Ultraviolet sky radiance distributions of translucent overcast skies

Summary The diffuse sky radiation component in the ultraviolet wavelengths is often at least 50% of the global irradiance under clear skies, and is the dominant component of ultraviolet global radiation under translucent overcast skies. The distribution of sky radiance was measured in a rural area and modeled for wavelength bands of ultraviolet-B (UVB, 280–320 nm) and ultraviolet-A (UVA, 320–400 nm). Sky radiance measurements were made during the summer of 1993 over a wide range of solar zenith angles using radiance sensors mounted on a hand-operated hemispherical rotation mount. UVB irradiance measurements were also made during each scan. Since the ratio of measured irradiance under overcast skies and that predicted for clear skies was not correlated with cloud base height, opaque cloud fraction, or solar zenith angle, it was concluded that the scattering from the clouds dominated the global irradiance, and this scattering was relatively unaffected by the scattering off opaque clouds in the translucent atmosphere.Analysis of the translucent overcast sky UVA and UVB radiance measurements using a semi-empirical distribution model showed that the spectral influences on multiple scattering, single scattering, and horizon brightening components of the distributions agreed with basic atmospheric radiation theory. The best model used solar zenith, the sky zenith, and the scattering angle with resultant coefficient of determination values of 0.62 and 0.25 for the UVA and UVB respectively. The developed equations can be applied directly to the diffuse sky irradiance on the horizontal to provide radiance distributions for the sky.With 6 Figures     

Cloud Transmission Estimates of UV-B Erythemal Irradiance

Summary Two UV-Biometer 501A instruments were used to estimate global erythemal irradiance at two locations in southwest Sweden; the Earth Sciences Centre, University of G?teborg (57.69° N; 11.92° E) and the island of Nordkoster, 200 km to the north (58.83° N; 10.72° E). A semi-empirical radiative transfer model was used to calculate the global erythemally effective irradiance under clear skies. A ratio of the hourly measured to clear-sky modelled irradiance was then derived for zenith angles 35–70°. Subsequent comparisons were then made with routine measurements of sunshine duration at G?teborg and sunshine duration, cloud cover, type and height at Nordkoster. Cloud transmission of UV-B irradiance decreases with increasing solar zenith angle, with cloud attenuation being 8% stronger at Nordkoster Island for zenith angles >>;60°. Transmission also decreases with increasing cloud cover such that overcast cloud conditions reduce transmissions by an average of 75%. In addition, cloud type affects the amount of ground incident irradiant flux. Fractus cloud afforded the least UV-B transmission (0.16), while cirrus filaments afforded the most (0.95). The spatial and temporal distribution of clouds appears tobe non-random. Under conditions of 1 to 3 octas, sky cover, clouds appear to be concentrated in line with the sensor and Sun on more occasions than that expected given a random cloud distribution. The same cloud cover condition also resulted in many instances of ground incident irradiance above clear-sky values. The presence of cumuliform clouds appears to increase the likelihood of the latter phenomena. Received January 4, 1998     

UV irradiance on arbitrarily oriented surfaces: variation with atmospheric and ground properties

Summary The dependency of erythemal weighted solar UV irradiance on tilted surfaces with different orientation is investigated with respect to solar zenith angle, variable atmospheric conditions and albedo of the location. For overcast conditions or a cloud in front of the sun, the irradiance on a horizontal surface in general is largest, with the consequence that it is reduced for surfaces with any tilted position. For cloud free conditions the irradiance on a tilted plane, in comparison to that on a horizontal flat surface, is increased for orientations towards the sun, but reduced for other orientations. The increase is strongest for low sun in combination with clear atmosphere and high ground albedo, as is typical for snow covered mountain conditions.     

Effect of clouds on UV and total irradiance at Paradise Bay, Antarctic Peninsula, from a summer 2000 campaign

Summary ?The analysis of ground-based measurements of solar erythemal ultraviolet (UV) irradiance with a Solar Light 501 biometer, and total (300–3000 nm) irradiance with an Eppley B&W pyranometer at the Argentine Antarctic Base “Almirante Brown”, Paradise Bay (64.9° S, 62.9° W, 10 m a.s.l.) is presented. Measurement period extends from February 16 to March 28 2000. A relatively high mean albedo and a very clean atmosphere characterise the place. Sky conditions were of generally high cloud cover percentage. Clear-sky irradiance for each day was estimated with model calculations, and the effect of the cloudiness was studied through the ratio of measured to clear-sky value (r). Two particular cases were analysed: overcast sky without precipitation and overcast sky with rain or slight snowfall, the last one presenting frequently dense fog. Total irradiance was more attenuated than UV by the homogeneous cloudiness, obtaining mean r values of 0.54 for erythemal irradiance and 0.30 for total irradiance in the first case (without precipitation) and 0.27 and 0.17 respectively in the second case (with precipitation). Mean r values for the complete period were 0.58 for erythemal irradiance and 0.43 for total irradiance. Erythemal and total daily insolations reduce quickly at this epoch due to the increase of the noon solar zenith angle and the decrease of daylight time. Additionally, they were strongly modulated by cloudiness. Measured maxima were 2.71 kJ/m² and 18.42 MJ/m² respectively. Measurements were compared with satellite data. TOMS-inferred erythemal daily insolation shows the typical underestimation with respect to ground measurements at regions of high mean albedo. Measured mean total daily insolation agrees with climatological satellite data for the months of the campaign. Received August 9, 2002; revised January 4, 2003; accepted January 28, 2003 Published online May 20, 2003     

Spectral dependence on the correction factor of erythemal UV for cloud,aerosol, total ozone,and surface properties: A modeling study

Radiative transfer model simulations were used to investigate the erythemal ultraviolet(EUV) correction factors by separating the UV-A and UV-B spectral ranges. The correction factor was defined as the ratio of EUV caused by changing the amounts and characteristics of the extinction and scattering materials. The EUV correction factors(CFEUV) for UV-A[CFEUV(A)] and UV-B [CFEUV(B)] were affected by changes in the total ozone, optical depths of aerosol and cloud, and the solar zenith angle. The differences between CFEUV(A) and CFEUV(B) were also estimated as a function of solar zenith angle, the optical depths of aerosol and cloud, and total ozone. The differences between CFEUV(A) and CFEUV(B) ranged from-5.0% to 25.0% for aerosols, and from-9.5% to 2.0% for clouds in all simulations for different solar zenith angles and optical depths of aerosol and cloud. The rate of decline of CFEUV per unit optical depth between UV-A and UV-B differed by up to 20% for the same aerosol and cloud conditions. For total ozone, the variation in CFEUV(A) was negligible compared with that in CFEUV(B) because of the effective spectral range of the ozone absorption band. In addition, the sensitivity of the CFEUVs due to changes in surface conditions(i.e., surface albedo and surface altitude) was also estimated by using the model in this study. For changes in surface albedo, the sensitivity of the CFEUVs was 2.9%–4.1% per 0.1 albedo change,depending on the amount of aerosols or clouds. For changes in surface altitude, the sensitivity of CFEUV(B) was twice that of CFEUV(A), because the Rayleigh optical depth increased significantly at shorter wavelengths.     

广州市地面太阳紫外线辐射观测和初步分析

通过对广州市2000～2001年两年的地面紫外线辐射观测资料的分析表明广州市全年紫外线辐射强度84.3%出现在"中等"以上级别,"强"、"很强"级别占65.1%,尤其是第二、三季度分别有46.2%、62.1%的时间紫外线出现"很强"级别,分析表明广州市属于紫外线高辐射地区;各候(旬)的最小紫外线指数在夏秋季节的涨落脉动比冬春季节要大,尤其是秋季更加明显.另外,定义了每月云衰减因子(CAF),12～4月为明显的云衰减加强期;6～9月为明显的云衰减减弱期,分析期间最大云衰减因子仅为0.11.     

On the use of a cloud modification factor for solar UV (290–385 nm) spectral range

Summary  Knowledge of ultraviolet radiation is necessary in different applications, in the absence of measurements, this radiometric flux must be estimated from available parameters. To compute this flux under all sky conditions one must consider the influence of clouds. Clouds are the largest modulators of the solar radiative flux 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 defining a cloud modification Factor. We have developed such cloud modification Factor considering two different types of clouds. The efficiency 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 first one is located at the University of Almería, 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 Almería 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 modification 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. Received November 15, 1999 Revised September 11, 2000     

The albedo of snow for partially cloudy skies

The albedo of snow for different cloudiness conditions is an important parameter in the Earth's radiation budget analysis and in the study of snowpack's thermal conditions. In this study an efficient approximate method is derived to calculate the incident spectral solar flux and snow-cover albedo in terms of different atmospheric, cloud, and snow parameters. The global flux under partially cloudy skies is expressed in terms of the clear sky flux and a coefficient which models the effect of scattering and absorption by cloud patches and multiple reflections between the cloud base and snowcover. The direct and the diffuse components of the clear sky flux are obtained using the spectral flux outside the atmosphere and the spectral transmission coefficients for absorption and scattering by molecules and aerosols.The spectral snow reflectance model considers both specular surface reflection and volumetric multiple scattering. The surface reflection is calculated by using a crystal-shape-dependent bidirectional reflectance distribution function; the volumetric multiple scattering is calculated by using a crystal-size-dependent approximate solution in the radiative transfer equation. The input parameters to the model are atmospheric precipitable water, ozone content, turbidity, cloud optical thickness, the size and shape of ice crystals of snow and surface pressure. The model yields spectral and integrated solar flux and snow reflectance as a function of solar elevation and fractional cloudcover.The model is illustrated using representative parameters for the Antarctic coastal regions. The albedo for a clear sky depends inversely on the solar elevation. At high elevations the albedo depends primarily upon the grain size; at low elevation the albedo depends on grain size and shape. The gradient of the albedo-elevation curve increases as the grains become larger and faceted. The albedo for a densely overcast sky is a few percent higher than the clear-sky albedo at high elevations. A simple relationship between grain size and the overcast albedo is obtained. For a set of grain size and shape, the albedo as a function of solar elevation and fractional cloud cover is tabulated.     

Effect of Macrophysical Parameters of Clouds on Broadband Solar Radiation (295-2800 nm) at a Subtropical Location

The present study describes the effect of clouds(macro-physical parameters) on global solar radiation(G).Data from four years of hourly measurements of G on a horizontal surface were used.These data were collected at the South Valley University(SVU) meteorological research station(26.2°N,32.7°E,96 m above mean see level.In addition,the cloud modification factor for G(CMFG) was estimated in three cases:high-level,mid-level,and low-level clouds.For every level,the variation of hourly CMFG as a function of cloud amount(CA) was illustrated.A third-order polynomial between hourly values of CMFG and CA was established.Furthermore,the effect of CA in the attenuation of G relative to its corresponding value in cloudless conditions is discussed.For cloud cover > 88%,G was reduced by 54%,34%,and 28% by low-,mid-,and high-level clouds,respectively.     

Retrospective cloud determinations from surface solar radiation measurements

Surface solar radiation measurements have been made at many meteorological sites for long periods. These data potentially provide higher temporal resolution cloud amount information than traditional estimations of cloud amount recorded in eighths of the sky obscured (oktas). To utilise existing solar radiation datasets for this, two quantities are derived from the surface measurements of global (G) and diffuse (D) solar irradiance, and top of atmosphere solar irradiance (E) — the Diffuse Fraction (DF = D/G) and the Opaqueness (Op = 1 - G/E). These are compared with subjective cloud observations made at Reading daily during 1997 to 2006. This shows that DF measurements are sensitive to cloud amount, particularly for low and moderate cloud coverage. Complimentary information is available in Op, which is particularly sensitive to cloud amount in moderate to overcast cloud coverage. In overcast conditions, DF = 1, during which Op provides a measure of overcast cloud thickness. As well as cloud amount, the variability in DF and Op provides a basis for discriminating between cumuliform and stratiform cloud: large variability indicates convective cloud whereas only small variability occurs under stratiform cloud.     

Evaluation on penetration rate of cloud for incoming solar radiation using geostationary satellite data

Solar surface insolation (SSI) represents how much solar radiance reaches the Earth??s surface in a specified area and is an important parameter in various fields such as surface energy research, meteorology, and climate change. This study calculates insolation using Multi-functional Transport Satellite (MTSAT-1R) data with a simplified cloud factor over Northeast Asia. For SSI retrieval from the geostationary satellite data, the physical model of Kawamura is modified to improve insolation estimation by considering various atmospheric constituents, such as Rayleigh scattering, water vapor, ozone, aerosols, and clouds. For more accurate atmospheric parameterization, satellite-based atmospheric constituents are used instead of constant values when estimating insolation. Cloud effects are a key problem in insolation estimation because of their complicated optical characteristics and high temporal and spatial variation. The accuracy of insolation data from satellites depends on how well cloud attenuation as a function of geostationary channels and angle can be inferred. This study uses a simplified cloud factor that depends on the reflectance and solar zenith angle. Empirical criteria to select reference data for fitting to the ground station data are applied to suggest simplified cloud factor methods. Insolation estimated using the cloud factor is compared with results of the unmodified physical model and with observations by ground-based pyranometers located in the Korean peninsula. The modified model results show far better agreement with ground truth data compared to estimates using the conventional method under overcast conditions.     

Measured Spectra of Solar Ultraviolet Irradiances at Athens Basin, Greece

Summary From a data archive of spectral energy distribution of global and diffuse solar irradiances measured in Athens during a field experiment, the impacts of changes in solar zenith angle, site altitude, and gaseous-aerosol pollutants loading, on spectral composition of ultraviolet (UV) radiation reaching the ground, has been determined for cloudless conditions. Measurements of spectral energy distribution of ultraviolet irradiance showed that in urban atmospheres there is a significant altitude effect on spectral UV irradiances which is more pronounced on shorter wavelength UVB than on longer wavelength UVA. In particular the largest attenuation in the UVB band, produced by the altitude effect, between the non-urban site of Mt. Hymettus and the urban site of Athens Museum, was about 27%; while the corresponding attenuation in the UVA band reached 20%. Correspondingly, the respective attenuation caused by altitude effect in the diffuse UVB band was 12%; whereas the altitude effect increases UVA band to as much as 9% between the summit of Mt. Hymettus and the urban site of Athens Museum. Depletion of UV irradiances by the urban atmosphere of downtown Athens was strongly related to aerosol-gaseous pollutants loading. The spectral UV measurements were found to be sensitive to both, changes in solar zenith angle and atmospheric turbidity. The spectral ratio of diffuse-to-direct irradiance critically depends on both solar zenith angle and aerosol-gaseous pollutants loading, increasing rapidly toward the shorter wavelengths. Finally, the hypothesis that the increased levels of aerosol-gaseous pollutants may act as a filter to the transfer of UV energy to the ground is supported, by the limited set of spectral measurements used in the present work. Received June 16, 1996 Revised April 18, 1997     

Determination of total vertical water vapor in the atmosphere

The comparison between the precipitable water vapor w obtained by classical sounding and that obtained by high resolution measurements of spectral solar direct irradiance in the 400–1000 nm spectral range is shown. Three different water vapor absorption functions in the πστ band are used to determine the water vapor w by optical measurements. An episode of attenuation of direct solar irradiance by cirrus clouds is also shown.     

Influence of the total atmospheric optical depth and cloud cover on solar irradiance components

Broadband solar irradiance data obtained in the spectral range 400–940 nm at Kwangju, South Korea from 1999–2000 have been analyzed to investigate the effects of cloud cover and atmospheric optical depth on solar radiation components. Results from measurements indicate that the percentage of direct and diffuse horizontal components of solar irradiance depend largely on total optical depth (TOD) and cloud cover. During summer and spring, the percentages of diffuse solar irradiance relative to the global irradiance were 5.0% and 4.9% as compared to 2.2% and 3.0% during winter and autumn. The diffuse solar irradiance is higher than the direct in spring and summer by 24.2%, and 40.6%, respectively, which may largely be attributed to the attenuation (scattering) of radiation by heavy dust pollution and large cloud amount. In cloud-free conditions with cloud cover ≤2/10, the fraction of the direct and diffuse components were 66.0% and 34.0%, respectively, with a mean daily global irradiance value of 7.92±2.91 MJ m⁻² day⁻¹. However, under cloudy conditions (with cloud cover ≥8/10), the diffuse and direct fractions were 97.9% and 2.2% of the global component, respectively. The annual mean TOD under cloudless conditions (cloud cover≤2/10) yields 0.74±0.33 and increased to as much as 3.15±0.67 under cloudy conditions with cloud amount ≥8/10. An empirical formula is derived for estimating the diffuse and direct components of horizontal solar irradiance by considering the total atmospheric optical depth (TOD). Results from statistical models are shown for the estimation of solar irradiance components as a function of TOD with sufficient accuracy as indicated by low standard error for each solar zenith angle (SZA).     

太阳短波辐射分光谱计算模式

本文采用太阳短波辐射分光谱模式计算了我国太阳辐射收支各分量。模式中主要考虑及计算了大气中各种成份在不同谱区对太阳辐射的吸收和衰减作用。如水汽、均匀混和气体的红外吸收，紫外和可见光区的臭氧吸收、雷利散射及大气气溶胶的削弱作用等。给出了大气中各种成份在不同波段对太阳辐射的削弱。并且分析了我国太阳辐射收支各分量1月、7月的分布特征。模式误差在4％～10％，比较理想。为无辐射观测的高原、沙漠、海洋等地区提供了一种较好的计算方法。     

广州夏季地基微波辐射计红外传感器观云性能分析

为进一步评估地基微波辐射计红外传感器的观云性能,收集了2017、2018两年5—7月布设在广州野外雷电实验基地的一台MWP967KV型地基微波辐射计红外传感器与一台K/LLX502B型激光测云仪观测的云底高度数据,并利用人工观测的天空状况、云状数据进行分类,在不同云类、天空状况下对地基微波辐射计红外传感器云识别和观测云底高度的性能进行分析。结果表明:①地基微波辐射计红外传感器具有良好的云识别能力,准确率达80.4%,由于观测视角的原因,未能识别出有云存在主要发生在"多云"情况下。②地基微波辐射计红外传感器与激光测云仪所测云底高度相关系数为0.63,有着较好的一致性。③因为观测原理不同,地基微波辐射计红外传感器所测云底高度总是高于激光测云仪,且两部仪器在低云阴天时所测的云底高度一致性要优于中、高云多云。综上所述,地基微波辐射计红外传感器用来识别云、观测云底高度具有一定的可行性,尤其是在低云、"阴天"条件下,有着较好的观云性能。     

Statistical Study and Prediction of Variability of Erythemal Ultraviolet Irradiance Solar Values in Valencia,Spain

The goal of this study was to statistically analyse the variability of global irradiance and ultraviolet erythemal (UVER) irradiance and their interrelationships with global and UVER irradiance, global clearness indices and ozone. A prediction of short-term UVER solar irradiance values was also obtained. Extreme values of UVER irradiance were included in the data set, as well as a time series of ultraviolet irradiance variability (UIV). The study period was from 2005 to 2014 and approximately 250,000 readings were taken at 5-min intervals. The effect of the clearness indices on global irradiance variability (GIV) and UIV was also recorded and bi-dimensional distributions were used to gather information on the two measured variables. With regard to daily GIV and UIV, it is also shown that for global clearness index (k_t) values lower than 0.6 both global and UVER irradiance had greater variability and that UIV on cloud-free days (k_t higher than 0.65) exceeds GIV. To study the dependence between UIV and GIV the χ² statistical method was used. It can be concluded that there is a 95% probability of a clear dependency between the variabilities. A connection between high k_t (corresponding to cloudless days) and low variabilities was found in the analysis of bi-dimensional distributions. Extreme values of UVER irradiance were also analyzed and it was possible to calculate the probable future values of UVER irradiance by extrapolating the values of the adjustment curve obtained from the Gumbel distribution.     

1/f noise in the UV solar spectral irradiance

The investigation of the intrinsic properties of the solar spectral irradiance as a function of the ultraviolet (UV) wavelength is attempted by exploiting rare observations performed at the Villard St. Pancrace station of the Lille University of Sciences and Technology ranging from 278 to 400 nm with a step of 0.05 nm every half an hour from nearly sunrise to sunset. To achieve this goal, the modern method of the detrended fluctuation analysis was applied on the solar spectral irradiance values versus wavelength. This analysis revealed that the solar incident flux at the top of the atmosphere and the solar spectral irradiance at the ground during two overcast sky days fluctuate with the UV wavelength exhibiting persistent long-range power-law behavior. More interestingly, the exponent of the power-law relationship between the fluctuations of the solar spectral irradiance versus UV wavelength at both the top of the atmosphere and the ground is consistently close to unity (of 1/f-type) throughout the day. This 1/f behavior has been detected in many complex dynamical systems, but despite much effort to derive a theory for its widespread occurrence in nature, it remains unexplained so far. According to the above-mentioned findings we speculate that the 1/f property of the incident solar UV flux at the top of the atmosphere could probably drive both the 1/f behavior depicted in the atmospheric components and the solar UV irradiance at the Earth's surface. The latter could influence the UV-sensitive biological ecosystems, giving rise to a 1/f-type variability in the biosphere, which has already been proven by recent observational data. We finally propose that Wien approximation could be multiplied by a 1/f function of wavelength (e.g., of the type of the fractional Brownian motion) in order to reproduce the aforementioned 1/f feature of the solar UV flux.     

Long-Term Trend in Ozone and Erythemal UV at Indian Latitudes

The role of atmospheric ozone to protect the living organisms and vegetation from the harmful effects of ultraviolet irradiation is well known. Depletion of the ozone layer is a great threat to the human society. In this paper we have discussed the lethal effects of ozone depletion and have presented the ozone and UV-B scenarios from 1979 to 2005 at different Indian latitudes using satellite data. The erythemal UV irradiance data obtained from Nimbus-7 and Earth probe total ozone mapping spectrometer (TOMS) and the tropospheric and stratospheric ozone data obtained from the convective cloud differential (CCD) method have been used to study the variability of erythemal UV irradiance and the stratospheric and tropospheric column ozone, respectively, over a period from 1979 to 2005. The observed results along with the expected upper and lower tolerance limits for tropospheric and stratospheric ozone, respectively, for different Indian latitudes, which have been estimated statistically using monthly mean CCD ozone data from 1979 to 2005 have been discussed in detail.     

西藏当雄地基紫外线指数观测研究

基于西藏当雄2009年9月—2011年8月地基紫外线指数 (UVI) 观测,结合TUV辐射传输模式分析, 该文检验太阳天顶角、云、臭氧、积雪和气溶胶对UVI的影响。结果显示,影响地面UVI主要因素是太阳天顶角和云。晴天地面UVI可简单用太阳天顶角拟合函数表征;地面UVI的云调制因子总体上随云量增加呈下降趋势,但间隙性、未遮蔽日面的云可增强太阳散射辐射,使云调制因子值平均增加约3%~6%,个别情形达40%。臭氧低谷使当雄UVI比同纬度平原地区增加约12%;冬季短期、浅层积雪使UVI增加16%或更低,也低于模式模拟值 (23%);气溶胶 (光学厚度为0.02~0.1) 对UVI衰减低于3%。因臭氧低谷导致青藏高原臭氧南北分布的差异,相同太阳天顶角下拉萨 (海拔为3650 m) UVI较瓦里关 (海拔为3810 m) 偏高7%~10%。与卫星产品比较表明:OMI卫星UVI产品在当雄、沱沱河、瓦里关和拉萨较地基测值总体偏高65%以上,而晴天则平均分别偏高8.6%,13%,9%和50%。云、地基与卫星像元地理位置差异应是卫星UVI产品偏高的原因。当雄地基UVI测值大于14时,卫星UVI产品反而低3%应与间隙性云有效增强了地面辐射有关。