Impact of Cloud Cover on Global UV Irradiance over Baghdad, Iraq (original) (raw)
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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.
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
Journal of Geophysical Research, 2000
Assessment of the effects of ozone depletion on biologically effective solar UV at ground level has been greatly advanced through the use of remote sensing data. Satellite data on atmospheric properties allow the construction of geographically distributed surface UV radiation maps based on radiative transfer calculations. In this respect, clouds play a dominant but rather complex role. We compared the reduction of daily UV doses due to clouds, as derived from satellite cloud data, with the reduction derived from routine ground-based measurements of global solar radiation (i.e., broadband total solar irradiances with wavelengths between 0.3 and 2.8 m). An empirical relationship is used to link the reduction due to clouds of global solar radiation and UV radiation. The abundance of global solar radiation measurements (data from over 125 stations in 30 satellite grid cells) for the European region ensured a sound basis for the data analysis for the period considered (May, June, and July of 1990, 1991. Approximately 6500 daily UV-reduction factors, defined as the ratio of daily UV doses calculated with and without clouds, were thus obtained applying both methods. The daily UV-reduction factors (and 10-day averaged UV reduction factors) from the two independent sources correlated well, with r 2 ϭ 0.83 (r 2 ϭ 0.89), and had a standard deviation of 0.06 (0.03). Over 90% of the satellite-derived results agreed within a range of Ϯ0.14 (Ϯ0.07) with the ground-based measurement-derived results. We evaluated sources of uncertainty related to spatial and temporal resolution, and optical properties, and estimated their consequences and range. Among these different sources the largest uncertainties are caused by the sampling error, i.e., grid-cell average versus station average, which is on average 0.10 for daily UV-reduction factors. Information on the atmospheric optical properties during the measurements may reduce the stated range of uncertainty from Ϯ0.14 to Ϯ0.07. The variation of the measurements from station to station is then the limiting factor. We concluded that the reduction of daily UV based on satellite-derived cloud cover and cloud optical thickness relates well with the UV reduction due to clouds derived from ground-based global solar radiation measurements.
Geophysical Research Letters, 2004
1] An extensive intercomparison has been made between the daily averaged reduction factors in UV irradiance due to the presence of clouds as derived from ground-based (pyranometer) and satellite based measurements for the summer months of six years over Europe. Two independent satellite datasets were used for this purpose (ISCCP D1 and TOMS) and the resulting correlations between daily, 10-day and monthly averaged values are presented. In general, the correlation between the daily averaged TOMS and groundbased values matches those for the corresponding ISCCP comparisons (with typical R 2 values of 0.85 and 0.8, respectively). Moreover, the monthly averaged TOMS values exhibit a more favorable agreement with groundbased values, with typical reduction factors ranging from between 0.61-0.77, with a typical standard deviation of <0.05 from the ideal.
Modification of spectral UV irradiance by clouds
Journal of Geophysical Research, 2002
1] Approximately 10,000 UV irradiance spectra resulting from 2 years of continuous measurements in Germany were used as a database to analyze the effect of cloudiness on spectral UV irradiance. Values of spectral cloud modification factors (CMF) were derived by modeling a corresponding clear-sky irradiance spectrum for every UV measurement under cloudy conditions. The total set of CMF values was used to train neural networks using different sets of input data ( parameter records) to describe the clouds, resulting in different, optimized, algorithms (CMF parameterizations). These different CMF parameterizations were evaluated by asking how the quality of the derived CMFs depended on the information content of different parameter records. It was shown that a visual description of cloudiness is not adequate to determine CMFs for an actual case (deviations of 50% and more), even if it was known whether or not the solar disk was obscured by clouds. Improvements for the determination of actual CMFs are possible, with deviations mostly below 15% if the parameter record comprises an actual broadband irradiance measurement. It was shown that such a CMF parameterization is able to provide a good estimation of actual CMFs, also for places with a different cloud climatology. The sensitivity of CMFs to wavelength and solar zenith angle was investigated on the basis of the derived CMF parameterizations. The relations found depend on the kind of CMF parameterization, i.e., the parameter record. In particular the separation of those cases when the solar disk is visible from those cases when the solar disk is obscured may lead to different dependencies of CMFs on solar zenith angle and wavelength.
Empirical models of UV total radiation and cloud effect study
International Journal of Climatology, 2009
Several empirical models of hourly ultraviolet total radiation (UVT) have been proposed in this study. Measurements of UVT radiation, 290-385 nm, have been recorded at ground level from February 2001 to June 2008 in Valladolid, Spain (latitude 41°40 N, longitude 4°50 W and 840 m a.s.l.). The empirical models have emerged due to the lack of some radiometric variables in measuring stations. Hence, good forecasts of them can be obtained from usual measures in these stations. Therefore, some advantages of the empirical models are that they allow the estimation of past missing data in the database and the forecast of future ultraviolet solar availability. In this study, reported models in the bibliography have been assessed and recalibrated. New expressions have been proposed that allow obtaining hourly values of ultraviolet radiation from global radiation measures and parameters as clearness index and relative optical air mass. The accuracy of these models has been assessed through the following statistical indices: mean bias, mean-absolute bias and root-mean-square errors whose values are close to zero, below 7% and below 10%, respectively. Two new clear sky models have been used to evaluate two new parameters: ultraviolet and global cloud modification factors, which can help to understand the role of the clouds on solar radiation. The ultraviolet cloud modification factor depends on cloudiness in such a way that its value under overcast skies is half of the cloudless skies one. Exponential and potential fits are the best relationships between both cloud factors. Finally, these parameters have been used to build new UV empirical models which show low values of the statistical indices mentioned above.
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.
On the use of a cloud modification factor for solar UV (290-385 nm) spectral range
Theoretical and Applied Climatology, 2001
Knowledge of ultraviolet radiation is necessary in different applications, in the absence of measurements, this radio-metric¯ux must be estimated from available parameters. To compute this¯ux under all sky conditions one must consider the in¯uence of clouds. Clouds are the largest modulators of the solar radiative¯ux reaching the Earth's surface. The amount and type of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received at the Earth's surface. This cloud radiative effect is different for the different solar spectral bands. In this work, we analyse the cloud radiative effect over ultraviolet radiation (290±385 nm). This could be done by de®ning a cloud modi®cation Factor. We have developed such cloud modi®cation Factor considering two different types of clouds. The ef®ciency of the cloud radiative effect scheme has been tested in combination with a cloudless sky empirical model using independent data sets. The performance of the model has been tested in relation to its predictive capability of global ultraviolet radiation. For this purpose, data recorded at two radiometric stations are used. The ®rst one is located at the University of Almerõ Âa, a seashore location (36.83 N, 2.41 W, 20 m a.m.s.l.), while the second one is located at Granada (37.18 N, 3.58 W, 660 m a.m.s.l.), an inland location. The database includes hourly values of the relevant variables that cover the years 1993±94 in Almerõ Âa and 1994±95 in Granada. Cloud cover information provided by the Spanish Meteorological Service has been include to compute the clouds radiative effect. After our study, it appears that the combination of an appropriate cloudless sky model with the cloud modi®cation Factor scheme provides estimates of ultraviolet radiation with mean bias deviation of about 5% that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar spectrum provides evidence for the spectral dependency of the cloud radiative effect.
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.
Atmospheric Research, 2010
The cloudiness effect on solar ultraviolet radiation (UV) has been analyzed in this study. Measurements of erythemal and UV total radiations have been registered in Valladolid, Central Spain (lat. 41°40′N, long. 4°50′W and 840 m a.s.l.). A statistical analysis of cloudiness has been carried out resulting clear skies (0-2 oktas) the most frequent conditions under low cloud cover, while cloudy skies (6-7 oktas) are the prevailing under total cloud cover. Hence, the dependences of erythemal UV (UVER) and UV total (UVT) radiations and CMF values (on both ranges) on total and low cloud covers have been analyzed. In all cases, low clouds show higher attenuation than total cloud cover. Moreover, an empirical formula proposed by other authors for several Spanish cities is verified with very similar coefficients for Valladolid database. Finally, the dependence of the ratio between CMF values on UVER and UVT radiations on cloud cover and solar elevation angle is analyzed. As a result, UVER and UVT radiations are not affected by the clouds in the same way. Actually, for low solar elevation angles, UVER is not as attenuated as UVT radiation. However, for high ones under cloudy (6-7 oktas) and, particularly, overcast (8 oktas) conditions, UVER presents smaller CMF values and, therefore, a higher attenuation. Due to the different spectral ranges between erythemal and UV total radiations, the photon reflections above the cloud, the Rayleigh scattering and the interaction of UV radiation with atmospheric components like ozone could explain these effects.