds
Photochemical & Photobiological Sciences, 2004
The variation of the biologically damaging solar UV (UVBE) enhanced by clouds above that of clear sky UVBE has been investigated. This was undertaken for summer through to winter for SZA of 5 to 60 o employing an integrated automatic cloud and spectral UV measurement system that recorded the solar UV spectra and the sky images at five minute intervals. The UVBE calculated with action spectra with higher relative effectiveness in the UVA produced the lower percentage of cloud enhanced cases. The DNA UVBE provided the highest percentage of cloud enhanced cases compared to the total number of UV scans with 2.2% cloud enhanced cases. As a comparison, the plant and fish melanoma UVBE provided the lowest percentage of cloud enhanced cases with 0.6% to 0.8% cloud enhanced cases. For the cases of cloud enhanced UVBE, the average ratio of the measured UVBE to calculated cloud free UVBE for the photokeratitis, cataracts, plant, generalized plant damage and fish melanoma action spectra was 1.21 to 1.25. In comparison, the highest value of 1.4 was for the DNA action spectrum.
A generally accepted value for the Radiation Amplification Factor (RAF), with respect to the erythemal action spectrum for sunburn of human skin, is −1.1, indicating that a 1.0% increase in stratospheric ozone leads to a 1.1% decrease in the biologically damaging UV radiation in the erythemal action spectrum reaching the Earth. The RAF is used to quantify the non-linear change in the biologically damaging UV radiation in the erythemal action spectrum as a function of total column ozone (O3). Spectrophotometer measurements recorded at ten US monitoring sites were used in this analysis, and over 71,000 total UVR measurement scans of the sky were collected at those 10 sites between 1998 and 2000 to assess the RAF value. This UVR dataset was examined to determine the specific impact of clouds on the RAF. Five de novo modeling approaches were used on the dataset, and the calculated RAF values ranged from a low of −0.80 to a high of −1.38.
The spectral effects of clouds on solar irradiance
Journal of Geophysical Research, 1998
Knowledge of th,e spectral attenuation associated with clouds is important for accurate estimates of natural irradiance at the Earth's surface. We compare spectral measurements of visible downwelling irradiance, under varying sky conditions at Halifax, Nova Scotia, Canada, with results from a clear-sky model. The spectral effect of clouds is estimated by taking the ratio of the measurements to the modeled irradiances and removing spectrally consistent instrumental effects and errors in the model. Empirical relationships derived between the spectral cloud effect and both CF, the cloud factor (the ratio of measured to modeled irradiances at 490 nm), and f, the fraction of sky covered by cloud, were found to follow a wavelength (•) dependence of the form a(C'F or f)+ b(C'F or f)(•/490) -4 in the 412-700 nm wavelength range. Both this relationship and a previously published linear relationship were found to be inadequate for describing cloudy irradiance data from the Bering Sea, indicating that the spectral effect of clouds can vary with cloud type and location. We show here that the spectral cloud effect can be mimicked by using a clear-sky model and changing the magnitude of the sky reflectivity or the spectral shape and magnitude of the ground albedo within the model. An investigation of the effects of cloud-dependent changes in irradiance spectra on calculations of bio-optical properties is also presented. Estimates of chlorophyll concentration from near-surface radia, nces are found to vary by up to 30%, whereas the effects on estimates of photosynthetically available and usable radiation at the sea surface are negligible. Paper number 1998JC900002. 0148-0227/98/1998JC900002509.00 els. The more complicated representations divide the atmosphere and clouds into several layers, requiring numerous parameters (e.g., LOWTRAN [Kneizys et al., 1983] and SBDART [Ricchiazzi et al., 1998]). Simpler methods compensate for clouds by applying a weighting to the irradiance for the clear-sky case [e.g., A twater and Ball, 1978]. A limited number of studies have shown that in the ultraviolet and visible wavelength regions (from 290 to 700 nm) clouds cause wavelengttldependent attenuation of downwelling solar irradiance [$pinhirne and GreenOnly the study of Siegel et al. [1998] provides a parameterization with coefficients that characterize the spectral effect of clouds on irradiance. However its applicability to a broad range of locations and times of year has not been determined. Different perceptions exist regarding the cause of spectral attenuation by clouds. One hypothesis is that the spectral change in downwelling irradiance is a result of irradiance reflection off the surface of the Earth and clouds [Wang and Lenoble, 1996; h'glling et al., 1997; Frederick, 1997; S. Madronich, personal communication, 1996]. The process is summarized as follows: Downwelling solar irradiance is reflected off the top sur-31,017 31,018 BARTLETT ET AL.: THE SPECTRAL EFFECTS OF CLOUDS face of clouds back to space. Since clouds are generally white or light gray in color, this process is spectrally neutral and simply decreases the magnitude of the downwelling irradiance. Part of this reflected irradiance is then scattered back toward the Earth's surface by the atmospheric constituents. The scattering is greater at shorter (blue) wavelengths than at longer (red) wavelengths, so the resulting downwelling irradiance is bluet relative to irradiance under clear sky.
The influence of cloudiness on UV global irradiance (295–385 nm)
Agricultural and Forest Meteorology, 2003
Two years of continuous measurements of UV (295-385 nm) irradiance recorded at Granada (37.18 • N, 3.58 • W, 660 m a.m.s.l.), Spain, were combined with concurrent synoptic cloud observations to establish the relative influence of clouds on UV irradiance. A marked influence of cloud cover on UV irradiance is evident, but negligible for fractional cloud coverage below 3 octas. The ratio of UV irradiance to the total solar irradiance increases with cloud cover, especially for cloud cover greater than 4 octas, highlighting the different effects of cloud on total solar irradiance and UV irradiance. In order to determine the effect of cloud on UV irradiance, we considered a cloud modification factor, defined as the ratio between the UV measurements and the corresponding clear sky UV irradiance that would be expected for the same time period and atmospheric conditions. It is shown that the effect of cloud for UV wavelengths is less than that for the whole solar spectrum and less than that for the visible part of the spectrum. On the other hand, the small influence of cloud cover for intermediate cloudiness was accompanied by low variability and variability increases with increasing cloud cover.
Effect of Cloud on UVA and Exposure to Humans¶
Photochemistry and Photobiology, 2001
The daily autumn and winter ultraviolet-A (320-400 nm) (UVA) exposures and 6 min UVA irradiance data for a southern hemisphere subtropical site (Toowoomba, Australia, 27.6؇S, 151.9؇E) are presented. This data is used to quantify the effect of cloud on UVA using an integrated sky camera and radiation system. Additionally, an estimate of the effect of enhanced UVA exposure on humans is made. The measurement system consisted of broad-band visible-infrared and UVA sensors together with a sun tracking, wide-angle video camera. The mean daily June exposure was found to be 409 kJ m Ϫ2 . Under the constraints of the uncertainty of both the UVA measurement system and clear-sky model, one case of enhanced UVA irradiance was found. Three cases of cloud enhancement of daily UVA exposure, approaching clearsky levels, were also determined using a calculated clearsky envelope. It was also determined that for a fulltime outdoor worker the additional UVA exposure could approach approximately that of one third of a full winter's day. For indoor workers with an outside lunch break of 12:00-1:00 P.M. the additional UVA exposure was on an average 6.9 kJ m Ϫ2 over three cloud-enhanced days. To the authors' knowledge this is the first paper to present some evidence of cloud-enhanced UVA human exposure.
Journal of Atmospheric and Solar-Terrestrial Physics, 2011
Measurements of UV spectra, total ozone, cloud cover, and cloud optical thickness, obtained at Lampedusa (central Mediterranean), are used to investigate the influence of clouds on the spectral UV irradiance, through the cloud modification factor (CMF), and on five biological processes. The CMF decreases with cloud optical thickness (COT), from about 0.5 for COT 15 to 0.25 for COT 45, and decreases with increasing wavelength above 315-320-nm. Observations display an increase in the CMF from 295 to 320-nm, which is related to enhanced absorption by tropospheric ozone due to the long photon path lengths under cloudy conditions. The use of a wavelength independent CMF instead of the experimentally determined spectral curves produces an overestimation of the biological effects of UV irradiance. The overestimation may be as large as 30% for the DNA damage, 20% for vitamin D synthesis, 12% for plant damage, and 8-10% for phytoplankton inhibition and erythema.