Homogenisation and trend detection analysis of broken series of solar UV-B data

Summary Statistical techniques have been developed to homogenise a broken series of clear-sky solar UV-B radiation, measured by a Robertson Berger (RB) meter over the period 1981–90 at Invercargill, New Zealand, and to analyse the series for long term trend. Statistical modelling of the quasi-linear UV-B/ozone relationship evident in the departures of daily clear-sky UV-B data and coincident satellite ozone data from their respective mean references has been used to provide a self-consistent de-seasonalised data set of UV-B and ozone departures, and to bridge a major gap in calibration that separates the data set into two periods, 1981–86 and 1988–90. The choice of UV-B reference is important to the quality of the results and particular attention was given to the methodology for defining it. Four alternative objective adjustment procedures for calibrating the 1988–90 period against the 1981–86 period were examined. Because our interest lies primarily in the higher values of summer, a UV-B-weighted procedure was chosen. The modelling and homogenisation techniques developed may have application in related analysis problems. Analysis of the data for the independent 1981–86 period showed large trends in ozone and UV-B, but this was mostly due to a period of very low ozone values during 1985. Over the whole period, 1981–90, the ozone trend was –4.7%/decade. The corresponding UV-B trend was +5.8%/decade, but this result is not independent of the corresponding ozone trend because the homogenisation procedure imposes the assumption of the derived UV-B/ozone relationship on the 1988–90 UV-B data. However, the evidence suggests there is little reason to doubt that solar UV-B radiation has increased at the site by about +6% per decade.With 10 Figures     

Changes to the Surface Level Solar Ultraviolet-B Radiation Due to Haze Perturbation

The South-East Asian region experienced a haze episode in 1994 which was widely believed to be due to widespread forest fires in Sumatra and Kalimantan (Indonesia). Broadband measurements of the surface level solar ultraviolet-B, UV-A and Global radiation at Penang (Malaysia) are used to study the effect of the 1994 haze on effective UV-B irradiance. We find that during the haze episode, there is enhanced absorption of surface level UV-B radiation. The effect of haze on UV-A and Global radiation is much less. The reduction in absolute noon time UV-B irradiance (mostly cloud free) during the 1994 haze period was 23% relative to the UV-B irradiance during thecorresponding haze-free period in 1995. Even though the noon time radiation data minimizes the cloud effect in the results presented some cloud effect is still present.     

Estimation of Hourly Solar Radiation at the Surface under Cloudless Conditions on the Tibetan Plateau Using a Simple Radiation Model

In this study,the clear sky hourly global and net solar irradiances at the surface determined using SUNFLUX,a simple parameterization scheme,for three stations(Gaize,Naqu,and Lhasa) on the Tibetan Plateau were evaluated against observation data.Our modeled results agree well with observations.The correlation coefficients between modeled and observed values were > 0.99 for all three stations.The relative error of modeled results,in average was < 7%,and the root-mean-square variance was < 27 W m 2.The solar irradiances in the radiation model were slightly overestimated compared with observation data;there were at least two likely causes.First,the radiative effects of aerosols were not included in the radiation model.Second,solar irradiances determined by thermopile pyranometers include a thermal offset error that causes solar radiation to be slightly underestimated.The solar radiation absorbed by the ozone and water vapor was estimated.The results show that monthly mean solar radiation absorbed by the ozone is < 2% of the global solar radiation(< 14 W m 2).Solar radiation absorbed by water vapor is stronger in summer than in winter.The maximum amount of monthly mean solar radiation absorbed by water vapor can be up to 13% of the global solar radiation(95 W m 2).This indicates that water vapor measurements with high precision are very important for precise determination of solar radiation.     

太阳模拟器在辐射仪器检测中的应用

该文主要介绍了由TM-500F太阳模拟器和旋转工作台组成的具有多功能辐射仪器室内检测设备的结构、原理及应用。利用该设备可对太阳辐射测量仪器进行灵敏度的校准, 由实验可知, 总辐射表室内外灵敏度检定结果非常一致, 其误差在±0.6%之内。也可进行余弦、方位响应误差, 倾斜误差, 非线性误差及响应时间等性能的测定。     

The daily and annual effects of dew, frost, and snow on a non-ventilated net radiometer

The formation of dew, deposition of frost and accumulation of snow mainly on the upper domes of a non-ventilated net radiometer seriously affect the measurement of available energy (net radiation). Net radiometers measure radiation, and energy balances and are widely used for estimation of evapotranspiration throughout the world. To study the effects of dew, frost, and snow on a non-ventilated net radiometer, a radiation station was set up which uses 2 CM21 Kipp & Zonen pyranometers (one inverted), 2 CG1 Kipp & Zonen pyrgeometers (one inverted), along with a Q7.1 net radiometer (Radiation & Energy Balance Systems, Inc.; REBS) in a semi-arid mountainous valley in Logan, Utah, U.S.A. The pyranometers and pyrgeometers were ventilated using 4 CV2 Kipp & Zonen ventilation systems. The net radiometer was not ventilated. The ventilation of pyranometers and pyrgeometers prevents dew and frost deposition and snow accumulation which otherwise would disturb measurements. All sensors were installed at about 3.0 m above the ground, which was covered with natural vegetation during the growing season (May–September). The incoming and outgoing solar or shortwave radiation, the incoming (atmospheric) and outgoing (terrestrial) longwave radiation, and the net radiation have been continuously measured by pyranometers, pyrgeometers and a net radiometer, respectively, since 1995. These parameters have been measured every 2 s and averaged into 20 min. To evaluate the effects of dew, frost, and snow, three days were chosen: 26 April 2004 with early morning dew, 6 January 2005 with an early morning frost, and the snowy day of 24 February 2005. Dew formation, frost deposition, and snow accumulation occurred mainly on the upper dome of the non-ventilated Q7.1 net radiometer on the related days, while the ventilated Kipp & Zonen system was free of dew, frost and snow. Net radiation measured by the non-ventilated net radiometer R_n,unvent. during dew and frost periods of the above-mentioned days was greater than ventilated ones R_n,vent. (− 0.2 MJ m^− 2 vs. − 0.8 MJ m^− 2 during almost 4 h on 26 April 2004, and − 0.2 MJ m^− 2 vs. − 0.7 MJ m^− 2 during almost 6.5 h on 6 January 2005). The reason for higher reading by the non-ventilated net radiometer during dew and frost periods was due to emission of additional longwave radiation from water and ice crystals formed mainly on the upper dome of the Q7.1 net radiometer. In contrast, during the snowy day of 24 February 2005, the R_n,unvent. was less than R_n,vent. (− 4.00 MJ m^− 2 vs. 0.77 MJ m^− 2, mainly from sunrise to sunset). The extremely low R_n,unvent. measured by the non-ventilated net radiometer on 24 February 2005 is due to blocking of the incoming solar radiation (mainly diffuse radiation) by the snow-covered upper dome.     

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.     

Building global and diffuse solar radiation series and assessing decadal trends in Girona (NE Iberian Peninsula)

Measurement of solar radiation was initiated in Girona, northeast of the Iberian Peninsula, in the late 1980s. Initially, two pyranometers were installed, one of them equipped with a shadowband for measuring the diffuse component. Two other pyranometers currently exist, both ventilated and one of them shadowed, with a sphere, and a pyrheliometer for measuring direct radiation. Additional instruments for other shortwave and longwave components, clouds, and atmospheric aerosols have been installed in recent years. The station is subject to daily inspection, data are saved at high temporal resolution, and instruments are periodically calibrated, all in accordance with the directions of the Baseline Surface Radiation Network. The present paper describes how the entire series of global solar radiation (1987–2014) and diffuse radiation (1994–2014) were built, including the quality control process. Appropriate corrections to the diffuse component were made when a shadowband was employed to make measurements. Analysis of the series reveals that annual mean global irradiance presents a statistically significant increase of 2.5 W m⁻² (1.4 %) decade⁻¹ (1988–2014 period), mainly due to what occurs in summer (5.6 W m⁻² decade⁻¹). These results constitute the first assessment of solar radiation trends for the northeastern region of the Iberian Peninsula and are consistent with trends observed in the regional surroundings and also by satellite platforms, in agreement with the global brightening phenomenon. Diffuse radiation has decreased at −1.3 W m⁻² (−2 %) decade⁻¹ (1994–2014 period), which is a further indication of the reduced cloudiness and/or aerosol load causing the changes.

     

1961～2009年我国地面太阳辐射变化特征及云对其影响的研究

基于我国100个地面站点的地面太阳总辐射、日平均云量资料分析1961～2009年我国地面太阳辐射(Surface Solar Radiation,SSR)变化特征及云在不同时期对SSR的影响。结果显示:1961～2009年我国SSR经历了先下降后上升的变化过程,其中1961～1990年SSR显著下降("变暗"),下降速率为-4.3%/10 a(7.87 W m–2(10 a)–1),各地SSR变化趋势比较一致;1990年后SSR开始上升("变亮"),上升速率为2.8%/10 a(2.4 W m–2(10 a)–1),各地SSR变化趋势不如前一阶段一致,但没有显著的地域分布特征。晴空条件的设置对1961～1990年各站点SSR变化特征影响不大,仍为大范围下降("变暗"),但对1990～2009年的结果影响显著。相比全天空条件的结果,晴空条件下1990～2009年我国SSR变化有明显的南北特征,南方地区以"变亮"为主,而北方地区大多继续"变暗",但"变暗"速度减缓。1961～1990年我国总云量总体呈小幅下降趋势,下降速率很慢,这一时期总云量与全天空SSR没有很好的对应关系;1990～2009年我国总云量总体呈小幅上升趋势,有显著的南北分布差异,北方地区以上升趋势为主,南方地区以下降趋势为主,期间云量与全天空情况下SSR有很好的对应关系。这些结果表明,在"变暗"阶段,云对SSR的作用不显著,而在"变亮"阶段,云的作用变得较为突出。     

支持向量机方法在南京太阳总辐射推算中的应用

利用1981、1996和2001年逐日南京站太阳总辐射和日照时数观测资料,建立了基于支持向量机(support vector machine,SVM)方法的太阳总辐射推算模型,预测了1982、1997和2002年的太阳总辐射,并把推算结果和采用线性的气候学方法所得到的推算结果分别与实测值进行对比。采用线性方法得到的1982、1997和2002年的太阳总辐射预测值与实测值间基于1:1线的决定系数(R~2)分别为0.800、0.859和0.838,均方根误差(RMSE)分别为3.250、2.649和2.925 MJ·m~(-2)·d~(-1)。采用SVM方法得到的1982、1997和2002年的R~2分别为0.894、0.938和0.936,RMSE分别为2.353、1.726和1.804 MJ·m~(-2)·d~(-1)。SVM方法得到的太阳总辐射预测值与实测值之间的误差较小,预测精度高于线性方法,更适用于实际太阳总辐射的计算。     

Surface Energy Balance Measurements Above an Exurban Residential Neighbourhood of Kansas City,Missouri

Previous measurements of urban energy balances generally have been limited to densely built, central city sites and older suburban locations with mature tree canopies that are higher than the height of the buildings. In contrast, few data are available for the extensive, open vegetated types typical of low-density residential areas that have been newly converted from rural land use. We made direct measurements of surface energy fluxes using the eddy-covariance technique at Greenwood, a recently developed exurban neighbourhood near Kansas City, Missouri, USA, during an intensive field campaign in August 2004. Energy partitioning was dominated by the latent heat flux under both cloudy and near clear-sky conditions. The mean daytime Bowen ratio (β) values were 0.46, 0.48, and 0.47 respectively for the cloudy, near clear-sky and all-sky conditions. Net radiation (R _n ) increased rapidly from dawn (−34 and −58W m⁻²) during the night to reach a maximum (423 and 630W m⁻²) after midday for cloudy and near clear-sky conditions respectively. Mean daytime values were 253 and 370W m⁻², respectively for the cloudy and near clear-sky conditions, while mean daily values were 114 for cloudy and 171W m⁻² for near clear-sky conditions, respectively. Midday surface albedo values were 0.25 and 0.24 for the cloudy and near clear-sky conditions, respectively. The site exhibited an angular dependence on the solar elevation angle, in contrast to previous observations over urban and suburban areas, but similar to vegetated surfaces. The latent heat flux (Q _E ), sensible heat flux (Q _H ), and the residual heat storage ΔQ _s terms accounted for between 46–58%, 21–23%, and 18–31% of R _n , respectively, for all-sky conditions and time averages. The observed albedo, R _n , and Q _E values are higher than the values that have been reported for suburban areas with high summer evapotranspiration rates in North America. These results suggest that the rapidly growing residential areas at the exurban fringe of large metropolitan areas have a surface energy balance that is more similar to the rural areas from which they were developed than it is to the older suburbs and city centres that make up the urban fabric to which they are being joined.     

Measurements of the HONO photodissociation constant

Measurements of the photodissociation constant for nitrous acid (j _HONO) were made at an urban site in Toronto, Canada, during the months of May–July 2005, using an optically thin actinometer. Operating details of the j _HONO monitor are reported, along with laboratory tests. Measurements of j _HONO were obtained for solar zenith angles ranging from 20–75^∘, under clear and cloudy skies. Maximum error estimates on j _HONO under clear skies range from 11% at sunrise, to 4% at solar noon, with a minimum detection limit of 5.7 × 10⁻⁴/sec for our actinometer. Measured clear-sky values of j _HONO were compared with values calculated by a four-stream discrete ordinate radiative transfer (RT) model (ACD TUV version 4.1), and were found to be within better than 10% agreement for solar zenith angles < 65^∘. For conditions of scattered cloud, enhancement and suppression of the j _HONO values occurred by as much as 16%–70%, and 59%–80%, respectively. The integrated band area of the nπ^∗ transition for gas-phase nitrous acid yields an oscillator strength, f = (1.06 ± 0.044)×10⁻³ (based on clear-sky data), 19.1% higher than the value reported by Bongartz et al. (1991).     

Solar eclipse-induced variations in solar flux, j(NO2) and surface ozone at Kannur, India

Surface ozone is mainly produced by the photodissociation of nitrogen dioxide (NO₂) by solar UV radiation. Subsequently, solar eclipses provide one of the unique occasions to explore the variations in the photolysis rate of NO₂ and their significant impact on the production of ozone at a location. This study aims to examine the diurnal variations in the photodissociation rate coefficient of NO₂, (j(NO₂*)), and mixing ratios of surface ozone and NO _X * (NO?+?NO₂*) during the solar eclipse that occurred on 15 January 2010 at Kannur (11.9°N, 75.4°E, 5?m amsl), a tropical coastal site on the Arabian Sea in South India. This investigation was carried out on the basis of the ground level observations of surface ozone and its prominent precursor NO₂*. The j(NO₂*) values were estimated from the observed solar UV-A flux data. A sharp decline in j(NO₂*) and surface ozone was observed during the eclipse phase because of the decreased efficiency of the ozone formation from NO₂. The NO₂* levels were found to increase during this episode, whereas the NO levels remained unchanged. The surface ozone concentration was reduced by 57.5%, whereas, on the other hand, that of NO _X * increased by 62.5% during the solar eclipse. Subsequently a reduction of *% in the magnitude of j(NO₂*) was found here during the maximum obscuration. Reductions in solar insolation, air temperature and wind speed were also observed during the solar eclipse event. The relative humidity showed a 6.4% decrease during the eclipse phase, which was a unique observation at this site.     

Factors affecting levels of aerosol sulfate and nitrate on the Western Mediterranean coast

The present study was carried out between October 2003 and September 2004 at an urban background station near the Mediterranean coast in southeastern Spain. The sulfate and nitrate content was determined in 332 PM10 and PM2.5 daily samples. The results show that the seasonal variation of nitrate measured in PM10 does not correspond with what has been observed at other locations in the Iberian Peninsula, where the minima are reached during the summer months due to decomposition of ammonium nitrate at high temperatures. The recorded levels of PM10 nitrate were slightly higher in summer due to an increase in the concentrations of coarse mode nitrate. On the contrary, both the concentrations and the percentages of nitrate in PM2.5 were lowest from June to September. The sulfate levels in both PM10 and PM2.5 were maxima in summer because the oxidation rate of SO₂ increases with solar radiation. An elevated correlation (0.72 < r < 0.92) between the monthly average concentrations and percentages of sulfate and solar radiation has been found. We have also investigated the influence that Saharan dust intrusions and high pollution episodes have upon the sulfate and nitrate concentrations. Both types of events increased NO₃⁻ and SO₄²⁻ levels, particularly the high pollution episodes.     

Characteristics of UV radiation at Tazhong of the Tarim Basin,west China

Tazhong station, located at the hinterland of the Taklimakan Desert in northwest China, experiences frequent dusty weather events during spring and summer seasons (its dusty season) caused by unstable stratified atmosphere, abundant sand source and strong low-level wind. On average, it has 246.2 dusty days each year, of which 16.2 days are classified as sand and dust storm days. To better understand the characteristic of solar ultraviolet (UV) radiation and factors influencing its variations under such an extreme environment, UV radiation data were collected continuously from 2007 to 2011 at Tazhong station using UVS-AB-T radiometer by Kipp and Zonen. This study documents observational characteristics of the UV radiation variations observed during the five-year period. Monthly UV radiation in this region varied in the range of 14.1–37.8 MJ m^?2 and the average annual amount was 320.7 MJ m^?2. The highest value of UV radiation occurred in June (62.5 W m^?2) while the lowest one in December (29.3 W m^?2). It showed a notable diurnal cycle, with peak value at 12:00–13:00 LST. Furthermore, its seasonal variation exhibited some unique features, with averaged UV magnitude showing an order of summer > spring > autumn > winter. The seasonal values were 37.0, 29.1, 24.9 and 15.9 MJ m^?2, respectively. In autumn and winter, its daily variations were relatively weak. However, significant daily variations were observed during spring and summer associated with frequent dust weather events occurring in the region. Further analysis showed that there was a significant correlation between the UV radiation and solar zenith angle under different weather conditions. Under the same solar zenith angle, UV radiation was higher during clear days while it was lower in sand and dust storm days. Our observations showed that there was a negative correlation between UV radiation and ozone, but such a relationship became absent in dusty days. The UV radiation was reduced by 6 % when cloud amount was 1–4 oktas, by 12 % when the cloud amount was 5–7 oktas, and by 24 % when the cloud amount was greater than 8 oktas. The relative reduction of UV radiation reached 26, 38, and 45 % in dust day, blowing sand day and sand and dust storm day, respectively. The results revealed that decrease in UV radiation can be attributed to cloud coverage and dust aerosols. Moreover, the reduction of UV radiation caused by dust aerosols was about 2–4 times greater than that caused by cloud coverage. These observational results are of value for improving our understanding of processes controlling UV radiation over sand desert and developing methods for its estimation and prediction.     

The Energy Balance Experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements

An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30 W m⁻² over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50 W m⁻²; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m⁻² across the field, rising to 50 W m⁻² or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25 W m⁻², 5%) per site during the day and 10 W m⁻² at night. The National Center for Atmospheric Research is supported by the National Science Foundation.     

El Chichón火山的太阳辐射效应

利用1964—1983年冬季无云条件下的太阳辐射资料分析发现,1982年3月底至4月初,墨西哥El Chichón(厄·奇冲)火山爆发后,对到达地球表面的太阳直接辐射、散射辐射以及总辐射产生了显著和持续的影响。这种影响特征与近百年来的几次巨大火山爆发所引起的辐射效应是一致的,这主要是因为火山气溶胶对太阳辐射的强大反向散射和吸收作用,改变了大气透明度所造成的。     

Evaluation of atmospheric thermal radiation algorithms for daylight hours

Existing simple but theoretically based clear-sky models for longwave down-welling radiation (LDR_c) and cloud impact algorithms transforming them to all-sky radiation (LDR) are checked against locally calibrated empirical algorithms. They are evaluated for daylight hours based on measurements in regionally differing climates of Germany. The Prata clear-sky scheme is additionally tested with adjusted coefficients so that LDR_c converges against a realistic emissivity for a completely dry atmosphere. This version is characterised by an improved modelled variance. Compared with locally calibrated schemes, bias and root mean square error (RMSE) of the more theoretical clear-sky schemes do not differ significantly and yield even better results at a mountain site. In contrast, the locally calibrated algorithms yield biases up to 9% and an increase in RMSE between 6% and 67%, if applied for other sites. For daylight hours, the cloud impact on LDR can be calculated via the ratio of observed to clear-sky global irradiation (CMF_sol). With CMF_sol, the Crawford and Duchon scheme reveals the lowest bias and a decrease in RMSE by 22% against the next best performing algorithms. Compared with synoptic cloud observations as input, the bias is reduced by 9 to 28 W m^?2 and the scattering of the residuals decreases by 20% to 30%. Based on published results for also non-European sites, it is inferred that the more theoretically based LDR_c schemes and cloud impact evaluated via CMF_sol are universally applicable and perform at least in the order of magnitude of locally calibrated empirical algorithms.     

晴天地表太阳辐射的参数化

讨论了晴天地表太阳总辐射和地表太阳净辐射瞬时值的参数化方法。首先利用辐射传输模式和中纬度夏季标准大气廓线,分谱带计算晴天各种大气条件下地表反射率取定值时的地表太阳总辐射,并把所得的结果作为标准资料,提出参数化方案。然后将地表反射率的影响作为误差项进行订正,从而得到各种地表反射率条件下的晴天地表太阳净辐射的计算方法。该方法的拟合精度较高,拟合值与辐射模式标准值的平均相对误差在0.3%以下。该参数化公式可以用于大尺度数值模式中地表辐射平衡的计算,以期达到地表辐射平衡计算与模式积分同步进行的目的。     

The characteristics of ultraviolet radiation in arid and semi-arid regions of China

Measurements of the broadband global solar radiation (R _S) and total ultraviolet radiation (the sum of UV-A and UV-B) were conducted from 2005 to 2010 at 9 sites in arid and semi-arid regions of China. These data were used to determine the temporal variability of UV and UV/R _S and their dependence on the water vapor content and clearness index. The dependence of UV/R _S on aerosol optical depth (AOD) and water vapor content was also investigated. In addition, a simple and efficient empirically model suited for all-weather conditions was developed to estimate UV from R _s. The annual average daily UV level in arid and semi-arid areas is 0.61 and 0.59 MJ m^?2 d^?1, respectively. The highest value (0.66?±?0.25 MJ m^?2 d^?1) was recorded at an arid area at Linze. The lowest value (0.53?±?0.22 MJ m^?2 d^?1) was recorded at a semi-arid area at Ansai. The highest daily value of UV radiation was measured in May, whereas the lowest value was measured in December. The monthly variation of the UV/R _s ratio ranged from 0.41 in Aksu to 0.35 in Qira. The monthly mean value of UV/R _s gradually increased from November and then decreased in August. A small decreasing trend of UV/R _s was observed in the arid and semi-arid regions due to recently increasing amounts of fine aerosol. A simple and efficient empirically model suit for all-weather condition was developed to estimate UV from R _s. The slope a and intercept b of the regression line between the estimated and measured values were close to 1 and zero, respectively. The relative error between the estimated and measured values was less than 11.5%. Application of the model to data collected from different locations in this region also resulted in reasonable estimates of UV.     

Transmissivity of solar radiation through crowns of single urban trees—application for outdoor thermal comfort modelling

Trees play an important role in mitigating heat stress on hot summer days, mainly due to their ability to provide shade. However, an important issue is also the reduction of solar radiation caused by trees in winter, in particular at high latitudes. In this study, we examine the transmissivity of total and direct solar radiation through crowns of single street trees in Göteborg, Sweden. One coniferous and four deciduous trees of species common in northern European cities were selected for case study. Radiation measurements were conducted on nine clear days in 2011–2012 in foliated and leafless tree conditions using two sunshine pyranometers—one located in shade of a tree and the other one on the roof of an adjacent building. The measurements showed a significant reduction of total and direct shortwave radiation in the shade of the studied trees, both foliated and leafless. Average transmissivity of direct solar radiation through the foliated and defoliated tree crowns ranged from 1.3 to 5.3 % and from 40.2 to 51.9 %, respectively. The results confirm the potential of a single urban tree to reduce heat stress in urban environment. However, the relatively low transmissivity through defoliated trees should be considered while planning street trees in high latitude cities, where the solar access in winter is limited. The results were used for parameterisation of SOLWEIG model for a better estimation of the mean radiant temperature (T_mrt). Measured values of transmissivity of solar radiation through both foliated and leafless trees were found to improve the model performance.