Calculation of cloud modification factors for the horizontal plane eye damaging ultraviolet radiation

This research examines the influence of cloud on the cataract effective UV (UV_Cat) irradiances on a horizontal plane over an extended period of 12months 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° or less provided an R² value of 0.85. The data in the first 6months of 2005 for an SZA of 70° or less that were at a different time to that when the technique was developed provided an R² value of 0.83 for the comparison of the measured and calculated UV_Cat irradiances.     

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     

南极中山站太阳紫外辐射测值比较

比较分析了2017年南极中山站3种仪器测量地面太阳紫外B（UVB）波段和紫外A（UVA）波段的辐照度。以Brewer光谱仪测值为参考,国产宽波段FSUVB日射表在UVB（波段280~315 nm）的辐照度相对误差为（55±75）%,误差随大气臭氧总量的增加呈上升趋势,但在南极“臭氧洞”期间偏低。Yankee UVB宽波段日射表在UVB（波段280~320 nm）的辐照度相对误差为（-31±22）%;国产宽波段FSUVA日射表在UVA（波段315~400 nm）的辐照度相对误差为（23±5.9）%。太阳天顶角低于80°的晴天以Tropospheric Ultraviolet Visible（TUV）辐射模式计算结果为参考时,FSUVB,Yankee UVB和FSUVA辐照度的平均相对误差分别为（30±37）%,（-22±19）%和（27±6.4）%,而Brewer相对误差未超过3.5%。国产宽波段UV日射表测值偏高,反映出波长较长的杂散光对太阳辐照度测值影响明显。     

Five years of cloud enhanced surface UV radiation measurements at two sites (in the Northern and Southern Hemispheres)

This is the first paper to report a detailed comparison between ground based measurements of cloud enhanced ultraviolet (UV) radiation at sites located in both hemispheres. The measurements were undertaken at a Northern Hemisphere site (at the campus of the University of Girona in Spain) and a Southern Hemisphere site (campus of the University of Southern Queensland, Toowoomba, Australia). These sites were chosen as they have both simultaneous sky camera and solar radiation (including UV) measurement campaigns operating for at least 5 years over the same time period. Although hemispheric comparisons of UV radiation are not new, this paper focuses on a study of enhancements, including a comparison of the frequency of occurrence and associated cloud types, at the locations with differing climatology. It is found that although there are commonalities between the sites, such as the correlation of the frequency of enhancements with larger solar zenith angle (SZA) both at Girona and Toowoomba, there are differences too, e.g. the frequency of occurrence of the enhancements, and the type of clouds that produce most enhancements (cirrus at Girona, cumulus at Toowoomba). It is suggested that this is due to the differences in latitude coupled with a large amount of Cirriform cloud, especially corresponding to large SZA, at Girona. Possible explanations for this major difference were examined, with the most likely hypothesis relating to multiple scattering due to Cirriform cloud and the subsequent effect on UV radiation on a horizontal surface at large SZA.     

Investigation about the dependence of spectral diffuse-to-direct-beam irradiance ratio on atmospheric turbidity and solar zenith angle

Summary The modifications of the solar spectral diffuse and direct-beam irradiances as well as the diffuse-to-direct-beam ratio, E_dλ/E_bλ, as a function of the aerosol optical depth, AOD, and solar zenith angle, SZA, is investigated. The E_dλ/E_bλ ratios decrease rapidly with wavelength and exponential curves in the form E_dλ/E_bλ = aλ^−b can be fitted with a great accuracy. These curves are strongly modified by the solar spectrum distribution, which is affected by the aerosol loading, aerosol optical properties and SZA. The spectral dependence of the above E_dλ/E_bλ ratios in logarithmic coordinates does not yield a straight line, while a significant departure from the linearity is revealed. The reasons for this departure are investigated in detail and it is established that the aerosol physical properties such as single scattering albedo and size distribution along with the effect of SZA are responsible. These parameters strongly affect the scattering processes in the atmosphere and as a consequence the diffuse spectral distribution. The E_dλ/E_bλ ratio, which is an indicator of the atmospheric transmittance (King, 1979), exhibits a strong wavelength and aerosol-loading dependence. The observed differences between turbid and clear atmospheres constitute a manifestation of contrasting air properties and influence solar irradiance spectra. The present work aims at investigating the effect of atmospheric turbidity and SZA on the E_dλ/E_bλ ratio. For this reason, two distinct cases are examined: one having different atmospheric turbidity conditions but same SZA and a second having different SZAs and same atmospheric turbidity levels.     

基于LBP算法的全天空红外云图的分类研究

利用局部二值模式（Local Binary Patterns,LBP）,结合局部云图的方差信息（VAR）,对全天空红外测云系统获得的红外云图进行了分类研究。首先建立了器测云状分类与传统分类的对应关系,其次分析了层状云、积状云、波状云、卷云和晴空5种类型天空的LBP谱和VAR特征,最后对274个样本进行了分类识别,并与人工观测结果比较,结果表明,层状云、积状云、波状云、卷云和晴空的识别正确率分别为100．0％、84．2％、70．3％、64．7％、99．0％,平均正确率达到87．2％。     

Reconstruction of erythemal UV irradiance and dose at Hohenpeissenberg (1968–2001) considering trends of total ozone, cloudiness and turbidity

Summary Erythemal ultraviolet (UV) doses reaching the earths surface depend in a complex manner on the amount of total ozone, cloud cover, cloud type and the structure of the cloud field. A statistical model was developed allowing the reconstruction of UV from measured total ozone and a cloud modification factor (CMF) for the GAW site Hohenpeissenberg, Germany (48°N, 11°E). CMF is derived from solar global radiation G, normalized against a Rayleigh scattering atmosphere. By this way the complex influence of the cloud field is accounted for by introduction of a measured parameter, exposed also to this complex field. The statistical relations are derived from the period 1990–1998 where UV measurements and relevant meteorological parameters are available. With these relations daily UV doses could be reconstructed back to 1968. Tests show that the model works remarkably well even for time scales of a minute except for situations with high albedo. The comparison of measured and calculated UV irradiances shows that the model explains 97% of the variance for solar elevations above 18° on average over the period 1968–2001. The reconstruction back to 1968 indicates that maximum UV irradiances (clear days) have increased due to long-term ozone decline. Clouds show seasonally depending long-term changes, especially an increase of cirrus. Consequently the UV doses have increased less or even decreased in some months in comparison to the changes expected from the ozone decline alone. In May to August total cloud frequency and cloud cover have decreased. Therefore, the average UV doses have increased much more than can be explained by the ozone decline alone. It is also shown that the optical thickness of cirrus clouds has increased since 1953. The higher frequency of cirrus is caused in part by more frequent contrails. Besides that an observed long-term rise and cooling of the tropopause favors an easier cirrus formation. However, whether climate change and an intensification of the water cycle is responsible for the cirrus trends has not been investigated in detail.     

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     

基于模糊纹理光谱的全天空红外图像云分类

为了对全天空红外测云系统获得的红外图像进行云类自动识别, 提出了基于模糊纹理光谱结合云物理属性的全天空云类识别方法。首先根据不同滤波窗口的模糊纹理光谱图像特征, 确定了滤波窗口大小, 然后通过分析不同天空类型下的FUTS谱 (fuzzy uncertainty texture spectrum) 以及同一种天空类型下的FUTS谱, 考察了FUTS进行云类识别的适用性, 最后利用最小距离分类法和云基本物理属性对全天空红外图像进行了分类测试。在200个测试样本中, 层状云、积云、高积云、卷云和晴空的识别率分别为100%, 100%, 90%, 100%, 100%, 平均识别率达到98%。基于模糊纹理光谱的云分类算法对单一云空具有很好的分类效果, 可进一步应用于全天空红外图像的云分类识别。     

Spectral radiance and sky luminance in Antarctica: a case study

Summary Sky luminance and spectral radiance has been characterised at Neumayer, Antarctica for selected situations during the austral summer 2003/04. Luminance has also been measured at Boulder, Colorado, USA in June 2003. The high reflectivity of the surface (albedo) in Antarctica, reaching values up to 100% in the ultraviolet (UV) and visible part of the solar spectrum due to snow cover, modifies the radiation field considerably when compared to mid-latitudes. A dependence of luminance and spectral radiance on solar zenith angle (SZA) and surface albedo has been identified. For snow and cloudless sky, the horizon luminance exceeds the zenith luminance by as much as a factor of 8.2 and 7.6 for a SZA of 86° and 48°, respectively. In contrast, over grass this factor amounts to 4.9 for a SZA of 86° and a factor of only 1.4 for a SZA of 48°. Thus, a snow surface with high albedo can enhance horizon brightening compared to grass by a factor of 1.7 for low sun at a SZA of 86° and by a factor of 5 for high sun at a SZA of 48°. For cloudy cases, zenith luminance and radiance exceed the cloudless value by a factor of 10 due to multiple scattering between the cloud base and high albedo surface. Measurements of spectral radiance show increased horizon brightening for increasing wavelengths and generally confirm the findings for luminance. Good agreement with model results is found for some cases; however there are also large deviations between measured and modelled values especially in the infrared. These deviations can only partly be explained by measurement uncertainties; to completely resolve the differences between model and measurement further studies need to be performed, which will require an improvement of modelling the spectral radiance. From the present study it can be concluded that a change in albedo conditions, which is predicted as a consequence of climate change, will significantly change the radiation conditions in polar regions as well.     

Trends in spectral UV radiation from long-term measurements at Hoher Sonnblick, Austria

High-quality long-term records of spectral UV irradiance from the Network for the Detection of Atmospheric Composition Change-affiliated Bentham spectroradiometer at the high-mountain site Hoher Sonnblick (47.05° N, 12.95° E, 3,106?m above sea level) from the period 1997?C2011 have been investigated for the existence of trends. Throughout the year, significant upward trends are found at wavelengths of 315?nm and longer. The magnitudes at 315?nm range from +9.3?±?4.5?%/dec at 45° solar zenith angle (SZA) to +14.2?±?3.7?%/dec at SZA 65° for all-sky conditions. The trend estimates at 305?nm are considerably smaller and less significant, yielding between +5.1?±?6.5 and +7.9?±?7.3?%/dec, depending on SZA. Seasonally, the largest trends are found during winter and spring. Total ozone has significantly increased by year-round +1.9?±?1.3?%/dec since 1997 and therefore cannot explain these significant increases. They are rather attributed to decreases in total cloud cover and aerosol optical depth.     

Ultraviolet radiation in overcast sky at the surface

Based on the analysis of one year of observation data of solar radiation at the ground in Beijing in 1990, a simple empirical formula for calculating UV radiation in overcast sky is established. The formula is Qlw/Quvo = A1S Ao, where Quv and Quvo are monthly mean daily sums of UV exposure in overcast sky and clear sky, respectively. S is the daily sunshine hours. The calculated results agree well with the observed. The maximum and minimum relative biases are 9.9% and 0.1%, respectively, and the yearly relative bias is 2.9%. The ratio of ultraviolet radiation of overcast sky to clear sky in 1990 is between 44.6% and 61.8%, and the yearly average is 53.9%. Thus, almost half of the UV energy is lost in the atmosphere in overcast sky in 1990.     

拉萨地区生物有效紫外辐射初步分析

根据1996～1998年由NILUV紫外辐射仪在西藏拉萨地区观测的紫外辐射资料,分析了青藏高原拉萨地区生物有效辐射的分布特征.结果表明,1997年日正午最大生物有效紫外辐射剂量率(UV dose rate)达到500mW m-2,最小值为9.7 mW m-2;晴天时生物有效辐射剂量率的日变化呈规则曲线,且早晚小,中午大;一年中紫外辐射变化的总趋势是由太阳天顶角决定的,紫外辐射的日变化和年变化是其最主要、最基本的变化;西藏拉萨地区的月平均红斑辐射剂量明显高于全球其他同纬度地区.     

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).     

Parameterisation of surface radiation flux at an Antarctic site

During the Antarctic summer 1994/95 the values of downward and upward flux densities of both solar and terrestrial radiation were recorded at 1200 m for a period of 1 month on the Reeves Nevè Glacier (lat 74°39′S, long 161°35′E), near the Italian base of Terra Nova Bay. The relations proposed by Swinbank [Swinbank, W.C., 1963. Long-wave radiation from clear skies. Q. J. R. Meteorol. Soc. 89, 339–348], Idso and Jackson [Idso, S.B., Jackson, R.D., 1969. Thermal radiation from the atmosphere. J. Geophys. Res. 74, 5397–5403] and Deacon [Deacon, E.L., 1970. The derivation of Swinbank's long-wave radiation formula. Q. J. R. Meteorol. Soc. 96, 313–319] associating the long-wave atmospheric radiation flux only to the air temperature at screen level were tested in extreme Antarctic climatological condition. A relation between the long-wave radiation flux and both screen air temperature and cloud cover fraction in accordance to the height of the cloud base was defined using the Kasten and Czeplak relationship that relates the solar radiation flux and the cloud cover index. The study of the incoming short-wave radiation flux from the clear sky and that reflected by the surrounding snow cover allowed for highlighting the role of surface geometry on the albedo measurements.     

临安与龙凤山辐射数据质量及初步结果比较

采用国际通用的辐射数据质量评估方法对2005—2011年浙江临安、黑龙江龙凤山两个区域大气本底站辐射观测资料进行质量评估与比较分析。结果表明:两站辐射数据通过物理可能限制检验与极端罕见限制检验的百分比均超过99.5%,但通过相关要素比较限制检验的百分比上,临安与龙凤山站分别降至97.9%与95.9%;双轴定位追踪太阳出现偏差是造成直接辐射与散射辐射数据精度降低的主要原因。CM21表的热偏移在-5 W·m-2以内,而其进行热偏移订正可以显著提高数据的通过率。对应同一太阳天顶角,临安站晴空总辐射与直接辐射均低于龙凤山站,散射辐射则相反,其原因是临安的大气透明系数较低,大气浑浊度较高。2006—2011年,临安站的大气向下长波辐射呈下降趋势,达到了0.01的显著性水平,且其多年平均值 (363.7±59.3 W·m-2) 显著高于龙凤山站 (274.9±77.6 W·m-2);龙凤山站晴空太阳总辐射呈增加趋势,达到了0.1的显著性水平。     

Airborne radar and in-situ observations of a shallow stratus with drizzle

Observations were made of a shallow stratus of upslope origin using an aircraft equipped with insitu probes and with a vertically-pointing radar of 3-mm wavelength. A cloud layer of 300 m thickness was found below the inversion; an additional layer of 100 m thickness was located within the inversion. The coldest temperature within the cloud was -2°C and the cloud contained no ice particles. Drizzle drops up to 180 Am were present in both cloud layers.The observations reveal precipitation and air motion structures of approximately 1 km horizontal dimensions. The origin of this organization appears to be weak convection. In addition, mixing played an important role in forming the cloud droplet and drizzle drop size distributions.     

Laser depolarization studies of simulated crystallized H2O/H2SO4 clouds

Laboratory experiments were carried out to investigate the linear depolarization ratios (LDRs) of crystallized H₂O/H₂SO₄ acid clouds at angles of 45°, 135° and 157° to the forward direction. LDR is the ratio of the returned energies in the planes of polarization perpendicular and parallel to that of the source (Sassen, 1974). Experiments were carried out for different concentrations of aqueous solution of sulfuric acid. A cloud of supercooled droplets is formed inside an experimental chamber kept in a walk-in cold room by heating an aqueous solution of sulfuric acid, and the ice crystal formation is initiated by momentarily introducing a rod dipped in liquid nitrogen into the cloud. A 2-mW polarized He:Ne laser beam is directed through the cloud and a photomultiplier tube measures the scattered intensity. It is found that the scattered energy is higher in the direction of 157° and 135° than that at 45°. Microscopic examination of the crystals formed from acid concentrations above 50% shows dark fuzzy spots on the surface of the ice crystals suggesting that this could be a film of acid drops. This film also alters the shape of the crystals, and the crystal boundaries are no longer sharp and well defined like that observed for pure water clouds and this is found to influence the LDR.     

Spectral investigation of the diffuse-to-direct solar beam irradiances ratio (UV–VIS) in the urban Athens atmosphere

This study explores the influence of air gaseous pollutants–aerosols and solar zenith angle (SZA) on the spectral diffuse-to-direct beam E _dλ/E _bλ irradiances ratio. It does so using ground-based spectroradiometric measurements taken over the Athens atmosphere during May 1995. It was found that the spectral E _dλ/E _bλ ratio decreases rapidly with increasing wavelength and regression curves of the form E _dλ/E _bλ = aλ^?b fitted the experimental data. These curves are strongly modified by aerosols–air pollutants, aerosol optical properties, and SZA. The log–log plot of E _dλ/E _bλ versus λ reveals a significant departure from linearity, which is likely to be associated with aerosol physical properties and SZA effects. The effect of atmospheric turbidity, as expressed through the aerosol optical at 500 nm and SZA on the spectral E _dλ/E _bλ ratio, is investigated in detail for two discernible atmospheric conditions observed in the urban Athens atmosphere. The first case includes different atmospheric turbidity levels under the same SZA, while the second corresponds to different SZA values under the same turbidity levels. It was found that the correlation between E _dλ/E _bλ and spectral aerosol optical depth can be a useful tool in determining the aerosol optical properties and aerosol types composition.     

Validation of a method to estimate direct normal irradiance of UVA and PAR bands from global horizontal measurements for cloudless sky conditions in Valencia, Spain, by a measurement campaign

A method is proposed to provide measurement of direct normal solar irradiance of bands with wavelength ranges (315?C400?nm, 400?C700?nm) from measurements of global horizontal band irradiance for cloudless sky conditions in Valencia. Global and normal direct irradiance data for every air mass were obtained by applying the SMART2 model to the atmosphere of Valencia. The direct normal to global irradiance ratio was parameterized versus the relative optical air mass. A measurement campaign of global horizontal and diffuse irradiance of UVA and PAR bands was carried out in Valencia, after which, the inferred direct normal irradiance was compared with those provided by the method. The result of the comparison shows that the method is acceptably accurate. The proposed model tends to underestimate the direct normal irradiance of the UVA band by 6%, although for values below 25?W/m² the model overestimates the direct irradiance by 6%, while for values above 25?W/m² the model underestimates it by 10%. The other two error estimators used ranging from 11% to 15% are similar in the defined interval measurements in relation to the whole UVA band. Regarding the PAR band, the model overestimates the direct normal irradiance of the PAR band by only 2.2%. With this, the results of the PAR band are more conclusive, as it has been found that for direct normal irradiance values higher than 280?W/m² the MBE error is almost zero and the other two estimator errors are small, about 5%.