Analysis of 40 year records of solar radiation data in Shanghai,Nanjing and Hangzhou in Eastern China

Summary In this paper, we analyze global, direct and diffuse solar radiation data on a horizontal surface observed at stations in Shanghai, Nanjing and Hangzhou for the period of 1961 to 2000. The data include monthly averages of the daily clearness index (G/G₀: the ratio of global to extraterrestrial solar radiation) and the diffuse fraction (D/G: the ratio of diffuse to global solar radiation. The present study has processed and analyzed the data, including variables or statistics of mean, and annual monthly and daily total, the diurnal variation and the frequency of daily totals of global solar radiation. A correlation between daily values of clearness index and diffuse fraction is obtained and recommended correlation equations were calculated. The annual variations and trend of yearly series are analyzed for daily global, direct and diffuse radiation on a horizontal surface, as well as for daily clearness index and diffuse fraction in Shanghai, Nanjing and Hangzhou. The results show: 1) the east China is characterized by a decrease in global and direct radiation and a little increase in diffuse radiation and a negative linear relationship was obtained between clearness index and diffuse fraction. 2) The annual variations of global, direct and diffuse radiation for Shanghai, Nanjing and Hangzhou are similar with relative low values of global and direct radiation in June due to the Meiyu period. 3) The acceleration of air pollution and decrease of relative sunshine are the possible causes for the decrease of global and direct radiation.     

华北地区光合有效辐射的计算方法研究

2004年9月—2006年10月,选择华北地区的4个观测站开展了太阳辐射（光合有效辐射PAR、可见光辐射VIS、总辐射Q等）、气象参数等的综合测量,得到了PAR、VIS等的变化特征,结果表明：PAR/Q、VIS/Q、PAR/VIS相对稳定,有明显的日、逐日和季节变化,并受到水汽、散射因子、云等因素的影响。2004—2006年禹城、栾城、香河、兴隆地区VIS/Q,PAR/Q和PAR/VIS的平均值分别为0.39,1.95 mol/MJ和4.97 mol/MJ;0.39,1.94 mol/MJ和4.95 mol/MJ;0.43,2.16 mol/MJ和4.97 mol/MJ;0.42,2.03 mol/MJ和4.89 mol/MJ。建立了计算华北地区实际天气PAR、VIS小时累计值的经验公式及PAR与VIS转换关系式,计算值与观测值符合得都比较好。考虑水汽和散射因子时,PAR、VIS计算值与观测值的相对偏差分别为13.0%、12.4%。由于某些站点可能缺少直接或散射辐射,也可以只考虑水汽因子,此时,PAR、VIS的相对偏差分别为13.2%、12.8%。对于PAR、VIS的传输和计算来说,水汽因子的作用最为重要;散射因子的作用虽弱于水汽因子,但也不应该忽视。     

Trends in solar radiation over Germany and an assessment of the role of aerosols and sunshine duration

Summary ¶Global, diffuse, and horizontal direct (beam) irradiances have been evaluated for 13 stations in Germany where the time series vary between 11 and 48 years. Global irradiance has decreased significantly at two stations and increased at four stations. The mean trend in global is an increase of 1.94Wm^–2 or 1.83% per decade. Diffuse irradiance has decreased at five stations, with a mean reduction of 2.44Wm^–2 or 3.46% per decade, while horizontal direct irradiance has increased an average of 4.86Wm^–2 or 10.40% per decade at five stations. Increases in global and direct are most common at stations in the southwest region of Germany, decreases in global were observed in the southeast, and there was an absence of spatial homogeneity in the diffuse trends. Spatial variability in irradiance over Germany is higher in the direct component compared to variability in global and diffuse.Trend analyses of concomitant time series of radiation, bright sunshine duration, and modeled estimates of Ångströms turbidity coefficient suggest that long-term decreases in aerosols are the most likely cause of increases in global irradiance observed at Mannheim, Norderney, and Trier; decreases in diffuse at Hohenpeissenberg, Kassel, Mannheim, and Trier; and increases in direct irradiance at Bocholt, Kassel, Mannheim, and Trier. An increase in sunshine duration at Freiburg likely contributed to an increase in global and direct irradiance observed at that station.     

Estimating solar radiation on tilted surfaces with various orientations: a study case in Karaj (Iran)

Summary The aim of this study is the evaluation of models that estimate daily global solar radiation on tilted surfaces from that measured on horizontal surfaces. Global solar radiation incident on a tilted plane consists of three components: beam radiation, diffuse radiation and reflected radiation from the ground. The Klein (1977) method, modified by Andersen (1980), was used for estimating direct solar radiation incident on tilted surfaces and an isotropic model was used for estimating reflected solar radiation incident on a tilted plane. In contrast models for the diffuse radiation component show major differences, which justifies a validation study which has been done. Eight models for derivation of daily slope diffuse irradiance from daily horizontal diffuse irradiance were tested against recorded slope irradiances at Karaj (35°55′ N; 50°56′ E), Iran. The following models were included: Badescu (2002), Tian et al. (2001), Reindl et al. (1990), Skartveit and Olseth (1986), Koronakis (1986), Steven and Unsworth (1980), Hay (1979) and Liu and Jordan (1962). All the models use the same method for calculating beam radiation as well as ground reflected radiation. However, only diffuse component of radiation was compared. Statistical indices showed that Reindl’s model gives the most accurate prediction for the south-facing surface and Koronakis’s model performs best for the west-facing surface. The Relative Root Mean Square Errors (%RMSE), except for Steven and Unsworth’s model that has unacceptable results, for whole data range from 1.02 to 10.42%. In general, Reindl’s model produces the best agreement with the measured tilted data.     

Indirect determination of broadband turbidity coefficients over Egypt

Long-term data from diffuse and global irradiances were used to calculate direct beam irradiance which was used to determine three atmospheric turbidity coefficients (Linke T _L , Ångström β and Unsworth–Monteith δ _a ) at seven sites in Egypt in the period from 1981 to 2000. Seven study sites (Barrani, Matruh, Arish, Cairo, Asyut, Aswan and Kharga) have been divided into three categories: Mediterranean climate (MC), desert Nile climate (DNC) and urban climate (UC, Cairo). The indirect method (i.e., global irradiance minus diffuse irradiance) used here allows to estimate the turbidity coefficients with an RMSE% ≤20 % (for β, δ _a and T _L ) and ~30 % (for β) if compared with those estimated by direct beam irradiance and sunphotometeric data, respectively. Monthly averages of T _L , β and δ _a show seasonal variations with mainly maxima in spring at all stations, due to Khamsin depressions coming from Sahara. Secondary maxima is observed in summer and autumn at DNC and MC (Barrani and Arish) stations in summer due to dust haze which prevails during that season and at UC (Cairo) in autumn, due to the northern extension of the Sudan monsoon trough, which is accompanied by small-scale depressions with dust particles. The mean annual values of β, δ _a , and T _L (0.216, 0.314, and 4.6, respectively) are larger in Cairo than at MC stations (0.146, 0.216, and 3.8, respectively) and DNC stations (0.153, 0.227, and 3.8, respectively). Both El-Chichon and Mt. Pinatubo eruptions were examined for all records data at MC, UC and DNC stations. The overburden caused by Mt. Pinatubo’s eruption was larger than El-Chichon’s eruption and overburden for β, and T _L at DNC stations (0.06, and 0.58 units, respectively) was more pronounced than that at MC (0.02, and 0.26, respectively) and UC (0.05 and 0.52 units, respectively) stations. The annual variations in wind speed and turbidity parameters show high values for both low and high wind speed at all stations. The wind directions have a clear effect on atmospheric turbidity, and consequently, largest turbidities occur when the wind carries aerosols from the main particle sources, such as industrial particle sources around Cairo or to some extent from the Sahara surrounding all study stations.     

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).     

Atmospheric turbidity of urban and desert areas of the Nile Basin in the aftermath of Mt. Pinatubo’s eruption

Summary  The Linke T _L , ?ngstr?m β and Unsworth-Monteith δ _a turbidity parameters are investigated for two sites in Egypt: Cairo, a densely populated urban area, and Aswan, an arid unpolluted area. These three turbidity parameters are calculated from broadband pyrheliometric measurements recorded hourly over the period 1992–96. Monthly averages of T _L , β and δ _a show relatively flat and identical seasonal variations with a marked main maxima during spring at both sites, due to Khamsin depressions coming from the Great Sahara. A secondary maximum is observed at Aswan in summer, due to dust haze which prevails during that season, and at Cairo in autumn, due to the northern extension of the Sudan monsoon trough, which is accompanied by small scale depressions with dust particles. Annual mean values of T _L , β and δ _a (5.59, 0.250 and 0.372, respectively) at Cairo are larger than at Aswan (3.89, 0.139 and 0.213, respectively). In the same way, the seasonal mean values of T _L , β and δ _a at Cairo are larger than at Aswan. More generally, the monthly and yearly average turbidity values are significantly larger in Cairo than in Aswan for the whole period 1992–96, which is attributable in part to the urbanization/industrialization effect of Cairo. An estimate of the corresponding overburden is obtained by comparison between the present data and older T _L data from 1922–27. It is also shown that turbidity over both sites is largest during 1992, just after the eruption of Mount Pinatubo in 1991. The dependence of β on some meteorological parameters such as wind speed and direction, precipitable water, relative humidity, temperature and visibility, is also analyzed. This reveals in particular that visibility is not a good predictor of turbidity at either site. Conversely, the wind direction and speed have a definite effect on turbidity, and consequently, largest turbidities occur when the wind carries aerosols from the main industrial particle source areas around Cairo. For any season of the year, the average turbidity at the latter site is larger than that at other big cities such as Athens, Rome, and Toronto, but is lower than at Dhahran, Saudi Arabia. Received February 3, 2000 Revised August 25, 2000     

Precise estimation of hourly global solar radiation for micrometeorological analysis by using data classification and hourly sunshine

We have developed a method for estimating hourly global solar radiation (GSR) from hourly sunshine duration data. This procedure requires only hourly sunshine duration as the input data and utilizes hourly precipitation and daily snow cover as auxiliary data to classify time intervals into six cases according to weather conditions. To obtain hourly GSR using a simple algebraic form, a quadratic function of the solar elevation angle and the sunshine duration ratio is used. Daily GSR is given by a sum of hourly GSRs. We evaluated the performance of the newly developed method using data obtained at 67 meteorological stations and found that the estimated GSR is highly consistent with that observed. Hourly and daily root-mean-square misfits are approximately 0.2 MJ/m²/h (~55 W/m²) and 1.4 to 1.5 MJ/m²/day (~16 to 17 W/m²), respectively. Our classification of weather conditions is effective for reducing estimation errors, especially under cloudy skies. Since the sunshine duration is observed at more meteorological stations than GSR, the proposed new method is a powerful tool for obtaining solar radiation with hourly resolution and a dense geographical distribution. One of the proposed methods, GSRgrn, can be applicable to hourly GSR estimations at different observation sites by setting local parameters (the precipitable water, surface albedo, and atmospheric turbidity) suitable to the sites. The hourly GSR can be applied for various micrometeorological studies, such as the heat budget of crop fields.     

Solar Radiation Climatology of Alaska

Summary There are only six locations in Alaska for which global radiation data of more than a year in duration are available. This is an extremely sparse coverage for a state which covers 1.5×10&⁶ km² and stretches over at least three climatic zones. Cloud observations are, however, available from 18 stations. We used fractional cloud cover and cloud type data to model the global radiation and thus obtain a more complete radiation coverage for Alaska. This extended data set allowed an analysis of geographic and seasonal trends. A simple 1-layer model based on Haurwitz’s semi-empirical approach, allowing for changes in cloud type and fractional coverage, was developed. The model predicts the annual global radiation fluxes to within 2–11% of the observed values. Estimated monthly mean values gave an average accuracy within about 6% of the measurements. The estimates agree well with the observations during the first four months of the year but less so for the last four. Changing surface albedo might explain this deviation. Previously, the 1993 National Solar Radiation Data Base (NSRDB) from the National Renewable Energy Laboratory (NREL) modeled global radiation data for 16 Alaskan stations. Although more complete and complex, the NREL model requires a larger number of input parameters, which are not available for Alaska. Hence, we believe that our model, which is based on cloud-radiation relationship and is specifically tuned to Alaskan conditions, produces better results for this region. Annual global solar radiation flux measurements are compared with results from global coverage models based on the International Satellite Cloud Climatology Project (ISCCP) data. Contour plots of seasonal and mean annual spatial distribution of global radiation for Alaska are presented and discussed in the context of their climatic and geographic settings. Received July 16, 1997 Revised May 18,1998     

The Diffuse Fraction of Global Solar Irradianceat a Tropical Location

Summary The annual and monthly mean diurnal variations of the diffuse fraction of global solar irradiance arriving on the ground at a tropical station in Sub-Sahel Africa is here been reported. The monthly mean hourly values of the diffuse fraction (K _d ) for such clear-sky months as February, March and November at this location, which approach a minimum at about local noon, are observed to lie generally below 0.50 during the period from 11:00 to 15:00 hrs (LST). Consequently, solar concentrators utilising parabolic mirrors are expected to have high performance during these months in this region. Like the mainly-cloudy and wet months (June to August) in which monthly mean hourly values of K _d higher than 0.62 have been recorded, the corresponding diffuse fraction for dust-haze months (mostly December and January) with high turbidity coefficients were generally above 0.50. Monthly mean hourly values of K _d for less cloudy months (April, May, September and October) ranged between 0.48 and 0.77 during the period from 11:00 to 15:00 hrs (LST). The effects of atmospheric dust-haze, clouds and albedo on the monthly mean diurnal variation of the diffuse fraction has been discussed. Also reported are the characteristic values of K _d for sets of months with relatively similar atmospheric and sky conditions at this location. The annual variations of the monthly mean daily values of K _d which exhibit strong seasonal dependence showed a peak in August for both years. Except for the months of February and March, the monthly mean daily totals of K _d exhibited similar annual marches during both years. The major discrepancy in the values of the monthly mean daily totals of K _d in both years were recorded in the months of February, November and December, with the corresponding K _d values for these months in both years agreeing only to within 32.9% in February, 15.4% in November and 16.2% in December. Apart from the aforementioned months, the corresponding monthly mean daily totals of K _d for the remaining nine months in both years agreed mostly to within less than 8.4%. The least monthly mean daily ratios of K _d were obtained in the relatively clear month of November for both years being 0.43 in 1993 and 0.49 in 1994. On an annual average, the diffuse component was found to constitute 59.6% of the global solar irradiance arriving on the ground at this region in 1993 and 60.9% in 1994. The results been reported here have been compared with a few others emanating from other tropical stations. Received February 5, 1998 Revised May 12, 1998     

1961-2010年河北省太阳直接辐射时空分布特征研究

利用1961-2010年河北省周边7个太阳辐射观测站的资料,拟合得出相对精确的河北省太阳直接辐射经验公式,分析了河北省太阳直接辐射时空变化特征。结果表明：河北省近50 a来太阳直接辐射年总量总体呈明显的下降趋势。其中,下降幅度相对较大的石家庄为25%,下降幅度相对较小的唐山为11%。河北省水平面太阳直接辐射年总量近50 a平均值空间差异较大,<2540MJ·m^-2 主要分布在邢台南部,>3510MJ·m^-2 主要分布在张家口西北部,大部分地区介于2540-3300MJ·m^-2 。河北省太阳能资源开发利用潜力比较大的区域主要在张家口的坝上地区。     

A Model to Estimate Global Radiation in Complex Terrain

Global radiation is an important parameter necessary for most ecological models. However, in situ data barely meets the needs of modelling mountainous ecosystems since most field stations are located in flat areas. Consequently, it is usually necessary to extrapolate radiation measurements obtained from an adjacent flat area to the complex terrain of concern. The distribution of radiation in complex terrain depends upon two factors: the local atmospheric conditions, which determine the radiation potentially available to a supposed flat surface in a given location, and the topographic effects on this possible radiation. The latter have been included in detail in most radiation models for complex terrain, but the former are often only simply treated as constant or estimated by over-simplified empirical algorithms. In this paper we propose a novel model that uses a parametric atmospheric model to calculate the potential radiation for a supposed flat surface in a given location, and then account for topographic effects. Direct radiation, diffuse radiation and reflected radiation are calculated separately in the model due to the distinctive characteristics of and the effects by topography. Based on the parametric model, this paper has investigated the relationship between radiation transmittance, clearness indices and altitude under a series of water vapour content and turbidity conditions. This combines three ratios, R _b, R _d, and R _r, defined as the direct radiation, diffuse radiation and reflected radiation received by the arbitrary surface, respectively, to their counterparts in the horizontal surface, to estimate the global radiation for any given location. The model has been validated with data from measurements in National Park Berchtesgaden, Germany, where six measurement sites with various altitudes and topographic characteristics have been deployed. The r ² of modelled and measured hourly global radiation are greater than 0.90 in all six sites, with RMSE varies from 16 to 100 W m⁻². Sensitivity analysis revealed that the model was not sensitive to change in water vapour content, which suggests the possibility to use an exponential algorithm of water vapour content when there is no in situ water vapour content information in complex terrains. The NRMSE was only reduced by 0.04, on average, in five of the six sites when water vapour content information was calculated from the in situ air temperature and relative humidity measurements.     

禹城地区光合有效辐射的计算方法

利用2005年7月-2006年10月在山东禹城对太阳辐射（包括光合有效辐射PAR、可见光辐射VIS、总辐射Q等）、气象要素等进行的4次综合观测资料,初步得到了朋R,VIS的日、季节变化特征并对其进行了分析,结果表明：各辐射的小时累计之比vIS／Q,PAR／Q,PAR／VIS相对稳定,其平均值分别为0．39,1．95（mol／MJ）,4．97（mol／MJ）,但它们均有明显的日、逐日、季节变化,并受水汽、气溶胶、云等因素的影响。同时建立了实际天气PAR,VIS的小时累计经验公式及其转换关系,计算值与观测值均比较接近。当考虑水汽和散射因子时,PAR,VIS的计算值与观测值的相对偏差分别为11．1％,10．6％。限于实际情况,可以只考虑水汽因子的作用来计算PAR和VIS的小时累计值,它们的相对偏差分别为13．3％,12．6％。水汽因子对于PAR,VIS的传输和计算具有重要作用。     

Modelling daily global radiation at the forest floor

A simple model is developed which estimates daily global radiation at the floor of a non-homogeneous Eucalyptus forest. Model input parameters are easily derived from field measurements and consist of individual tree location, tree height, maximum canopy width and its corresponding height, height of the lowest branch and trunk thickness. In addition, the model requires values for global and diffuse irradiance in the open. The tree canopy is represented as a series of spheres containing leaves which are homogeneously spaced but are oriented in the vertical plane. This configuration closely approaches that of actual eucalyptus trees. A Monte-Carlo approach is used to estimate the albedo of the unit sphere as a function of solar zenith angle. At a given combination of solar zenith and azimuth angle, the model estimates the solar irradiance at a specific forest floor location.The model, when tested against pyranometer measurements, predicted daily solar irradiance with a correlation of 0.98 and a standard error of 0.98 MJ m^-2 day^-1. This good performance is attributed to the spatial averaging of the radiation fluxes over the entire day, and the relatively low sensitivity of the calculated solar irradiance to sphere albedo.     

Simulation and Regionalization of Daily Global Solar Radiation: A Case Study in Quebec,Canada

Global solar radiation (GSR) is essential for agricultural and plant growth modelling, air and water heating analyses, and solar electric power systems. However, GSR gauging stations are scarce compared with stations for monitoring common meteorological variables such as air temperature and relative humidity. In this study, one power function, three linear regression, and three non-linear models based on an artificial neural network (ANN) are developed to extend short records of daily GSR for meteorological stations where predictors (i.e., temperature and/or relative humidity) are available. The seven models are then applied to 19 meteorological stations located across the province of Quebec (Canada). On average, the root-mean-square errors (RMSEs) for ANN-based models are 0.33–0.54?MJ?m^?2?d^?1 smaller than those for the power function and linear regression models for the same input variables, indicating that the non-linear ANN-based models are more efficient in simulating daily GSR. Regionalization potential of the seven models is also evaluated for ungauged stations where predictors are available. The power function and the three linear regression models are tested by interpolating spatially correlated at-site coefficients using universal kriging or by applying a leave-one-out calibration procedure for spatially uncorrelated at-site coefficients. Regional ANN-based models are also developed by training the model based on the leave-one-out procedure. The RMSEs for regional ANN models are 0.08–0.46?MJ?m^?2?d^?1 smaller than for other models using the same input conditions. However, the regional ANN-based models are more sensitive to new station input values compared with the other models. Maps of interpolated coefficients and regional equations of the power function and the linear regression models are provided for direct application to the study area.     

Clear-sky aerosol radiative forcing effects based on multi-site AERONET observations over Europe

Summary One of the great unknowns in climate research is the contribution of aerosols to climate forcing and climate perturbation. In this study, retrievals from AERONET are used to estimate the direct clear-sky aerosol top-of-atmosphere and surface radiative forcing effects for 12 multi-site observing stations in Europe. The radiative transfer code sdisort in the libRadtran environment is applied to accomplish these estimations. Most of the calculations in this study rely on observations which have been made for the years 1999, 2000, and 2001. Some stations do have observations dating back to the year of 1995. The calculations rely on a pre-compiled aerosol optical properties database for Europe. Aerosol radiative forcing effects are calculated with monthly mean aerosol optical properties retrievals and calculations are presented for three different surface albedo scenarios. Two of the surface albedo scenarios are generic by nature bare soil and green vegetation and the third relies on the ISCCP (International Satellite Cloud Climatology Project) data product. The ISCCP database has also been used to obtain clear-sky weighting fractions over AERONET stations. The AERONET stations cover the area 0° to 30° E and 42° to 52° N. AERONET retrievals are column integrated and this study does not make any seperation between the contribution of natural and anthropogenic components. For the 12 AERONET stations, median clear-sky top-of-atmosphere aerosol radiative forcing effect values for different surface albedo scenarios are calculated to be in the range of −4 to −2 W/m². High median radiative forcing effect values of about −6 W/m² were found to occur mainly in the summer months while lower values of about −1 W/m² occur in the winter months. The aerosol surface forcing also increases in summer months and can reach values of −8 W/m². Individual stations often have much higher values by a factor of 2. The median top-of-atmosphere aerosol radiative forcing effect efficiency is estimated to be about −25 W/m² and their respective surface efficiency is around −35 W/m². The fractional absorption coefficient is estimated to be 1.7, but deviates significantly from station to station. In addition, it is found that the well known peak of the aerosol radiative forcing effect at a solar zenith angle of about 75° is in fact the average of the peaks occurring at shorter and longer wavelengths. According to estimations for Central Europe, based on mean aerosol optical properties retrievals from 12 stations, the critical threshold of the aerosol single scattering albedo, between cooling and heating in the presence of an aerosol layer, is close between 0.6 and 0.76.     

Relationship between Net Radiation and Broadband Solar Radiation in the Tibetan Plateau

The characteristics of net radiation (R_n)(0.3--10 μm) in Lhasa and Haibei in the Tibetan Plateau were analyzed based on long-term in-situ measurements of surface radiation data. The monthly average of daily R_n reached a minimum during the winter period followed by an increase until May and then a decline until January. This variation is consistent with solar activity. The annual mean daily total R_n values were 0.92 MJ m^-2 d^-1 and 0.66 MJ m^-2d^-1 in Lhasa and Haibei, respectively. A relationship between R_n and broadband solar radiation (R_s) was demonstrated by a good linear correlation at the two sites. R_n can be an accurate estimate from R_s. The estimated R_n values were similar to the observed values, and the relative deviations between the estimates and measurements of R_n were 2.8% and 3.8% in Lhasa and Haibei, respectively. The application of the R_n estimating model to other locations showed that it could provide acceptable estimated R_n values from the R_s data. Furthermore, we analyzed the influence of clouds on R_n by different clear index (K_s), defined as the ratio of R_s to the extraterrestrial solar irradiance on a horizontal surface. The results indicate that more accurate results are associated with increased cloudy conditions. The influence of the albedo was also considered, but its inclusion in the model resulted in only a slight improvement. Because surface albedo is not usually measured, an expression based solely on global solar radiation could be of more extensive use.     

On the annual and monthly mean diurnal variations of diffuse solar radiation at a meteorological station in west Africa

Summary In addition to global solar radiationE ^g , the hourly diffuse componentE ^d incident on a horizontal surface has been measured from February 1993 to January 1995 at a meteorological station in tropical West Africa. The measured diffuse solar irradiance data was corrected for shadow band effects. The monthly mean diurnal variations of diffuse solar irradiance obtained for identical months in the two years have been compared and found to be generally consistent. The corresponding monthly mean hourly values ofE ^d for identical months in 1993 and 1994 agreed to within 9% while yielding correlation coefficients greater than 0.960. In addition, the monthly mean daily totals ofE ^d for identical months were found to agree mostly to within 6% and showed virtually the same annual variations in both years. The monthly mean daily total values of diffuse solar radiation for most months in the two years ranged between 7.94 MJm^–2d^–1 and 10.50 MJm^–2d^–1. The monthly mean of daily hourly maximum values ofE ^d obtained for identical months in the two years have been discussed in relation to the dominant atmospheric conditions during these months. The results been presented here have been compared with those of some investigators within and outside the Africa region.With 8 Figures     

Spatial variability in UV radiation during the growing season across the continental USA

Summary ?A major limitation in predicting the ultraviolet-B irradiance on humans, plant leaves and flowers and aquatic organisms is the difficulty in estimating exposure. This study analyzes the spatial variability in the daily exposure of narrow band 300 nm and 368 nm and broadband 290–315 nm (UVB) solar radiation between twelve paired locations in the United States Department of Agriculture (USDA) UVB Climate Network over two summer growing seasons (May through August of 2000 and 2001). The spatial correlation of the UVB, 300 nm and 368 nm daily exposures between locations was approximately 0.7 to 0.8 for spacing distances of 100 km. The 300 nm daily exposure was typically more highly correlated between locations than the 368 nm daily exposure. Both the diffuse and direct beam components to the 300 nm daily exposure were similarly correlated with distance between locations. The 368 nm diffuse component of the daily exposures was less correlated with distance than the direct beam component, limiting the ability to interpolate daily exposures from measurement locations. In general the variability in daily exposures of UVB in the USDA UVB Climate Network is too large to interpolate daily exposures of solar radiation, with estimated 300 nm, 368 nm and broadband UVB errors at one-half the mean station spacing of the USDA Network of 22%, 21% and 16% respectively. More accurate interpolations of UVB exposure from this network will require either the incorporation of cloud cover variability from satellite imagery for daily exposure or the use of longer periods of accumulated exposure. Received May 14, 2002; revised October 25, 2002; accepted November 16, 2002     

Parameterization and mapping of solar radiation in data sparse regions

Knowledge of temporal and spatial variation of solar radiation is essential for many applications. In this work, a simple and feasible procedure is conducted to map the daily solar radiation for Liaoning province, one of the most important agricultural areas in China, but with sparsely measured solar radiation data. The daily sunshine duration are interpolated to the whole area, subsequently, solar radiation are calculated by ?ngstr?m-Prescott model, the generic parameters of which are determined by least square to minimize the overall fitting residual between the ratio of actual to potential sunshine duration and the ratio of actual to extra-terrestrial solar radiation of the sites where solar radiation are available. In other local regions with sparse data, mapping of the solar radiation could be done following the simple procedure. In the present study area, using the interpolated daily sunshine duration data by ANUSPLIN, ?ngstr?m-Prescott model with the generic parameters (a = 0.505, and b = 0.204) returns reasonable results, with the overall RMSE of 2.255 MJ m^?2, and RRMSE of 16.54%. The daily solar radiation varies between 5.26 in December and 22.74 MJ m^?2 in May, and shows an obviously spatial variation which is mainly contributed to the climate and topography. The substitution of solar radiation from nearby station is preferred to estimation by ?ngstr?m-Prescott model if the distance between the stations falls below the threshold of 135 ± 15 km. The RMSE of such substitution increases by approximately 0.157 MJ m^?2 per 10 km.